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TM
MP2305
2A, 23V Synchronous Rectified Step-Down Converter
The Future of Analog IC Technology
TM
DESCRIPTION
The MP2305 is a monolithic synchronous buck regulator. The device integrates 130m MOSFETS that provide 2A continuous load current over a wide operating input voltage of 4.75V to 23V. Current mode control provides fast transient response and cycle-by-cycle current limit. An adjustable soft-start prevents inrush current at turn-on. Shutdown mode drops the supply current to 1A. This device, available in an 8-pin SOIC package, provides a very compact system solution with minimal reliance on external components.
FEATURES
* * * * * * * * * * * * * * * 2A Output Current Wide 4.75V to 23V Operating Input Range Integrated 130m Power MOSFET Switches Output Adjustable from 0.923V to 20V Up to 93% Efficiency Programmable Soft-Start Stable with Low ESR Ceramic Output Capacitors Fixed 340KHz Frequency Cycle-by-Cycle Over Current Protection Input Under Voltage Lockout Distributed Power Systems Networking Systems FPGA, DSP, ASIC Power Supplies Green Electronics/ Appliances Notebook Computers
APPLICATIONS
EVALUATION BOARD REFERENCE
Board Number EV2305DS-00A Dimensions 2.0"X x 1.5"Y x 0.5"Z
"MPS" and "The Future of Analog IC Technology" are Trademarks of Monolithic Power Systems, Inc.
TYPICAL APPLICATION
INPUT 4.75V to 23V C5 10nF
100 95 VOUT = 3.3V VOUT = 2.5V 90
Efficiency vs Load Current
EFFICIENCY (%)
2 7 IN EN
1 BS 3 SW
MP2305
8 SS GND 4 FB COMP 6 5
OUTPUT 3.3V 2A
85 80 75 70 65 60 55 50 0
C3 3.3nF
0.5 1.0 1.5 2.0 LOAD CURRENT (A)
2.5
MP2305-TAC01
MP2305-EC01
MP2305 Rev. 1.7 4/17/2006
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
PACKAGE REFERENCE
ABSOLUTE MAXIMUM RATINGS (1)
Supply Voltage VIN ....................... -0.3V to +26V Switch Voltage VSW .................. -1V to VIN +0.3V Boost Voltage VBS ..........VSW - 0.3V to VSW + 6V All Other Pins................................. -0.3V to +6V Junction Temperature...............................150C Lead Temperature ....................................260C Storage Temperature .............-65C to +150C
TOP VIEW
BS IN SW GND 1 2 3 4 8 7 6 5 SS EN COMP FB
Recommended Operating Conditions
(2)
MP2305_PD01
Input Voltage VIN ............................ 4.75V to 23V Output Voltage VOUT .................... 0.923V to 20V Ambient Operating Temperature .... -40C to +85C
Thermal Resistance
Part Number* MP2305DS * Package SOIC8 Temperature
(3)
SOIC8..................................... 90 ...... 45... C/W -40 to +85C
Notes: 1) Exceeding these ratings may damage the device. 2) The device is not guaranteed to function outside of its operating conditions. 3) Measured on approximately 1" square of 1 oz copper.
JA
JC
For Tape & Reel, add suffix -Z (eg. MP2305DS-Z) For Lead Free, add suffix -LF (eg. MP2305DS-LF-Z)
ELECTRICAL CHARACTERISTICS
VIN = 12V, TA = +25C, unless otherwise noted.
Parameter Shutdown Supply Current Supply Current Feedback Voltage Feedback Overvoltage Threshold Error Amplifier Voltage Gain (4) Error Amplifier Transconductance High-Side Switch On Resistance (4) Low-Side Switch On Resistance (4) High-Side Switch Leakage Current Upper Switch Current Limit Lower Switch Current Limit COMP to Current Sense Transconductance Oscillation Frequency Short Circuit Oscillation Frequency Maximum Duty Cycle Minimum On Time (4) EN Shutdown Threshold Voltage EN Shutdown Threshold Voltage Hysteresis EN Lockout Threshold Voltage EN Lockout Hysterisis
MP2305 Rev. 1.7 4/17/2006
Symbol Condition VEN = 0V VEN = 2.0V; VFB = 1.0V VFB AEA GEA RDS(ON)1 RDS(ON)2 VEN = 0V, VSW = 0V Minimum Duty Cycle From Drain to Source GCS Fosc1 Fosc2 DMAX VFB = 0V VFB = 1.0V VEN Rising IC = 10A 4.75V VIN 23V
Min
Typ 1 1.3
Max 3.0 1.5 0.946
Units A mA V V V/V A/V m m A A A A/V KHz KHz % ns V mV
0.900
0.923 1.1 400 800 130 130
10 2.4 3.4 1.1 3.5 340 100 90 220 1.5 210 2.2 2.5 210 2.7
1.1
2.0
V mV 2
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
ELECTRICAL CHARACTERISTICS (continued)
VIN = 12V, TA = +25C, unless otherwise noted.
Parameter Input Under Voltage Lockout Threshold Input Under Voltage Lockout Threshold Hysteresis Soft-Start Current Soft-Start Period Thermal Shutdown (4)
Note: 4) Guaranteed by design, not tested.
Symbol Condition VIN Rising
Min 3.80
Typ 4.10 210
Max 4.40
Units V mV A ms C
VSS = 0V CSS = 0.1F
6 15 160
PIN FUNCTIONS
Pin # 1 2 Name BS IN Description High-Side Gate Drive Boost Input. BS supplies the drive for the high-side N-Channel MOSFET switch. Connect a 0.01F or greater capacitor from SW to BS to power the high side switch. Power Input. IN supplies the power to the IC, as well as the step-down converter switches. Drive IN with a 4.75V to 23V power source. Bypass IN to GND with a suitably large capacitor to eliminate noise on the input to the IC. See Input Capacitor. Power Switching Output. SW is the switching node that supplies power to the output. Connect the output LC filter from SW to the output load. Note that a capacitor is required from SW to BS to power the high-side switch. Ground. Feedback Input. FB senses the output voltage to regulate that voltage. Drive FB with a resistive voltage divider from the output voltage. The feedback threshold is 0.923V. See Setting the Output Voltage. Compensation Node. COMP is used to compensate the regulation control loop. Connect a series RC network from COMP to GND to compensate the regulation control loop. In some cases, an additional capacitor from COMP to GND is required. See Compensation Components. Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to turn on the regulator, drive it low to turn it off. Pull up with 100k resistor for automatic startup. Soft-Start Control Input. SS controls the soft start period. Connect a capacitor from SS to GND to set the soft-start period. A 0.1F capacitor sets the soft-start period to 15ms. To disable the soft-start feature, leave SS unconnected.
3 4 5
SW GND FB
6
COMP
7 8
EN SS
MP2305 Rev. 1.7 4/17/2006
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 12V, VO = 3.3V, L = 10H, C1 = 10F, C2 = 22F, TA = +25C, unless otherwise noted.
Steady State Test
VIN = 12V, VOUT = 3.3V IOUT = 0A, IIN= 8.2mA
VIN 20mV/div. VOUT 20mV/div. IL 1A/div. VEN 5V/div. VOUT 2V/div. IL 1A/div. VSW 10V/div.
Startup through Enable
VIN = 12V, VOUT = 3.3V IOUT = 1A (Resistance Load)
VEN 5V/div. VOUT 2V/div. IL 1A/div. VSW 10V/div.
Shutdown through Enable
VIN = 12V, VOUT = 3.3V IOUT = 1A (Resistance Load)
VSW 10V/div.
MP2305-TPC01
2ms/div.
MP2305-TPC02
2ms/div.
MP2305-TPC03
Heavy Load Operation
2A Load
VIN, AC 200mV/div. VO, AC 20mV/div. IL 1A/div. VSW 10V/div. VIN, AC 200mV/div. VO, AC 20mV/div. IL 1A/div.
Medium Load Operation
1A Load
VIN, AC 20mV/div.
Light Load Operation
No Load
VO, AC 20mV/div. IL 1A/div. VSW 10V/div.
VSW 10V/div.
MP2305-TPC04
MP2305-TPC05
MP2305-TPC06
Short Circuit Protection
Short Circuit Recovery
Load Transient
VOUT 2V/div.
VOUT 2V/div.
VOUT 200mV/div.
IL 2A/div.
IL 1A/div. IL 2A/div. ILOAD 1A/div.
MP2305-TPC07
MP2305-TPC08
MP2305-TPC09
MP2305 Rev. 1.7 4/17/2006
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
OPERATION
FUNCTIONAL DESCRIPTION
The MP2305 is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages from 4.75V to 23V down to an output voltage as low as 0.923V, and supplies up to 2A of load current. The MP2305 uses current-mode control to regulate the output voltage. The output voltage is measured at FB through a resistive voltage divider and amplified through the internal transconductance error amplifier. The voltage at the COMP pin is compared to the switch current measured internally to control the output voltage. The converter uses internal N-Channel MOSFET switches to step-down the input voltage to the regulated output voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor connected between SW and BS is needed to drive the high side gate. The boost capacitor is charged from the internal 5V rail when SW is low. When the MP2305 FB pin exceeds 20% of the nominal regulation voltage of 0.923V, the over voltage comparator is tripped and the COMP pin and the SS pin are discharged to GND, forcing the high-side switch off.
+ OVP 1.1V FB 5 0.3V -OSCILLATOR + 100/340KHz ---SS 8 0.923V + + ERROR AMPLIFIER + S R Q Q 3 SW RAMP CLK CURRENT SENSE AMPLIFIER 2 + -1 BS 5V IN
CURRENT COMPARATOR
COMP 6 2.5V + EN OK -LOCKOUT COMPARATOR IN EN 7 + INTERNAL REGULATORS 1.5V -SHUTDOWN COMPARATOR 1.2V EN OVP
4 IN < 4.10V
GND
5V
MP2305_F01_BD01
Figure 1--Functional Block Diagram
MP2305 Rev. 1.7 4/17/2006
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
APPLICATIONS INFORMATION
COMPONENT SELECTION
Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio:
VFB = VOUT R2 R1 + R2
Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by:
ILP = ILOAD + VOUT V x 1 - OUT 2 x fS x L VIN
Where ILOAD is the load current. The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI requirements. Optional Schottky Diode During the transition between high-side switch and low-side switch, the body diode of the lowside power MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 1 lists example Schottky diodes and their Manufacturers. Table 1--Diode Selection Guide
Part Number B130 SK13 MBRS130 Voltage/Current Rating 30V, 1A 30V, 1A 30V, 1A Vendor Diodes, Inc. Diodes, Inc. International Rectifier
Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is:
VOUT = 0.923 x R1 + R2 R2
R2 can be as high as 100k, but a typical value is 10k. Using the typical value for R2, R1 is determined by:
R1 = 10.83 x ( VOUT - 0.923 ) (k)
For example, for a 3.3V output voltage, R2 is 10k, and R1 is 26.1k. Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by:
L= VOUT V x 1 - OUT f S x I L VIN
Input Capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choose X5R or X7R dielectrics when using ceramic capacitors. Since the input capacitor (C1) absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by:
I C1 = ILOAD x VOUT VOUT x1- VIN VIN
Where VOUT is the output voltage, VIN is the input voltage, fS is the switching frequency, and IL is the peak-to-peak inductor ripple current.
MP2305 Rev. 1.7 4/17/2006
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER The worst-case condition occurs at VIN = 2VOUT, where IC1 = ILOAD/2. For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1F, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple for low ESR capacitors can be estimated by:
VIN = ILOAD V V x OUT x 1 - OUT C1 x fS VIN VIN
The characteristics of the output capacitor also affect the stability of the regulation system. The MP2305 can be optimized for a wide range of capacitance and ESR values. Compensation Components MP2305 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. COMP pin is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. The DC gain of the voltage feedback loop is given by:
A VDC = R LOAD x G CS x A EA x VFB VOUT
Where C1 is the input capacitance value. Output Capacitor The output capacitor is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by:
VOUT = VOUT V x 1 - OUT fS x L VIN 1 x R ESR + 8 x f S x C2
Where AVEA is the error amplifier voltage gain; GCS is the current sense transconductance and RLOAD is the load resistor value. The system has two poles of importance. One is due to the compensation capacitor (C3) and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at:
fP1 = fP2 = GEA 2 x C3 x A VEA 1 2 x C2 x R LOAD
Where C2 is the output capacitance value and RESR is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be estimated by:
VOUT = VOUT 8 x fS
2
Where GEA is transconductance.
the
error
amplifier
The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at:
f Z1 = 1 2 x C3 x R3
V x 1 - OUT VIN x L x C2

In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to:
VOUT = VOUT V x 1 - OUT fS x L VIN x R ESR
The system may have another zero of importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero, due to the ESR and capacitance of the output capacitor, is located at:
fESR = 1 2 x C2 x R ESR
MP2305 Rev. 1.7 4/17/2006
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER In this case (as shown in Figure 2), a third pole set by the compensation capacitor (C6) and the compensation resistor (R3) is used to compensate the effect of the ESR zero on the loop gain. This pole is located at:
fP 3 = 1 2 x C6 x R3
3. Determine if the second compensation capacitor (C6) is required. It is required if the ESR zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid:
f 1 The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where the feedback loop has the unity gain is important. Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system instability. A good rule of thumb is to set the crossover frequency below one-tenth of the switching frequency. To optimize the compensation components, the following procedure can be used. 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation:
R3 = 2 x C2 x fC VOUT 2 x C2 x 0.1 x fS VOUT x < x GEA x GCS VFB GEA x GCS VFB
If this is the case, then add the second compensation capacitor (C6) to set the pole fP3 at the location of the ESR zero. Determine the C6 value by the equation:
C6 = C2 x R ESR R3
External Bootstrap Diode It is recommended that an external bootstrap diode be added when the system has a 5V fixed input or the power supply generates a 5V output. This helps improve the efficiency of the regulator. The bootstrap diode can be a low cost one such as IN4148 or BAT54.
5V
BS
MP2305
SW
10nF
Where fC is the desired crossover frequency which is typically below one tenth of the switching frequency. 2. Choose the compensation capacitor (C3) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, fZ1, below one-forth of the crossover frequency provides sufficient phase margin. Determine the C3 value by the following equation:
C3 > 4 2 x R3 x f C
MP2305_F02
Figure 2--External Bootstrap Diode This diode is also recommended for high duty cycle operation (when output voltage
VOUT >65%) and high VIN
(VOUT>12V)
applications
Where R3 is the compensation resistor.
MP2305 Rev. 1.7 4/17/2006
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MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
TYPICAL APPLICATION CIRCUIT
INPUT 4.75V to 23V C5 10nF
2 7 IN EN
1 BS 3 SW
MP2305
8 SS GND 4 FB COMP 6 5
OUTPUT 3.3V 2A
C6
(optional)
C3 3.3nF
D1 B130
(optional)
MP2305-F03
Figure 3--MP2305 with 3.3V Output, 22F/6.3V Ceramic Output Capacitor
MP2305 Rev. 1.7 4/17/2006
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TM
MP2305 - 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
PACKAGE INFORMATION
SOIC8
NOTICE: The information in this document is subject to change without notice. Please contact MPS for current specifications. Users should warrant and guarantee that third party Intellectual Property rights are not infringed upon when integrating MPS products into any application. MPS will not assume any legal responsibility for any said applications.
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