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19-1006; Rev 2; 10/08
KIT ATION EVALU BLE AVAILA
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter
General Description Features
Four Internal MOSFET True H-Bridge Buck/Boost Glitch-Free, Buck-Boost Transitions Minimal Output Ripple Variation on Transitions Up to 92% Efficiency
MAX8625A
The MAX8625A PWM step-up/down regulator is intended to power digital logic, hard disk drives, motors, and other loads in portable, battery-powered devices such as PDAs, cell phones, digital still cameras (DSCs), and MP3 players. The MAX8625A provides either a fixed 3.3V or adjustable output voltage (1.25V to 4V) at up to 0.8A from a 2.5V to 5.5V input. The MAX8625A utilizes a 2A peak current limit. Maxim's proprietary H-bridge topology* provides a seamless transition through all operating modes without the glitches commonly seen with other devices. Four internal MOSFETs (two switches and two synchronous rectifiers) with internal compensation minimize external components. A SKIP input selects a low-noise, fixedfrequency PWM mode, or a high-efficiency skip mode where the converter automatically switches to PFM mode under light loads for best light-load efficiency. The internal oscillator operates at 1MHz to allow for a small external inductor and capacitors. The MAX8625A features current-limit circuitry that shuts down the IC in the event of an output overload. In addition, soft-start circuitry reduces inrush current during startup. The IC also features True ShutdownTM, which disconnects the output from the input when the IC is disabled. The MAX8625A is available in a 3mm x 3mm, 14-pin TDFN package.
37A (typ) Quiescent Current in Skip Mode 2.5V to 5.5V Input Range Fixed 3.3V or Adjustable Output 1A (max) Logic-Controlled Shutdown True Shutdown Output Overload Protection Internal Compensation Internal Soft-Start 1MHz Switching Frequency Thermal-Overload Protection Small 3mm x 3mm, 14-Pin TDFN Package
Ordering Information
PART MAX8625AETD+ PINPACKAGE 14 TDFN-EP** (3mm x 3mm) TOP MARK ABQ
Applications
PDAs and Smartphones DSCs and Camcorders MP3 Players and Cellular Phones Battery-Powered Hard Disk Drive (HDD)
Note: The device is specified over the -40C to +85C extended temperature range. +Denotes a lead-free package. **EP = Exposed pad.
Pin Configuration
TOP VIEW
GND GND OUT OUT REF IN IN
Typical Operating Circuit
14
13
12
11
10
9
8
INPUT 2.7V TO 5.5V
LX1 IN
LX2 OUT GND
OUTPUT 3.3V
MAX8625A
PWM
+ 1 LX1 2 LX1 3 LX2 4 LX2 5 ON 6 SKIP EP
SKIP
7 FB
SKIP
MAX8625A
FB
ON OFF
REF ON
TDFN-EP
EP = EXPOSED PAD.
*US Patent #7,289,119. True Shutdown is a trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
ABSOLUTE MAXIMUM RATINGS
IN, OUT, SKIP, ON to GND ......................................-0.3V to +6V REF, FB, to GND...............................................-0.3V, (IN + 0.3V) LX2, LX1 (Note 1).........................................................1.5ARMS Continuous Power Dissipation (TA = +70C) Single-Layer Board (derate 18.5mW/C above TA = +70C) ...................................................1482mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Note 1: LX1 and LX2 have internal clamp diodes to IN, PGND and OUT, PGND, respectively. Applications that forward bias these diodes should take care not to exceed the device's power-dissipation limits.
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = 3.6V, ON = SKIP = IN, FB = GND, VOUT = 3.3V, LX_ unconnected, CREF = C5 = 0.1F to GND, Figure 4. TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 2)
PARAMETER Supply Range UVLO Threshold Quiescent Supply Current, FPWM Mode, Switching Quiescent Supply Current, Skip Mode, Switching Quiescent Supply Current, No Switching, Skip Mode Shutdown Supply Current SYMBOL VIN UVLO IIN IIN IIN IIN VIN rising, 60mV hysteresis No load, VOUT = 3.2V SKIP = GND, no load SKIP = GND, FB = 1.3V ON = GND, TA = +25C TA = +85C PWM mode, VIN = 2.5V to 5.5V Output Voltage Accuracy (Fixed Output) IOUT = 0 to 0.5A, VIN = 2.5V to 5.5V, TA = -40C to +85C (Note 3) SKIP mode, valley regulation value Average skip voltage Load step +0.5A Output Voltage Range (Adjustable Output) Maximum Output Current Soft-Start Load Regulation Line Regulation OUT Bias Current REF Output Voltage REF Load Regulation FB Feedback Threshold VFB IOUT VREF VIN = 3.6V L = 3.3H; COUT = C3 + C4 = 44F IOUT = 0 to 500mA VIN = 2.5V to 5.5V VOUT = 3.3V VIN = 2.5V to 5.5V IREF = 10A IOUT = 0 to full load, PWM mode; VIN = 2.5V to 5.5V 1.244 1.244 1.25 0.80 250 0.1 0.03 3 1.25 1 1.25 1.258 1.256 -1 3.28 3.285 -3 4.00 % V A mA/ms %/A %/V A V mV V CONDITIONS MIN 2.5 2.20 15 37 35 0.1 0.2 3.30 +1 45 1 TYP MAX 5.5 2.49 22 UNITS V V mA A A A V % V
2
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, ON = SKIP = IN, FB = GND, VOUT = 3.3V, LX_ unconnected, CREF = C5 = 0.1F to GND, Figure 4. TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 2)
PARAMETER FB Dual-Mode Threshold FB Leakage Current ON, SKIP Input High Voltage ON, SKIP Input Low Voltage ON Input Leakage Current SKIP Input Leakage Current Peak Current Limit Fault Latch-Off Delay Each MOSFET, TA = +25C MOSFET On-Resistance Rectifier-Off Current Threshold Idle-Mode Current Threshold (Note 4) LX1, LX2 Leakage Current RON ILX1OFF ISKIP Each MOSFET, VIN = 2.5V to 5.5V, TA = -40C to +85C SKIP = GND SKIP = GND, load decreasing Load increasing VIN = VOUT = 5.5V, VLX1 = 0V to VIN, VLX2 = 0V to VOUT, TA = +25C TA = +85C Out Reverse Current Minimum TON OSC Frequency Thermal Shutdown ILXLKGR TONMIN FOSCPWM 15C hysteresis 850 VIN = VLX1 = VLX2 = 0V, VOUT = 5.5V, measure I (LX2), TA = +25C TA = +85C 125 100 300 0.01 0.2 0.01 0.5 25 1000 +165 1150 % kHz C 1 A 1 SYMBOL VFBDM IFB VIH VIL IIHL ISKIPH ISKIPL ILIMP VFB = 1.3V, TA = +25C VFB = 1.3V, TA = +85C 2.5V < VIN < 5.5V 2.5V < VIN < 5.5V 2.5V < VIN < 5.5V, TA = +25C TA = +85C VSKIP = 3.6V VSKIP = 0V LX1 PMOS -2 1700 0.001 0.01 3 -0.2 2000 100 0.05 0.1 0.2 mA mA 2300 12 1.6 0.45 1 CONDITIONS MIN 75 TYP 100 0.001 0.01 MAX 125 0.1 UNITS mV A V V A A mA ms
MAX8625A
ILXLKG
A
Note 2: Devices are production tested at TA = +25C. Specifications over the operating temperature range are guaranteed by design and characterization. Note 3: Limits are guaranteed by design and not production tested. Note 4: The idle-mode current threshold is the transition point between fixed-frequency PWM operation and idle-mode operation. The specification is given in terms of output load current for an inductor value of 3.3H. For the step-up mode, the idle-mode transition varies with input to the output-voltage ratios.
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3
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
Typical Operating Characteristics
(VIN = 3.6V, SKIP = GND, TA = +25C, Figure 4, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT SKIP AND FPWM MODES
MAX8625A toc01
SKIP-MODE EFFICIENCY vs. INPUT VOLTAGE
MAX8625A toc02
EFFICIENCY vs. LOAD CURRENT FPWM MODE (FIGURE 3)
90 80 EFFICIENCY (%) 70 60 50 40 30 VOUT = 2.8V VIN = 2.7V 3.0V, 3.3V, 3.6V, 4.2V, 5.0V 0.1 1 10 100 1000
MAX8625A toc03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 100 VOUT = 3.3V VIN = 2.7V 3.0V, 3.3V, 3.6V, 4.2V, 5.0V
100 95 90 EFFICIENCY (%) 85 100mA 80 75 70 65 60 2.0 2.5 3.0 3.5 VOUT = 3.3V LOAD CURRENT = 100mA, 300mA, 500mA 4.0 4.5 5.0 5.5 500mA 300mA
100
20 10 0
1000
6.0
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
EFFICIENCY vs. LOAD CURRENT FPWM MODE (FIGURE 3)
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 100 1000 LOAD CURRENT (mA) VOUT = 3.45V VIN = 2.7V 3.0V, 3.3V, 3.6V, 4.2V, 5.0V
MAX8625A toc04
OUTPUT VOLTAGE (3.3V INTERNAL FB) vs. LOAD CURRENT
MAX8625A toc05
OUTPUT VOLTAGE (2.8V EXTERNAL FB) vs. LOAD CURRENT (FIGURE 3)
1.5 1.0 DEVIATION (%) 0.5 0 -0.5 -1.0
MAX8625A toc06
100
2.0 1.5 1.0 DEVIATION (%) 0.5 0 -0.5 -1.0 -1.5 -2.0 0.1 VOUT = 3.3V TA = +25C, TA = -40C, TA = +85C, 1 10 100
2.0
-1.5 -2.0 1000 0.1
VOUT = 2.8V TA = +25C, TA = -40C, TA = +85C 1 10 100 1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
OUTPUT VOLTAGE vs. INPUT VOLTAGE WITH INTERNAL FB RESISTORS
MAX8625A toc07
OUTPUT VOLTAGE vs. INPUT VOLTAGE WITH EXTERNAL FB RESISTORS
MAX8625A toc08
SUPPLY CURRENT vs. INPUT VOLTAGE WITH NO LOAD
MAX8625A toc09
3.33 3.32 OUTPUT VOLTAGE (V) 3.31 3.30 3.29 3.28 3.27 3.0 3.5 4.0 4.5 5.0 5.5 LOAD: 500mA, VOUT = 3.3V TA = +25C, TA = -40C, TA = +85C
2.82 2.81 OUTPUT VOLTAGE (V) 2.80 2.79 2.78 2.77 2.76 2.75 LOAD: 500mA, VOUT = 2.8V TA = +25C, TA = -40C, TA = +85C (FIGURE 3)
100 FPWM MODE SUPPLY CURRENT (mA) 10
1
0.1
NO LOAD VOUT = 3.3V
0.01 3.0 3.5 4.0 4.5 5.0 5.5 6.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V)
6.0
INPUT VOLTAGE (V)
4
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter
Typical Operating Characteristics (continued)
(VIN = 3.6V, SKIP = GND, TA = +25C, Figure 4, unless otherwise noted.)
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE
MAX8625A toc10
MAX8625A
SWITCHING WAVEFORMS VIN = 3V, LOAD = 500mA, VOUT = 3.3V
MAX8625A toc11
1000 900 MAXIMUM LOAD CURRENT (mA) 800 700 600 500 400 300 200 100 0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 VOUT = 3.3V
VOUT 50mV/div (AC-COUPLED) VLX1 2V/div VLX2 2V/div
ILX 500mA/div
6.0
1s/div
INPUT VOLTAGE (V)
SWITCHING WAVEFORMS VIN = 3.3V, LOAD = 500mA, VOUT = 3.3V
MAX8625A toc12
SWITCHING WAVEFORMS VIN = 3.6V, LOAD = 500mA, VOUT = 3.3V
MAX8625A toc13
VOUT 50mV/div (AC-COUPLED) VLX1 2V/div VLX2 2V/div VLX1 2V/div VLX2 2V/div
VOUT 50mV/div (AC-COUPLED)
ILX 500mA/div
ILX 500mA/div
1s/div
1s/div
SKIP MODE VIN = 3V, LOAD = 20mA, VOUT = 3.288V
MAX8625A toc14
FPWM MODE VIN = 3V, LOAD = 20mA, VOUT = 3.308V
VOUT 20mV/div (AC-COUPLED)
MAX8625A toc15
OUT
20mV/div (AC-COUPLED) VLX1 2V/div VLX2 2V/div
CH1 = VLX1 2V/div CH2 = VLX2 2V/div
ILX 500mA/div 1s/div
ILX 500mA/div
10s/div
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5
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
Typical Operating Characteristics (continued)
(VIN = 3.6V, SKIP = GND, TA = +25C, Figure 4, unless otherwise noted.)
STARTUP WAVEFORMS VIN = 3.6V, LOAD = 5, VOUT = 3.288V
MAX8625A toc16
STARTUP WAVEFORMS (FIGURE 3) VIN = 3.6V, LOAD = 30, VOUT = 1.5V
MAX8625A toc17
SHDN 2V/div VOUT 20mV/div ILX 500mA/div IBATT 500mA/div
SHDN 2V/div VOUT 500mA/div ILX 500mA/div
IBATT 100mA/div
2ms/div
2ms/div
LOAD TRANSIENT VOUT = 3.3V
MAX8625A toc18
LINE TRANSIENT VOUT = 3.3V, LOAD = 5.5, VIN RAMP 3V TO 4V
MAX8625A toc19
VOUT 100mV/div (DC OFFSET = 3.3V) IBATT 250mA/div CH2 = VOUT 50mV/div (AC-COUPLED)
ILX 500mA/div
CH1 = VIN 500mV/div 3V OFFSET
400s/div
1ms/div
BODE PLOT GAIN AND PHASE vs. FREQUENCY
40 30 20 10 GAIN (dB) 0 -10 -20 -30 -40 -50 -60 1 10 100 1000 FREQUENCY (kHz) VIN = 3.6 VOUT = 3.3V LOAD = 200mA PHASE GAIN
MAX8625A toc20
OSCILLATOR FREQUENCY vs. TEMPERATURE
144 108 72 36 0 -36 -72 -108 -144 -180 PHASE (DEG) OSCILLATOR FREQUENCY (MHz) 1.04 1.02 1.00 0.98 0.96 0.94 0.92 0.90 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
MAX8625A toc21
180
1.06
6
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter
Typical Operating Characteristics (continued)
(VIN = 3.6V, SKIP = GND, TA = +25C, Figure 4, unless otherwise noted.)
MINIMUM STARTUP VOLTAGE vs. TEMPERATURE
MAX8625A toc22
MAX8625A
REFERENCE vs. TEMPERATURE NO LOAD
MAX8625A toc23
2.48 2.46 MINIMUM STARTUP VOLTAGE (V) 2.44 2.42 2.40 2.38 2.36 2.34 2.32 2.30 2.28 -50 -25 0 25 50 75 VOUT = 3.3V, NO LOAD
1.28 1.27 REFERENCE (V) 1.26 1.25 1.24 1.23 1.22
VOUT = 3.3V VIN = 3.0V, 3.6V, 4.2V, 5.0V -40 -20 0 20 40 60 80 100
100
TEMPERATURE (C)
TEMPERATURE (C)
REFERENCE vs. TEMPERATURE WITH 300mA LOAD
MAX8625A toc24
SHUTDOWN DUE TO OVERLOAD VIN = 3.6V, VOUT = 3.288V
MAX8625A toc25
1.28 1.27 REFERENCE (V) 1.26 1.25 1.24 1.23 1.22 -40 -20 0 20 40 60 80 VOUT = 3.3V VIN = 3.0V, 3.6V, 4.2V, 5.0V
VLX2 2V/div VLX2 2V/div VOUT 500mV/div ILX 500mA/div 100s/div
100
TEMPERATURE (C)
BOOST-TO-BUCK TRANSITION FPWM MODE VIN = 3.6V, VOUT = 3.288V
MAX8625A toc26
VOUT 100mV/div AC-COUPLED VIN 1V/div DC OFFSET = 3V
ILX 200mA/div
2s/div
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
Pin Description
PIN 1, 2 3, 4 5 NAME LX1 LX2 ON FUNCTION Inductor Connection 1. Connect the inductor between LX1 and LX2. Both LX1 pins must be connected together externally. LX1 is internally connected to GND during shutdown. Inductor Connection 2. Connect the inductor between LX1 and LX2. Both LX2 pins must be connected together externally. LX2 is internally connected to GND during shutdown. Enable Input. Connect ON to the input or drive high to enable the IC. Drive ON low to disable the IC. Mode Select Input. Connect SKIP to GND to enable skip mode. This mode provides the best overall efficiency curve. Connect SKIP to IN to enable forced-PWM mode. This mode provides the lowest noise, but reduces lightload efficiency compared to skip mode. Feedback Input. Connect to ground to set the fixed 3.3V output. Connect FB to the center tap of an external resistor-divider from the output to GND to set the output voltage to a different value. VFB regulates to 1.25V. Reference Output. Bypass REF to GND with a 0.1F ceramic capacitor. VREF is 1.25V and is internally pulled to GND during shutdown. Power Output. Bypass OUT to GND with two 22F ceramic capacitors. Both OUT pins must be connected together externally. Ground. Connect the exposed pad and GND directly under the IC. Power-Supply Input. Bypass IN to GND with two 22F ceramic capacitors. Connect IN to a 2.5V to 5.5V supply. Both IN pins must be connected together externally. Exposed Pad. Connect to GND directly under the IC. Connect to a large ground plane for increased thermal performance.
6
SKIP
7
FB
8 9, 10 11, 12 13, 14 --
REF OUT GND IN EP
Detailed Description
The MAX8625A step-up/down architecture employs a true H-bridge topology that combines a boost converter and a buck converter topology using a single inductor and output capacitor (Figure 1). The MAX8625A utilizes a pulse-width modulated (PWM), current-mode control scheme and operates at a 1MHz fixed frequency to minimize external component size. A proprietary H-bridge design eliminates mode changes when transitioning from buck to boost operation. This control scheme provides very low output ripple using a much smaller inductor than a conventional H-bridge, while avoiding glitches that are commonly seen during mode transitions with competing devices. The MAX8625A switches at an internally set frequency of 1MHz, allowing for tiny external components. Internal compensation further reduces the external component count in cost- and space-sensitive applications. The MAX8625A is optimized for use in HDDs, DSCs, and other devices requiring low-quiescent current for optimal light-load efficiency and maximum battery life.
8
Control Scheme
The MAX8625A basic noninverting step-up/down converter operates with four internal switches. The control logic determines which two internal MOSFETs operate to maintain the regulated output voltage. Unlike a traditional H-bridge, the MAX8625A utilizes smaller peakinductor currents, thus improving efficiency and lowering input/output ripple. The MAX8625A uses three operating phases during each switching cycle. In phase 1 (fast-charge), the inductor current ramps up with a di/dt of VIN/L. In phase 2 (slow charge/discharge), the current either ramps up or down depending on the difference between the input voltage and the output voltage (VIN - VOUT)/L. In phase 3 (discharge), the inductor current discharges at a rate of VOUT/L through MOSFETs P2 and N1 (see Figure 1). An additional fourth phase (phase 4: hold) is entered when the inductor current falls to zero during phase 3. This fourth phase is only used during skip operation. The state machine (Figure 2) decides which phase to use and when to switch phases. The converter goes through the first three phases in the same order at all
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
LX1 IN P1 LX2 P2 OUT
N1 UVLO P1 CURRENT SENSE
N2
ON SKIP REF 1.25V Gm
PWM/PFM CONTROL OSCILLATOR
REFERENCE
GND
125mV
MAX8625
FB
Figure 1. Simplified Block Diagram
times. This reduces the ripple and removes any mode transitions from boost-only or buck-only to hybrid modes as seen in competing H-bridge converters. The time spent in each phase is set by a PWM controller, using timers and/or peak-current regulation on a cycle-by-cycle basis. The heart of the PWM control block is a comparator that compares the output voltage-error feedback signal and the sum of the currentsense and slope compensation signals. The currentmode control logic regulates the inductor current as a function of the output error voltage signal. The currentsense signal is monitored across the MOSFETs (P1, N1, and N2). A fixed time delay of approximately 30ns occurs between turning the P1 and N2 MOSFETs off, and turning the N1 and P2 MOSFETs on. This dead time prevents efficiency loss by preventing "shootthrough" current.
Step-Down Operation (VIN > VOUT) During medium and heavy loads and V IN > V OUT , MOSFETs P1 and N2 turn on to begin phase 1 at the clock edge and ramp up the inductor current. The duration of phase 1 is set by an internal timer. During phase 2, N2 turns off, and P2 turns on to further ramp up inductor current and also transfer charge to the output. This slow charge phase is terminated on a clock edge and P1 is turned off. The converter now enters the fast discharge phase (phase 3). In phase 3, N1 turns on and the inductor current ramps down to the valley current-regulation point set by the error signal. At the end of phase 3, both P2 and N1 turn off and another phase 1 is initiated and the cycle repeats. With SKIP asserted low, during light loads when inductor current falls to zero in phase 3, the converter switches to phase 4 to reduce power consumption and avoid
9
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
OFF ON = 0 P1, P2 = OFF N1, N2 = ON IQ = 0A TPUP POWER-UP ON = 1, P1, P2 = OFF, N1, N2 = ON, OSC = ON AND REF = ON IF UVLO OK TRUN REFOK = 0 OR UVLO = 0 (ASYNCHRONOUS FROM ANYWHERE)
FAULT TIMEOUT (ASYNCHRONOUS FROM ANYWHERE)
ERROR ON = 1 P1, P2 = OFF N1, N2 = ON
ON = 0 (ASYNCHRONOUS FROM ANYWHERE)
PHASE 2 SLOW CHARGE/ DISCHARGE OSC = ON P1, P2 = ON N1, N2 = OFF T1-3 T2-3
T1-2
PHASE 1 FAST-CHARGE OSC = ON P1, N2 = ON P2, N1 = OFF
T3-1
T4-1
PHASE 3 FAST DISCHARGE OSC = ON P2, N1 = ON P1, N2 = OFF
T3-4 (SKIP)
PHASE 4 HOLD OSC = OFF N1, N2 = ON P1, P2 = OFF
Figure 2. State Diagram
shuttling current in and out of the output capacitor. If SKIP is asserted high for forced-PWM mode, phase 4 is not entered and current shuttling is allowed (and is necessary to maintain the PWM operation frequency when no load is present).
Step-Up Operation (VIN < VOUT) During medium and heavy loads when VIN < VOUT, MOSFETs P1 and N2 turn on at the clock edge to ramp up the inductor current. Phase 1 terminates when the inductor current reaches the peak target current set by the PWM comparator and N2 turns off. This is followed by a slow-discharge phase (phase 2) instead of a charge phase (since VIN is less than VOUT) when P2 turns on. The slow-discharge phase terminates on a clock edge. The converter now enters the fast-discharge phase (phase 3). During phase 3, P1 turns off
10
and N1 turns on. At the end of the minimum time, both P2 and N1 turn off and the cycle repeats. If SKIP is asserted low, during light loads when inductor current falls to zero in phase 3, the converter switches to phase 4 (hold) to reduce power consumption and avoid shuttling current in and out of the output. If SKIP is high to assert forced-PWM mode, the converter never enters phase 4 and allows negative inductor current.
Step-Up/Down Transition-Zone Operation (VIN = VOUT) When VIN = VOUT, the converter still goes through the three phases for moderate to heavy loads. However, the maximum time is now spent in phase 2 where inductor current di/dt is almost zero, since it is proportional to (VIN - VOUT). This eliminates transition glitches
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter
or oscillation between the boost and buck modes as seen in other step-up/down converters. See the switching waveforms for each of the three modes and transition waveforms in the Typical Operating Characteristics section.
Soft-Start
Soft-start prevents input inrush current during startup. Internal soft-start circuitry ramps the peak inductor current with an internal DAC in 8ms. Once the output reaches regulation, the current limit immediately jumps to the maximum threshold. This allows full load capability as soon as regulation is reached, even if it occurs before the 8ms soft-start time is complete. When using the MAX8625A at low input voltages (close to UVLO and < 3V), it is recommended that the ON pin should not be tied to the BATT or supply voltage node directly. The ON pin should be held low for > 1ms after power to the MAX8625A is applied before it is driven high for normal operation.
MAX8625A
Forced-PWM Mode
Drive SKIP high to operate the MAX8625A in forcedPWM mode. In this mode, the IC operates at a constant 1MHz switching frequency with no pulse skipping. This scheme is desirable in noise-sensitive applications because the output ripple is minimized and has a predictable noise spectrum. Forced PWM consumes higher supply current at light loads due to constant switching.
Skip Mode
Drive SKIP low to operate the MAX8625A in skip mode to improve light-load efficiency. In skip mode, the IC switches only as necessary to maintain the output at light loads, but still operates with fixed-frequency PWM at medium and heavy loads. This maximizes light-load efficiency and reduces the input quiescent current to 37A (typ). Do not dynamically transition between skip and FPWM. The MAX8625A is not designed for dynamic transitions between FPWM and skip modes. Spikes of negative inductor current are possible when making these types of dynamic transitions. The magnitude of the spike depends on the load and output capacitance. The MAX8625A has no protection against these types of negative current spikes.
Shutdown
Drive ON low to place the MAX8625A in shutdown mode and reduce supply current to less than 1A. During shutdown, OUT is disconnected from IN, and LX1 and LX2 are connected to GND. Drive ON high for normal operation.
Fault and Thermal Shutdown
The MAX8625A contains current-limit and thermal shutdown circuitry to protect the IC from fault conditions. When the inductor current exceeds the current limit (2A for the MAX8625A), the converter immediately enters phase 3 and an internal 100ms timer starts. The converter continues to commutate through the three phases, spending most of its time in phase 1 and phase 3. If the overcurrent event continues and the output is out of regulation for the duration of the 100ms timer, the IC enters shutdown mode and the output latches off. ON must then be toggled to clear the fault. If the overload is removed before the 100ms timer expires, the timer is cleared and the converter resumes normal operation. The thermal-shutdown circuitry disables the IC switching if the die temperature exceeds +165C. The IC begins soft-start once the die temperature cools by 15C.
Load Regulation and Transient Response
During a load transient, the output voltage instantly changes due to the ESR of the output capacitors by an amount equal to their ESR times the change in load current (VOUT = RESR x ILOAD). The output voltage then deviates further based on the speed at which the loop compensates for the load step. Increasing the output capacitance reduces the output-voltage droop. See the Capacitor Selection section. The typical application circuit limits the output transient droop to less than 3%. See the Typical Operating Characteristics section.
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11
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
Applications Information
Selecting the Output Voltage
The MAX8625A output is nominally fixed at 3.3V. Connect FB to GND to select the internally fixed-output voltage. For an adjustable output voltage, connect FB to the center tap of an external resistor-divider connected from the output to GND (R1 and R2 in Figure 3). Select 100k for R2 and calculate R1 using the following equation: V R1 = 100k x OUT - 1 VFB where VFB = 1.25V and VOUT is the desired output regulation voltage. VOUT must be between 1.25V and 4V. Note that the minimum output voltage is limited by the minimum duty cycle. VOUT cannot be below 1.25V. 22F ceramic capacitors at the input. Select two 22F ceramic output capacitors. For best stability over a wide temperature range, use X5R or better dielectric.
Inductor Selection
The recommended inductance range for the MAX8625A is 3.3H to 4.7H. Larger values of L give a smaller ripple, while smaller L values provide a better transient response. This is because, for boost and stepup/down topologies, the crossover frequency is inversely proportional to the value of L for a given load and input voltage. The MAX8625A is internally compensated, and therefore, the choice of power components for stable operation is bounded. A 3.3H inductor with 2A rating is recommended for the 3.3V fixed output with 0.8A load.
PCB Layout and Routing
Good PCB layout is important to achieve optimal performance from the MAX8625A. Poor design can cause excessive conducted and/or radiated noise. Conductors carrying discontinuous currents and any high-current path should be made as short and wide as possible. Keep the feedback network (R1 and R2) very close to the IC, preferably within 0.2 inches of the FB and GND pins. Nodes with high dv/dt (switching nodes) should be kept as small as possible and routed away from FB. Connect the input and output capacitors as close as possible to the IC. Refer to the MAX8625A evaluation kit for a PCB layout example.
L 3.3H
Calculating Maximum Output Current
The maximum output current provided by the MAX8625A circuit depends on the inductor value, switching frequency, efficiency, and input/output voltage. See the Typical Operating Characteristics section for the Maximum Load Current vs. Input Voltage graph.
Capacitor Selection
The input and output ripple currents are both discontinuous in this topology. Therefore, select at least two
1 INPUT 2.7V TO 5.5V C1, C2 22F 13 14 LX1 IN IN
2 LX1
3 LX2
4 LX2 9 OUT 10 OUT OUTPUT 3V C3, C4 22F
U1
MODE SELECTION INPUT OFF 8 C5 0.1F 6 SKIP ON 5 ON
R1 140k FB 7
MAX8625A
R2 100k
REF
11 GND GND 12
Figure 3. Typical Application Circuit (Adjustable Output)
12 ______________________________________________________________________________________
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
L 3.3H
1 INPUT 2.7V TO 5.5V C1, C2 22F 13 14 LX1 IN IN
2 LX1
3 LX2
4 LX2 9 OUTPUT 3.3V C3, C4 22F
OUT 10 OUT
U1
MODE SELECTION INPUT OFF 8 C5 0.1F 6 SKIP ON 5 ON
MAX8625A
FB
7
REF
11 GND 12 GND
Figure 4. Typical Application Circuit (Fixed 3.3V Output)
Chip Information
PROCESS: BiCMOS
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13
High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 14 TDFN-EP PACKAGE CODE T1433-2 DOCUMENT NO. 21-0137
6, 8, &10L, DFN THIN.EPS
14
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
MAX8625A
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 0.20 MAX. 0.80 3.10 3.10 0.05 0.40
PACKAGE VARIATIONS PKG. CODE T633-2 T833-2 T833-3 T1033-1 T1033-2 T1433-1 T1433-2 N 6 8 8 10 10 14 14 D2 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.700.10 1.700.10 E2 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 e 0.95 BSC 0.65 BSC 0.65 BSC 0.50 BSC 0.50 BSC 0.40 BSC 0.40 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEC MO229 / WEEC MO229 / WEED-3 MO229 / WEED-3 ------b 0.400.05 0.300.05 0.300.05 0.250.05 0.250.05 0.200.05 0.200.05 [(N/2)-1] x e 1.90 REF 1.95 REF 1.95 REF 2.00 REF 2.00 REF 2.40 REF 2.40 REF
0.25 MIN. 0.20 REF.
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High-Efficiency, Seamless Transition, Step-Up/Down DC-DC Converter MAX8625A
Revision History
REVISION NUMBER 0 1 2 REVISION DATE 3/08 5/08 10/08 Initial release Added PCB Layout and Routing section Updated Skip Mode and Soft-Start sections DESCRIPTION PAGES CHANGED -- 12 2, 11
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2008 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. Inc.

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