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ZXLD1937
ADJUSTABLE LED DRIVER WITH INTERNAL SWITCH IN TSOT23-5
DESCRIPTION
The ZXLD1937 is a PFM inductive boost converter designed for driving 2, 3 or 4 series connected white LEDs from a Li-Ion cell and up to 8 LEDs from a 5V supply. The device operates from an input supply of between 2.5V and 5.5V and provides an adjustable output current of up to 50mA. The ZXLD1937 includes the output switch and peak current sense resistor, and can operate with a maximum output voltage of 28V. Quiescent current is typically 60 A and a shutdown function is provided to reduce this current to less than 500nA in the 'off' state. Output current can be adjusted by applying a PWM control signal to the 'Enable' pin. Depending upon the control frequency, this will provide either a continuous or a 'chopped' output current. The PWM filter components are contained within the chip. The device is assembled in the TSOT23-5 pin package with 1mm maximum height profile.
ADVANCED FEATURES
* True Analog Dimming via PWM
APPLICATIONS
* Mobile phones * Digital cameras * PDAs * LCD modules * Portable internet appliances * Palmtop computers
FEATURES
* * * * * * * * * *
1mm height profile TSOT23-5 pin package Internal PWM filter for flicker free output High efficiency (80% typ) Wide input voltage range: 2.5V to 5.5V Up to 50mA output current Low quiescent current: (60 A typ) 500nA maximum shutdown current Up to 1MHz switching frequency Low external component count Inherently matched LED currents
TYPICAL APPLICATION CIRCUIT
PIN CONNECTIONS
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ORDERING INFORMATION
DEVICE ZXLD1937ET5 DEVICE DESCRIPTION Boost converter in TSOT23-5 TEMPERATURE RANGE -40C to +85C PART MARK 1937 TAPING OPTIONS TA, TC
ZXLD1937ET5TA for 7" reel of 3,000 devices ZXLD1937ET5TC for 13" reel of 10,000 devices
ABSOLUTE MAXIMUM RATINGS (Voltages to GND unless otherwise stated)
PARAMETER Input voltage LX output voltage Switch output current Power dissipation Operating temperature Storage temperature Junction temperature SYMBOL (V IN ) (V LX ) (I LX ) (PD) (T OP ) (T ST ) (T j MAX ) LIMIT 7 30 500 300 -40 to 85 -55 to 150 125 UNIT V V mA mW C C C
NOTES: Operation above the absolute maximum may cause device failure. Operation at the absolute maximum ratings for extended periods may reduce device reliability.
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ELECTRICAL CHARACTERISTICS (at Vin = 3V, Tamb = 25C unless otherwise stated(1))
SYMBOL V IN I IN PARAMETER Input voltage Supply current Quiescent Shutdown V FB I FB f LX T OFF T ON I LXpk R LX I LX(leak) V OUT V ENH V ENL I ENL I ENH T EN(hold) T/T f LPF A LPF T/T FB pin control voltage FB pin input current Operating frequency LX output 'OFF' time LX output 'ON' time (2) Switch peak current limit Switch 'On' resistance Switch leakage current Controller output voltage EN pin High level Input voltage EN pin Low level Input voltage EN pin Low level input current EN pin High level input current EN pin turn off delay (3) PWM duty cycle range at `EN' input for filtered PWM control (4) Internal PWM low pass filter cut-off frequency Filter attenuation PWM duty cycle range at `EN' input for `gated' output current control (5) f=30kHz f < 1kHz, V ENH =V IN 0 V LX =20V Normal operation Device active Device in shutdown V EN =0V V EN =V IN V EN switched from high to low 10kHz < f < 100kHz, V ENH =V IN 20 4 52.5 100 120 100 1.5 L=10 H, V OUT =10V, I OUT =20mA 320 1.5 1 28 VIN 0.4 -100 1 A V V V nA A s % kHz dB % L=10 H, V OUT =10V, I OUT =20mA 350 0.35 500 5 V EN = V IN , I LX = 0, Output not switching V EN = 0V 90.5 60 100 500 109.5 100 1 A nA mV nA MHz ns s mA CONDITIONS MIN. 2.5 TYP. MAX. UNIT 5.5 V
NOTES: (1) Production testing of the device is performed at 25C. Functional operation of the device over a -40C to +85C temperature range is guaranteed by design, characterisation and process control. (2) Nominal 'on' time (TONnom) is defined by the input voltage (VIN), coil inductance (L) and peak current (ILXpkdc) according to the expression: TONnom = {ILX(pkdc) x L/VIN} +200ns. (3) This is the time for which the device remains active after the EN pin has been asserted low. This delay is necessary to allow the output to be maintained during dc PWM mode operation. (4) The minimum PWM signal frequency during this mode of operation is to ensure that the device remains active during PWM control. This provides a continuous dc output current. For lower frequencies, the device will be gated 'on' and 'off' during PWM control. (5) The maximum PWM signal frequency during this mode of operation should be kept as low as possible to minimise errors due to the turn-off delay of the device (see Enable pin turn-off delay).
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PIN DESCRIPTION
PIN NO. 1 2 3 4 NAME LX GND FB EN DESCRIPTION Output of NDMOS switch Ground (0V) Feedback pin for current control loop (connect resistor R1 from this pin to GND for output current I=100mV/ R1) Enable input (active high to turn on device) Also used to adjust output current by PWM signal Connect to Vin for permanent operation 5 V IN Input voltage (2.5V to 5.5V). Decouple with capacitor close to device
BLOCK DIAGRAM
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DEVICE DESCRIPTION The device is a PFM flyback dc-dc boost converter, working in discontinuous mode. With reference to the chip block diagram and typical application circuit, the operation of the device is as follows: Control loop When 'EN' is high, the control circuits become active and the low side of the coil (L1) is switched to ground via NDMOS transistor (MN). The current in L1 is allowed to build up to an internally defined level (nominally 320mA) before MN is turned off. The energy stored in L1 is then transferred to the output capacitor (C2) via schottky diode (D1). When the voltage on C2 has risen above the threshold voltage of the series connected LEDs, current will flow through external sense resistor R1. The voltage developed across R1 is sensed at pin 'FB' and compared to a 100mV reference voltage (V REF ). A comparator senses when the feedback voltage is above VREF and its output is used to control the 'off' time of the output switch. The control loop is self-oscillating, producing pulses of up to 5 s maximum duration (switch 'on'), at a frequency that varies in proportion to the LED current. The feedback loop maintains a voltage of VREF at the FB pin and therefore defines a maximum LED current equal to VREF divided by R1. The minimum 'off' time of the output switch is fixed at 0.5 s nominal, to allow time for the coil's energy to be dissipated before the switch is turned on again. This maintains stable and efficient operation in discontinuous mode. Filtered PWM operation The input of an internal low pass filter is switched to VREF when the EN pin is high and switched to ground when the EN pin is low. The output of this filter drives the comparator within the control loop. A continuous high state on EN therefore provides a filtered voltage of value VREF to the comparator. However, by varying the duty cycle of the EN signal at a suitably high frequency (f>10kHz), the control loop will see a voltage, that has an average value equal to the duty cycle multiplied by VREF. This provides a means of adjusting the output current to a lower value. It also allows the device to be both turned on and adjusted with a single signal at the 'EN' pin. The output during this mode of operation will be a dc current equal to (VREF /R1) x duty cycle Gated PWM operation The internal circuitry of the ZXLD1937 is turned off when no signal is present on the 'EN' pin for more than 120 s (nominal). A low frequency signal applied to the EN pin will therefore gate the device 'on' and 'off' at the gating frequency and the duty cycle of this signal can be varied to provide a 'chopped' output current equal to (VREF /R1) x duty cycle. For best accuracy, the gating frequency should be made as low as possible (e.g. below 1kHz), such that the turn off delay of the chip is only a small proportion of the gating period Further details of setting output current are given in the applications section under brightness control.
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TYPICAL CHARACTERISTICS (For typical application circuit at VIN=3V and TA=25C unless otherwise stated)
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TYPICAL OPERATING CHARACTERISTICS (For typical applications circuit at VIN=3V, L=10 H Coilcraft DO1608C Series, 3 series LEDs, ILED=15mA, TA=25C unless otherwise stated)
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APPLICATIONS
Programming the maximum LED current The maximum LED current is programmed by adding a single resistor in series with the LED chain. The current is determined by the resistor value and feedback voltage and is given by: ILED = VFB/R1 where VFB=100mV The table below gives recommended resistor values for required LED currents:
LED Current 10mA 15mA 20mA 30mA R1 Value 10 6.8 5 3.3
Pulsed Dimming - Gated Mode If a lower frequency of 1kHz or less is applied to the EN pin, the device will be gated 'on' and 'off' at a duty cycle (D) corresponding to that of the input signal. The average output current is then given by: IOUTavg 0.1D/R1 This mode may be preferred over dc current control if the purest white output is required. However, note the 120 s nominal turn-off delay of the device, when using the device in this mode.
Dimming Control via a PWM signal on the EN pin A Pulse Width Modulated (PWM) signal can be applied to the EN pin in order to adjust the output current to a value below the maximum LED current. Two modes of adjustment are possible as described below. True Analog Dimming - Filtered `DC' mode If a PWM signal of 10kHz or higher is applied to the EN pin, the device will remain active when the EN pin is low. However, the input to the internal low pass filter will be switched alternately from VREF to ground, with a duty cycle (D) corresponding to that of the PWM signal. This will present a filtered dc voltage equal to the duty cycle multiplied by VREF to the control loop and will produce a dc output current lower than the maximum set value. This current is given by: IOUTdc = 0.1D/R1 This mode of adjustment minimizes flicker in the light output and system noise.
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Dimming Control using a DC voltage For applications where the EN 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:
MOSFET `off'
I LED ( MIN ) =
MOSFET `on' 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.
V FB RLED V FB RLED
+ I LED ( MIN )
I LED ( MAX ) =
where VFB = 100mV
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Open circuit protection For applications where the LED chain can 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 current in the Zener diode defines the output current as: Capacitor selection A ceramic capacitor grounded close to the GND pin of the package is recommended at the output of the d e v i ce . S u r f a ce m o u n t t y p e s o f f e r t h e b e s t performance due to their lower inductance. A minimum value of 0.22 F is advised, although higher values will lower switching frequency and improve efficiency especially at lower load currents. A higher value will also minimise ripple when using the device to provide an adjustable dc output current. A good quality, low ESR capacitor should also be used for input decoupling, as the ESR of this capacitor is effectively in series with the source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. A minimum value of 1 F is acceptable if the input source is close to the device, but higher values will improve performance at lower input voltages, when the source impedance is high. The input capacitor should be mounted as close as possible to the IC. For maximum stability over temperature, capacitors with X7R dielectric are recommended, as these have a much smaller temperature coefficient than other types. A table of recommended manufacturers is provided below:
Manufacturer Murata Taiyo Yuden Kemet AVX Website www.murata.com www.t-yuden.com www.kement.com www.avxcorp.com
IZ =
V FB RZ
To limit battery drain in this fault condition the Zener diode current should be less than 1mA.
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Inductor selection The choice of inductor will depend on available board space as well as required performance. Small value inductors have the advantage of smaller physical size and may offer lower series resistance and higher saturation current compared to larger values. A disadvantage of lower inductor values is that they result in higher frequency switching, which in turn causes reduced efficiency due to switch losses. Higher inductor values can provide better performance at lower supply voltages. However, if the inductance is too high, the output power will be limited by the internal oscillator, which will prevent the coil current from reaching its peak value. This condition will arise whenever the ramp time (I LX(peak) x L/V IN ) exceeds the nominal 5 s maximum 'on' time limit for the LX output. The graphs opposite show the ZXLD1937 performance for given inductor values and different manufacturers. Recommended inductor values for the ZXLD1937 are in the range 6.8 H to 22 H. The inductor should be mounted as close to the device as possible with low resistance connections to the LX and VIN pins. Suitable coils for use with the ZXLD1937 are shown in the table below:
Part No. CMD4D11-100MC DO1608-103 LQH31CN100 LB2012Y100MR L ( H) 10 10 10 10 DCR () 0.457 0.16 1.3 0.5 I SAT (A) 0.5 1.1 0.23 0.1 Manufacturer Sumida www.sumida.com Coilcraft www.coilcraft.com Murata www.murata.com Taiyo Yuden www.t-yuden.co
Diode selection The rectifier diode (D1) should be a fast low capacitance schottky diode with low reverse leakage at the working voltage. It should also have a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. The table below gives some typical characteristics for diodes that can be used with the ZXLD1937:
Layout considerations PCB tracks should be kept as short as possible to minimise ground bounce, and the ground pin of the device should be soldered directly to the ground plane. It is particularly important to mount the coil and the input/output capacitors close to the device to minimise parasitic resistance and inductance, which will degrade efficiency. The FB pin is a high impedance input so PCB track lengths to this should also be kept as short as possible to reduce noise pickup. Excess capacitance from the FB pin to ground should be avoided.
Ic (mA) 400 500 I R at 30V ( A) 15 15 Package SOD323 SOT23
Diode ZHCS400 ZHCS500
V F @ 100mA (mV) 300 300
I FSM (mA) 1000 1000
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REFERENCE DESIGNS 3 LED Driver for Handset LCD Backlight Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref U1 D1 L1 R1 R2 1 C1 C2 LEDs 400mA 10 H 6.8 100k 1F 1F 0603 0603 0603 0603 Value Package TSOT23-5 SOD323 Part Number ZXLD1937ET5 ZHCS400 CMD4D11-100MC Generic Generic Generic Generic NSCW215 Manufacturer Zetex Zetex Sumida Generic Generic Generic Generic Nichia 3pcs per board Notes LED Driver IC 400mA Schottky Diode 1mm Height Profile
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
Performance Graphs
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ZXLD1937
4 LED Driver for Handset LCD Backlight Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref U1 D1 L1 R1 R2 1 C1 C2 LEDs 400mA 10 H 6.8 100k 1F 1F 0603 0603 0603 0603 Value Package TSOT23-5 SOD323 Part Number ZXLD1937ET5 ZHCS400 CMD4D11-100MC Generic Generic Generic Generic NSCW215 Manufacturer Zetex Zetex Sumida Generic Generic Generic Generic Nichia 4pcs per board Notes LED Driver IC 400mA Schottky Diode 1mm Height Profile
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
Performance Graphs
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ZXLD1937
5 LED Driver for Handset Main and Sub Display LCD Backlight Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref U1 D1 L1 R1 R2 1 C1 C2 400m A 10 H 6.8 100k 1F 1F 0603 0603 0603 0603 Value Package TSOT23-5 SOD323 Part Number ZXLD1937ET5 ZHCS400 CMD4D11-100MC Generic Generic Generic Generic Manufacturer Zetex Zetex Sumida Generic Generic Generic Generic Notes LED Driver IC 400mA Schottky Diode 1mm Height Profile
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
Performance Graphs
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ZXLD1937
6 LED Driver for LCD Backlight Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref U1 D1 L1 R1 R2 1 C1 C2 LEDs 400mA 10 H 6.8 100k 1F 1F 0603 0603 0603 0603 Value Package TSOT23-5 SOD323 Part Number ZXLD1937ET5 ZHCS400 CMD4D11-100MC Generic Generic Generic Generic NSCW215 Manufacturer Zetex Zetex Sumida Generic Generic Generic Generic Nichia 6pcs per board Notes LED Driver IC 400mA Schottky Diode 1mm Height Profile
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
Performance Graphs
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PACKAGE OUTLINE - TSOT23-5
Controlling dimensions are in millimeters. Approximate conversions are given in inches
PACKAGE DIMENSIONS
Millimeters DIM Min A A1 A2 b c D E 0.01 0.84 0.30 0.12 Max 1.00 0.10 0.90 0.45 0.20 Min 0.0003 0.0330 0.0118 0.0047 Max 0.0393 0.0039 0.0354 0.0177 0.0078 E1 e e1 L L2 a Inches DIM Min Max Min Max 1.60 BSC 0.95 BSC 1.90 BSC 0.30 0.50 0.062 BSC 0.0037 BSC 0.074 BSC 0.0118 0.0196 Millimeters Inches
0.25 BSC 4 12 -
0.010 BSC 4 12 -
2.90 BSC 2.80 BSC
0.114 BSC 0.110 BSC
(c) Zetex Semiconductors plc 2004
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