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ZXSC400
LED DRIVER BOOST CONVERTER
DESCRIPTION
The ZXSC400 is a voltage mode boost converter in the SOT23-6 package. Its low feedback voltage allows the current in a chain of LEDs to be set and accurately monitored with a single resistor giving minimal losses. Its excellent load and line regulation means that for the full supply range from lithium-ion cells, the LED current will typically change by less than 1%. Using high efficiency Zetex switching transistors with ratings of 20V and higher allow many LEDs to be chained in series for the best LED current matching possible.
FEATURES
* 1.8V to 8V supply range * Typical output regulation of 1% * Over 80% typical efficiency * 4.5 A typical shutdown current * Series connection for ultimate LED current matching
SOT23-6
APPLICATIONS
* White LED backlighting for colour LCD panels * General LED backlighting * High performance white LED flashlights * General LED driving from batteries
PINOUT
ORDERING INFORMATION
DEVICE
ZXSC400E6TA
REEL SIZE
7"
TAPE WIDTH
8mm
QUANTITY PER REEL
3000 units
VCC GND STDN
Top View
DRIVE VFB SENSE
DEVICE MARKING
* C400
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ZXSC400
ABSOLUTE MAXIMUM RATINGS
VCC DRIVE EOR STDN SENSE Operating Temp. Storage Temp. Power Dissipation -0.3V to -0.3V to -0.3V to -0.3V to -0.3V to -40C to -55C to 450mW +10V VCC + 0.3V VCC + 0.3V The lower of (+5.0V) or (VCC + 0.3V) The lower of (+5.0V) or (VCC + 0.3V) +85C +125C
ELECTRICAL CHARACTERISTICS Test Conditions VCC= 3V, T= -40C to 85C unless otherwise stated.
Symbol Parameter Conditions Min Supply parameters V IN Iq 1 I STDN Eff 1 Acc REF TCO REF T DRIVE F OSC V SENSE I SENSE V FB I FB 2 V IH V IL dV LN I OUT 3 I DRIVE V DRIVE C DRIVE dILD 4
Note
1 2 3 4
Limits Typ Max
Units
V CC Range Quiescent Current Shutdown Current Efficiency Reference tolerance Reference Temp Co Discharge pulse width Operating Frequency 1.8V < V CC < 8V 30mA > I LED > 10mA 1.8V < V CC < 8V V CC = 8V
1.8
8 220 4.5 80
V A A %
-3.0 0.005 1.7
3.0
% %/ C s
200
kHz
Input parameters Sense voltage Sense input current Feedback voltage Feedback input current Shutdown high voltage Shutdown low voltage Line voltage regulation V FB = 0V, V SENSE = 0V V FB = 0V, V SENSE = 0V 22 -1 291 -1.2 1.5 0 0.5 28 -7 300 34 -15 309 -4.5 VCC 0.55 V V %/V mV A mV A
Output parameters Output current Transistor drive current Transistor voltage drive Mosfet gate drive cpbty Load current regulation Vin > 2V, Vout = 10.5V Vdrive = 0.7V 1.8V < V CC < 8V 50 2 0 300 0.1 3.5 5 V C C -0.4 mA mA V pF mA/V
Excluding gate/base drive current. IFB is typically half these values at 300mV. System not device spec, including recommended transistors. 10.5V represents 3 white LEDs. Change in LED current with changing supply voltage (LED VF x number of series LEDs).
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TYPICAL CHARACTERISTICS
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ZXSC400
DESIGN INFORMATION IC operation description
Bandgap Reference All threshold voltages and internal currents are derived from a temperature compensated bandgap reference circuit with a reference voltage of 1.22V nominal. Dynamic Drive Output Depending on the input signal, the output is either "LOW" or "HIGH". In the high state a 2.5mA current source (max drive voltage = VCC-0.4V) drives the base or gate of the external transistor. In order to operate the external switching transistor at optimum efficiency, both output states are initiated with a short transient current in order to quickly discharge the base or the gate of the switching transistor. Switching Circuit The switching circuit consists of two comparators, Comp1 and Comp2, a gate U1, a monostable and the drive output. Normally the DRIVE output is "HIGH"; the external switching transistor is turned on. Current ramps up in the inductor, the switching transistor and external current sensing resistor. This voltage is sensed by comparator, Comp2, at input ISENSE. Once the current sense voltage across the sensing resistor exceeds 30mV, comparator Comp2 through gate U1 triggers a re-triggerable monostable and turns off the output drive stage for 2 s. The inductor discharges to the load of the application. After 2 s a new charge cycle begins, thus ramping the output voltage. When the output voltage reaches the nominal value and FB gets an input voltage of more than 300mV, the monostable is forced "on" from Comp1 through gate U1, until the feedback voltage falls below 300mV. The above action continues to maintain regulation. Block diagram
2s
Pin functions
Pin No. 1 2 3 4 5 6 Name V CC GND STDN SENSE V FB DRIVE Description Supply voltage, 1.8V to 8V. Ground Shutdown Inductor current sense input. Internal threshold voltage set to 30mV. Connect external sense resistor Reference voltage. Internal threshold set to 300mV. Connect external resistor to program LED current Drive output for external switching transistor. Connect to base or gate of external switching transistor.
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APPLICATIONS INFORMATION
Switching transistor selection The choice of switching transistor has a major impact on the converter efficiency. For optimum performance, a bipolar transistor with low VCE(SAT) and high gain is required. The VCEO of the switching transistor is also an important parameter as this sees the full output voltage when the transistor is switched off. Zetex SuperSOTTM transistors are an ideal choice for this application. Schottky diode selection As with the switching transistor, the Schottky rectifier diode has a major impact on the converter efficiency. A Schottky diode with a low forward voltage and fast recovery time should be used for this application. The diode should be selected so that the maximum forward current rating is greater or equal to the maximum peak current in the inductor, and the maximum reverse voltage is greater or equal to the output voltage. The Zetex ZHCS Series meet these needs. Combination devices To minimise the external component count Zetex recommends the ZX3CDBS1M832 combination of NPN transistor and Schottky diode in a 3mm x 2mm MLP package. This device is recommended for use in applications using 1 to 4 white LEDs. The IC is also capable of driving MOSFETs. Zetex recommends the ZXMNS3BM832 combination of low threshold voltage N-Channel MOSFET and Schottky diode in a 3mm x 2mm MLP package. This device is recommended for use in applications using 1 to 8 white LEDs. Capacitor Selection A small value, low ESR ceramic capacitor is required to filter the output, typically 1 F to 4.7 F. Generally an input capacitor is not required, but a small ceramic capacitor may be added to aid EMC, typically 1 F to 4.7 F. Inductor Selection The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution. Inductor selection has a significant impact on the converter performance. For applications where efficiency is critical, an inductor with a series resistance of 250m or less should be used. A list of recommended inductors is listed in the table below:
Part No. CMD4D11-100MC CMD4D11-220MC LPO2506OB-103 ST2006103
Manufacture Sumida Sumida Coilcraft Standex Electronics Inc
L 10 H 22 H 10 H 10 H
I PK (A) 0.5 0.4 1.0 0.6
R DC ( ) 0.457 0.676 0.24 0.1
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ZXSC400
Peak current definition In general, the IPK value must be chosen to ensure that the switching transistor, Q1, is in full saturation with maximum output power conditions, assuming worse-case input voltage and transistor gain under all operating temperature extremes. Once I PK is decided the value of R SENSE can be determined by: RSENSE = Sense Resistor A low value sense resistor is required to set the peak current. Power in this resistor is negligible due to the low sense voltage threshold, VISENSE. At the bottom of the page there is a table of recommended sense resistors. Output power calculation By making the above assumptions for inductance and peak current the output power can be determined by: POUT = IAV x VIN x where IAV = and TON and TDIS = and TOFF 1.7 s (internally set by ZXSC400) and = efficiency e.g. 100% = 1 Operating frequency can be derived by: F=
Manufacture Cyntec IRC
Programming LED current Once the required output power is determined, the LED current can be programmed by adding a single resistor in the LED chain. The resistor value is determined by the following:
VSENSE IPK
ILED = Where VFB = 300mV RLED selection table
ILED 40mA 30mA
VFB RLED
(Watts)
IPK (TON + TDIS) x 2 (TON + TOFF) IPK x L = VIN
RLED 7.5 10 15 20 30
20mA 15mA 10mA
IPK x L (VLED - VIN)
Shutdown Control The ZXSC400 offers a shutdown mode that consumes a standby current of less than 5 A. When the voltage at the STDN pin between 1V and 8V (and also open circuit), the ZXSC400 is enabled and the driver is in normal operation. When the voltage at the STDN pin is 0.7V or lower, the ZXSC400 is disabled and the driver is in shutdown mode. The SHDN input is a high impedance current source of 1 A typ. The driving device can be an open collector or an open drain or a logic output with a "High" voltage of 5V max. The device shutdown current depends of the supply voltage, see typical characteristics graph
1 TON + TOFF
Series RL1220 LR1206 RDC( ) Range 0.022 - 10 0.010 - 1.0 Size 0805 1206 Tolerance 5% 5% URL www.cyntec.com www.irctt.com
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Open-circuit protection For applications where the LED chain might go open-circuit a Zener diode can be connected across the LED chain preventing over-voltage and possible damage to the main switching transistor. The Zener diodes should be selected by ensuring its voltage rating is higher than the combined forward voltage of the LED chain. Under open circuit conditions the curren in the Zener diode defines the output current as: IZ = VFB RZ Dimming Control using the shutdown pin The first method uses the shutdown pin. By injecting a PWM waveform on this pin and varying the duty cycle, LED current and hence LED brightness can be adjusted. To implement this method of brightness control on the ZXSC400, apply PWM signal with amplitude of between 0.7V and Vcc at a frequency of 120Hz or above (to eliminate LED flicker). The LED current and hence LED brightness is linearly proportional to the duty cycle ratio, so for brightness control adjust duty cycle ratio as necessary. For example, a 10% duty cycle equates to 10% of full LED brightness.
The circuit example below give an open circuit output current of 300 A.
Dimming Control There are four types of dimming control that can be implemented for the ZXSC400.
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ZXSC400
Dimming Control using a DC voltage For applications where the shutdown pin is not available a DC voltage can be used to control dimming. By adding resistors R2 and R3 and applying a DC voltage, the LED current can be adjusted from 100% to 0%. As the DC voltage increases, the voltage drop across R2 increases and the voltage drop across R1 decreases, thus reducing the current through the LEDs. Selection of R2 and R3 should ensure that the current from the DC voltage is much less than the LED current and much larger than the feedback current. The component values in the diagram below represent 0% to 100% dimming control from a 0 to 2V DC voltage. Dimming Control using a logic signal For applications where the LED current needs to be adjusted in discrete steps a logic signal can be applied as shown in the diagram below. When Q1 os `off', R1 sets the minimum LED current. When Q1 is `on', R2 sets the LED current that will be added to the minimum LED current. The formula for selecting values for R1 and R2 are given below:
ZXSC400 VFB
R2
ZXSC400 VFB
VDC
R3
67k
R2
10k
LOGIC SIGNAL
R1
MOSFET `off' VFB ILED(MIN) = RLEB
Q1
R1
Dimming Control using a filtered PWM signal The filtered PWM signal can be considered as an adjustable DC voltage by applying a RC filter. The values shown in the diagram below are configured to give 0% to 100% dimming for a 1kHz to 100kHz PWM signal with a 2V amplitude. e.g. a 50% duty cycle will give 50% dimming.
MOSFET `on' VFLB ILED(MAX) = + ILED(MIN) RLEB Where VFB = 300mV
ZXSC400 VFB
PWM
R4
10k
R3
67k
R2
10k
R1
C1
0.1F
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ZXSC400
Layout considerations Layout is critical for the circuit to function in the most efficient manner in terms of electrical efficiency, thermal considerations and noise. For `step-up converters' there are four main current loops, the input loop, power-switch loop, rectifier loop and output loop. The supply charging the input capacitor forms the input loop. The power-switch loop is defined when Q1 is `on', current flows from the input through the inductor, Q1, RSENSE and to ground. When Q1 is `off', the energy stored in the inductor is transferred to the output capacitor and load via D1, forming the rectifier loop. The output loop is formed by the output capacitor supplying the load when Q1 is switched back off. To optimise for best performance each of these loops kept separate from each other and interconnected with short, thick traces thus minimising parasitic inductance, capacitance and resistance. Also the RSENSE resistor should be connected, with minimum trace length, between emitter lead of Q1 and ground, again minimising stray parasitics.
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ZXSC400
REFERENCE DESIGNS
Li - Ion to 2 LED converter
100 90
30
Load Current (mA)
3.5 4.0
2 White LEDs ILED=20mA
25 20 15 10 5 0 2.5
2 White LEDs
Efficiency (%)
80 70 60
2.5
3.0
3.0
3.5
4.0
Input voltage (V)
Input Voltage (V)
Input Voltage vs Efficiency
Input Voltage vs LED Current
Bill of Materials
Ref U1 U2 L1 R1 R2 C1 C2 LED 10 H 100m 15 2.2 F/6V3 2.2 F/16V Value Part Number ZXSC400 ZX3CDBS1M832 CMD4D11-100MC LR1206 / RL1220 Generic GRM Series GRM Series NSCW215 Manufacture Zetex Zetex Sumida IRC / Cyntec Generic Murata Murata Nichia Comments LED driver IC NPN + Schottky dual 1mm height profile 1206 / 0805 size 0603 size 0805 size 1206 size White SMT LED
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ZXSC400
Li - Ion to 3 LED converter
100 90 80 70 60 2.5
30
LED Current (mA)
3 White LEDs ILED=20mA
3 White LEDs
Efficiency (%)
20
10
3.0
3.5
4.0
0 2.5
3.0
3.5
4.0
Input Voltage v Efficiency
Input Voltage (V)
Input Voltage v LED Current
Input Voltage (V)
Bill of Materials
Ref U1 U2 L1 R1 R2 C1 C2 LED 10 H 68m 15 2.2 F/6V3 2.2 F/16V Value Part Number ZXSC400 ZX3CDBS1M832 CMD4D11-100MC LR1206 / RL1220 Generic GRM Series GRM Series NSCW215 Manufacture Zetex Zetex Sumida IRC / Cyntec Generic Murata Murata Nichia Comments LED driver IC NPN + Schottky dual 1mm height profile 1206 / 0805 size 0603 size 0805 size 1206 size White SMT LED
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ZXSC400
Li - Ion to 4 LED converter
90
30
80
Load Current (mA)
4.0
4 White LEDs ILED=20mA
Efficiency (%)
L=22H RSENSE=80m
25 20 15 10 5 0 2.5
4 White LEDs
70
L=10H RSENSE=47m
60 2.5
3.0
3.5
3.0
3.5
4.0
Input voltage (V)
Input Voltage (V)
Input Voltage vs Efficiency
Input Voltage vs LED Current
Bill of Materials
Ref U1 U2 L1 R1 R2 C1 C2 LED 10 H 47m 15 2.2 F/6V3 2.2 F/16V Value Part Number ZXSC400 ZX3CDBS1M832 CMD4D11-100MC LR1206 / RL1220 Generic GRM Series GRM Series NSCW215 Manufacture Zetex Zetex Sumida IRC / Cyntec Generic Murata Murata Nichia Comments LED driver IC NPN + Schottky dual 1mm height profile 1206 / 0805 size 0603 size 0805 size 1206 size White SMT LED
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ZXSC400
Li - Ion to 6 LED converter
90
30
80
Load Current (mA)
3.5 4.0
6 White LEDs ILED=20mA
25 20 15 10 5 0 2.5
6 White LEDs
Efficiency (%)
70
60 2.5
3.0
3.0
3.5
4.0
Input voltage (V)
Input Voltage (V)
Input Voltage vs Efficiency
Bill of Materials
Ref U1 Q1 D1 L1 R1 R2 C1 C2 LED 22 H 47m 15 2.2 F/6V3 2.2 F/25V Value Part Number ZXSC400 FMMT619 ZHCS1000 CMD4D11-220MC LR1206 / RL1220 Generic GRM Series GRM Series NSCW215
Input Voltage vs LED Current
Manufacture Zetex Zetex Zetex Sumida IRC / Cyntec Generic Murata Murata Nichia
Comments LED driver IC 40V NPN 1A Schottky diode 1mm height profile 1206 / 0805 size 0603 size 0805 size 1206 size White SMT LED
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ZXSC400
Li - Ion to 8 LED converter
90
30
80
Load Current (mA)
3.5 4.0
8 White LEDs ILED=20mA
25 20 15 10 5 0 2.5
8 White LEDs
Efficiency (%)
70
60 2.5
3.0
3.0
3.5
4.0
Input voltage (V)
Input Voltage (V)
Input Voltage vs Efficiency
Bill of Materials
Ref U1 Q1 D1 L1 or R1 R2 C1 C2 LED 22 H 33m 15 2.2 F/6V3 2.2 F/35V Value Part Number ZXSC400 FMMT619 ZHCS1000 CDRH4D28-220 D01608C-223 LR1206 / RL1220 Generic GRM Series GRM Series NSCW215
Input Voltage vs LED Current
Manufacture Zetex Zetex Zetex Sumida Coilcraft IRC / Cyntec Generic Murata Murata Nichia
Comments LED driver IC 40V NPN 1A Schottky diode 0.7A, 0.235 0.7A, 0.37 1206 / 0805 size 0603 size 0805 size 1206 size White SMT LED
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Notes
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ZXSC400
PACKAGE OUTLINE PAD LAYOUT DETAILS
b
e L2
E
E1
e1 D
a
DATUM A
C
A
A2
A1
CONTROLLING DIMENSIONS IN MILLIMETRES APPROX CONVERSIONS INCHES.
PACKAGE DIMENSIONS
Millimetres DIM Min A A1 A2 b C D 0.90 0.00 0.90 0.35 0.09 2.80 Max 1.45 0.15 1.30 0.50 0.20 3.00 Min 0.35 0 0.035 0.014 0.0035 0.110 Max 0.057 0.006 0.051 0.019 0.008 0.118 E E1 L e e1 L Inches DIM Min 2.60 1.50 0.10 Max 3.00 1.75 0.60 Min 0.102 0.059 0.004 Max 0.118 0.069 0.002 Millimetres Inches
0.95 REF 1.90 REF 0 10
0.037 REF 0.074 REF 0 10
(c) Zetex plc 2003
Europe Zetex plc Fields New Road Chadderton Oldham, OL9 8NP United Kingdom Telephone (44) 161 622 4422 Fax: (44) 161 622 4420 uk.sales@zetex.com Zetex GmbH Streitfeldstrae 19 D-81673 Munchen Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Americas Zetex Inc 700 Veterans Memorial Hwy Hauppauge, NY11788 USA Telephone: (631) 360 2222 Fax: (631) 360 8222 usa.sales@zetex.com Asia Pacific Zetex (Asia) Ltd 3701-04 Metroplaza, Tower 1 Hing Fong Road Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com
These offices are supported by agents and distributors in major countries world-wide. This publication is issued to provide outline information only which (unless agreed by the Company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. The Company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. For the latest product information, log on to
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