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ZXLB1600
LCD AND OLED BIAS BOOST CONVERTER - STN, CSTN SCREENS
DESCRIPTION
The ZXLB1600 inductive switching boost converter accepts an input voltage of between 1.6V and 5.5V and provides an adjustable output voltage of up to 28V for LCD and OLED bias. The device contains an output switch and a second switch to isolate the coil from the input to provide true isolation in shutdown mode. The output can be adjusted by means of an externally applied dc voltage, a PWM control signal, or external feedback resistors and can supply typically 10mA of output current at maximum output voltage. Higher current is available at lower output voltages. The input voltage range accepts a number of battery solutions, including dual dry cell and single Li-Ion cells and PFM operation mode allows the output to be regulated with high efficiency under light or no load conditions. The switching frequency range permits the use of miniature surface mount inductors. A low battery comparator is provided to indicate when the input voltage has fallen to within 2% of a preset threshold. This threshold is set internally , but can be adjusted externally to any voltage within the supply voltage range.
FEATURES
* Wide input voltage range: 1.6 to 5.5V * Adjustable output voltage up to 28V, using PWM
or analog control voltage Internal PWM filter True shutdown (output isolated from input) Internal output switch and current sense Low quiescent current: (75 A max) 5 A (max) shutdown current (including low battery comparator) Up to 500kHz switching frequency High efficiency Small MSOP10 package Low external component count
* * * * * * * * * * * * * *
APPLICATIONS
PDAs Mobile phones - OLED sub displays Digital cameras Portable internet appliances Palmtop computers
* GPS terminals
TYPICAL APPLICATION CIRCUIT
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ABSOLUTE MAXIMUM RATINGS (Voltages to GND unless otherwise stated)
Output voltage Input voltage Switch output current Power dissipation Operating temperature Storage temperature Junction temperature 30V 7V 500mA 800mW 0 to 70 C -55 to 125 C 150 C
ELECTRICAL CHARACTERISTICS Test conditions unless otherwise stated: VIN = 3.0V, TAMB = 25C
Symbol General V IN I IN Input voltage (1) Supply current Shutdown Quiescent Isolating switch R ISO I ISO V SW(max) LX Switch I LX R LX I LX(leak) V LX V OUT I OUT V LNR V LDR f LX T ON T OFF Switch peak current limit (3) `On' resistance Switch leakage current Operating voltage on LX pin -0.5 0.15 0.5 0.35 2 1 30 V A A `On' resistance Leakage current Maximum voltage on SW pin during normal operation V EN = V IN V EN = 0V 0.7 2 1 VIN+0.5 V A
(2)
Parameter
Conditions
Min
Typ
Max
Units
1.6 V EN = 0V V EN = V IN , I OUT = 0V, Not switching 3.5 30
5.5 5 75
V A A
Controller output Output voltage range Output current Line regulation Load regulation Operating frequency Output `ON' time Output `OFF' time Efficiency (4) LX output low LX output off Lx = 22 H, V OUT = 20V 0.9 80 Lx = 22 H, V OUT = 28V I OUT = 1mA, 2V < V IN <5.5V V OUT = 28V, 100 A < I OUT < 5mA VIN 5 10 0.1 0.15 500 10 28 V mA %/V %/mA kHz s s %
Note: 1) Minimum supply voltage should be maintained above 2V for operation at minimum temperature. 2) Shutdown current includes the operating current for the low battery comparator, which remains active in shutdown mode. 3) This is the dc value. The dynamic value may exceed 350mA during normal operation, due to switching delays, coil inductance and supply voltage. 4) Efficiency is dependent upon the choice of external components, input/output voltages and load current.(see typical operating curves).
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ELECTRICAL CHARACTERISTICS (Cont.) Test conditions unless otherwise stated: VIN = 3.0V, TAMB = 25C
Symbol Parameter Conditions Min Typ Max Units Output voltage control by dc voltage applied to `ADJ' pin V ADJ (nom) V ADJ Internal reference voltage Temperature coefficient of V ADJ External overdrive voltage range on `ADJ' pin for output voltage control Input current into V ADJ pin Default Output voltage `ADJ' pin floating, `EN'= V IN 0.5 1.23 40 VADJ (NOM) -10 27 29 V V ppm/ C V
I ADJ
0.5Note: 5) 6)
A
PWM duty cycle range at `EN' input Internal low pass filter cut-off frequency Filter attenuation
10kHz(5) < f < 100kHz, VENH =VIN
40 4
100
% kHz dB
f=30kHz
52.5
Internal voltage setting resistors for output voltage Ref. Block diagram 317 6.93 k M
Low level Input voltage High level Input voltage Low level input current High level input current Enable active hold time (6)
Device in shutdown Device active V EN =0V V EN =V IN V EN switched from high to low V IN falling V IN rising 1.94 20 20 1.21 120 1.4
0.4 Vin -100 100
V V nA nA s
Low-battery detection circuit Detection threshold Temperature coefficient of V BLT Hysteresis Internal reference voltage Maximum voltage on LBT pin Low level output current Low level output voltage High level output current High level output voltage Output `on' I BLOL = 0.5mA Output `off' , V BLOH = 29V Output `off' 2.02 V ppm/ C mV V Vin-0.5 V 1 0.4 2 29 V mA V A
Internal voltage setting resistors for low-battery detection circuit Ref. block diagram 1.56 2.44 M M
This is the minimum PWM frequency to maintain a continuous output. Lower frequencies can be used, but will result in gated operation of the device i.e. device enters shutdown when EN is low (see Note 6). This is the time for which the device remains active after the EN pin has been driven low. This delay allows a continuous output to be maintained during PWM mode operation at frequencies higher than 10kHz.
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PIN CONNECTIONS
PIN DESCRIPTION
Pin # 1 2 3 4 5 6 7 Name ADJ EN VIN SW SENSE LX LBF Description Internal (or external) reference voltage. Can be overdriven to adjust output voltage Enable input (active high) Also used to adjust output voltage by PWM signal Input voltage Output of high side PMOS isolation switch Output voltage sense Output of NMOS switch Low battery flag output: open drain (active low for low battery voltage) Active when EN is high or low 8 9 10 GND FB LBT Ground Voltage feedback pin for output (threshold 1.23V) Low battery flag threshold adjust input (threshold 1.21V)
ORDERING INFORMATION
DEVICE ZXLB1600X10TA ZXLB1600X10TC
TA reels 1k, TC reels 4k devices.
DEVICE DESCRIPTION Boost converter for LCD bias in MSOP10 Boost converter for LCD bias in MSOP10
TEMPERATURE RANGE 0 C to 70 C 0 C to 70 C
PART MARK ZXLB1600 ZXLB1600
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BLOCK DIAGRAM
Device description
The device is a flyback 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:
Setting output voltage
With external voltage The output voltage is equal to the voltage present on the 'ADJ' pin of the device multiplied by the internal resistor network factor (R1+R2)/R1. The ADJ pin is connected to the internal reference voltage of value VADJ(nom) and by default will give a nominal output of 28V. However, if required, the ADJ pin may be overdriven with an external dc voltage VADJ, in order to adjust the output voltage to a value lower than the default value.
Control loop
When 'EN' is high, the control circuits become active. The high side of the coil is connected to the input via a large PMOS isolating switch (MP) and the low side to ground via NDMOS transistor (MN). The current in the coil is allowed to build up to an internally defined level (nominally 200 to 300mA) before MN is turned off. The energy stored in the coil is then transferred to the output capacitor (C2) via diode (D1). The output voltage is sensed at pin 'SENSE' by internal resistors R1 and R2 (which may be shunted externally at pin 'FB') and compared to a reference voltage (1.23V nominal). A comparator senses when the output voltage is above that set by the reference and its output is used to control the 'off' time of the output switch. The control loop is self-oscillating, producing pulses of 10 s maximum duration (switch 'on'), at a frequency that varies in proportion to the output load current. The minimum 'off' time of the output switch is fixed at 1.25 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. ISSUE 3 - SEPTEMBER 2003 5
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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 and the control loop regulates the output to a nominal value of 28V. However, by varying the duty cycle (D) 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 voltage 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 voltage equal to D x 28V
Low battery detection
A comparator driving an open drain NMOS output transistor performs the low battery flag function. The detection threshold (battery voltage falling) is set to 1.98V nominal with internal resistors R3 and R4, but this can be changed by shunting the internal potential divider with two external resistors at pin 'LBT'. Operation is such that the output transistor will be turned on when the battery voltage falls below the detection threshold. VBLT = 1.21 x (R3+R4)/R4 A small amount (nominal 20mV) of hysteresis is provided to aid clean switching. The low battery detection circuit remains active when 'EN' is low.
Gated PWM operation
The internal circuitry of the ZXLB1600 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 voltage equal to D x 28V. 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 application notes.
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ZXLB1600
TYPICAL OPERATING CHARACTERISTICS
(For typical applications circuit at Vin = 3V and TA = 25 C unless otherwise stated)
Plot 1 Start-up response at max output voltage 1mA load Bottom: Enable input (1V/div), Top: Output voltage (10V/div)
Plot 2 Start-up response at max output voltage 5mA load Bottom: Enable input (1V/div), Top: Output voltage (10V/div)
Plot 3 Operating waveforms at max output voltage 1mA load Bottom: LX Output (10V/div), Top: Output voltage (50mV/div ac)
Plot 4 Operating waveforms at max output voltage 10mA load Bottom: LX Output (10V/div), Top: Output voltage (50mV/div ac)
Plot 5 Line rejection at 1mA load Bottom: Input voltage (1V/div), Top: Output voltage (0.2V/div ac)
Plot 6 Load rejection Bottom: Load current (1mA/div), Top: Output voltage (0.1V/div ac)
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TYPICAL OPERATING CHARACTERISTICS (Continued) VIN = 3V and TA = 25 C
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TYPICAL OPERATING CHARACTERISTICS (Continued) VIN = 3V and TA = 25 C
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TYPICAL OPERATING CHARACTERISTICS (Continued) VIN = 3V and TA = 25 C
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APPLICATION NOTES
Adjusting output voltage When connected as shown in the typical application circuit, the ZXLB1600 will produce a nominal output voltage of 28V. This can be adjusted by one of the three methods described below. 1) Output voltage adjustment by external resistors The internal reference and resistor divider network R1 and R2 set a nominal output of 28V. However, this network is accessible at the FB pin and can be shunted by means of external resistors to set different nominal output voltages. The potential divider defines output voltage according to the relationship: VOUT(dc) = (R1+R2)/R1 x 1.23V When using external resistors, these should be chosen with lower values than the internal resistors to minimize errors caused by the 25% absolute value variation of the internal resistors. The internal resistors have high values in order to minimize these errors. The following table gives suggested E24/E96 resistor values for various output voltages. in this way. However, some non-linearity in the above expression may occur at values of VADJ below approximately 0.5V. Also note that when driving the ADJ pin, the control voltage must have sufficiently low impedance to sink the bias current of the internal reference.(10 A max). 3) PWM output adjustment A Pulse Width Modulated (PWM) signal can be applied to the EN pin in order to adjust the output voltage to a value below the value set in in 1) or 2). This method of adjustment permits the device to be turned on and the output voltage set by a single logic signal applied to the EN pin. No external resistors or capacitors are required and the amplitude of the control signal is not critical, providing it conforms to the limits defined in the electrical characteristics. Two modes of adjustment are possible as described below: 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 voltage lower than the maximum set value. This voltage is given by: VOUT = 28 x D A square wave signal applied to the EN pin, for example, will turn the device on and produce a nominal regulated output of 14V.
Required output voltage 5V 12V 18V 20V 22V 25V
External resistor across R2 280k 715k 1M 1.15 M 1.15M 1.2 M
External resistor across R1 91k 82k 75k 75k 68.1k 62k
2) Output adjustment by external voltage The internal voltage reference (Pin ADJ) may be overdriven by an external control voltage to set the output voltage. The relationship between applied voltage (VADJ) and output voltage (VOUT) is: VOUT = 22.86 x VADJ Note that the output can be set to any value between the input voltage and the maximum operating voltage ISSUE 3 - SEPTEMBER 2003 11
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Gated mode The ZXLB1600 contains a timing circuit that switches the device on a few microseconds after the application of a rising edge to EN and turns it back off again nominally 120 s after the falling edge of EN. So, 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 voltage is then given by: VOUT(avg) ~ 28 x D Output voltage can be adjusted all the way down to the input voltage by means of PWM control, but for best results, the duty cycle range should be kept within the specified range of 0.4 to 1. Lower duty cycles may result in increased output ripple and non-linearity in the relationship between duty cycle and output voltage. If a greater control range, or reduced ripple is required, the nominal output can be adjusted by one of the other methods before the PWM signal is applied. Negative output The ZXLB1600 can be used to provide a negative output voltage (in addition to the normal positive output) as shown in the application circuit below. In this circuit, the external resistors R3 an R4 are used to set the output voltage to 22V as described in the previous section. These resistors and output capacitor C2 have relatively low values in this circuit in order to give a short time constant. This improves the regulation of the negative voltage. Capacitor selection A low ESR ceramic capacitor grounded close to the GND pin of the package is recommended at the output of the device. Surface mount types offer the best performance due to their lower inductance. A minimum value of 1 F is advised, although higher values will lower switching frequency and improve efficiency especially at lower load currents. A higher value will also minimize ripple when using the device to provide an adjustable dc output voltage. 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 3.3 F is acceptable if the input source is close to the device, but higher values are recommended 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.
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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 smaller inductors 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 ILX(peak) x L/VIN exceeds the preset 10s maximum `on' time limit for the LX output. The ZXLB1600 has been optimized for use with inductor values in the range 10 H to 100 H. The typical characteristics show how efficiency and available output current vary with input voltage and inductance. The inductor should be mounted as close to the device as possible with low resistance connections to the LX and SW pins. Suitable coils for use with the ZXLB1600 are those in the LP02506 and DO1608 series, made by Coilcraft. Diode selection The rectifier diode (D1) should be a fast low capacitance switching type 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. Small schottky diodes such as the BAT54 are suitable for use with the ZXLB1600 and this diode will give good all round performance over the output voltage and current range. At lower output voltages, a larger schottky diode such as the ZHCS500 or MBR0540 will provide a smaller forward drop and higher efficiency. At higher output voltages, where forward drop is less important, a silicon switching diode such as the 1N4148 can be used, however this will give lower efficiency. The BAT54S device specified in the application circuit contains a second diode (D2) as one half of a series connected pair. This second diode is used here to clamp possible negative excursions (due to coil ringing) from driving the drain of the output transistor below -0.5V. This prevents internal coupling effects, which might otherwise affect output regulation. The table below gives some typical characteristics for various diodes.
Diode BAT54 ZHCS500 MBR0540 1N4148
Forward voltage at 100mA (V) 530 300 390 950
Peak current (mA) 300 1000 1000 450
Continuous current (mA) 200 500 500 200
Reverse leakage (A) 2 15 1 0.025
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Increased efficiency If isolation of the coil from the supply is not needed, the high side of this can be connected directly to VIN to improve efficiency. This prevents power loss in the internal PMOS switch and typical efficiency gains of 5% can be achieved. (See efficiency vs. load curves). Some applications may require the coil to be fed from a separate supply with a different voltage to VIN. In this case, the SW pin should be left floating. Layout considerations PCB tracks should be kept as short as possible to minimize 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 minimize parasitic resistance and inductance, which will degrade efficiency and increase output ripple. The FB and LBT pins are high impedance inputs, so PCB track lengths to these should also be kept as short as possible to reduce noise pickup. Output ripple is typically only 50mV p-p, but a small feed-forward capacitor (~100pF) connected from the FB pin to the output may help to reduce this further. Capacitance from the FB pin to ground should be avoided, but a capacitor can be connected from the LBT pin to ground to reduce noise pickup into the low battery comparator if required. Low battery detection circuit The device contains an independent Low Battery Detection Circuit that remains powered when the device is shutdown. The detection threshold is set internally to a default value of 1.98V, but can be adjusted by means of external resistors as described below. Low battery threshold adjustment, LBT The internal potential divider network R3/R4 sets the detection threshold. This is accessible at the LBT pin and can be shunted by means of external resistors to set different nominal threshold voltages. The potential divider defines threshold voltage according to the relationship: VLBT = (R3+R4)/R4 x 1.21V When using external resistors, these should be chosen with lower values than the internal resistors to minimize errors caused by the 25% absolute value variation of the internal resistors. The internal resistors have high values in order to minimize these errors. It is suggested to use values less than half those shown for R3, R4 at the bottom of page 3. Low battery flag output, LBF This is an open drain output that switches low when the battery voltage falls below the detection threshold. An external pull-up resistor can be connected to this pin to allow it to interface to any voltage up to a maximum of 29V. Current in the pull-up resistor should be limited to a value below IBLOL.
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Notes:
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PACKAGE OUTLINE PACKAGE DIMENSIONS
DIM A A1 A2 D E E1 b c e 0.75 MILLIMETER MIN. MAX. 1.10 0.15 0.95 0.0295 INCHES MIN. MAX. 0.43 0.006 0.037
3.00 BSC 4.90 BSC 3.00 BSC 0.17 0.08 0.27 0.23
0.118 BSC 0.1929 BSC 0.118 BSC 0.0066 0.003 0.0106 0.009
0.50 BSC 0 15 0.80
0.0196 BSC 0 0.015 15 0.031
L L1
0.40
0.95 BSC
0.037 BSC
Controlling dimensions are in millimeters. Approximate conversions are given in inches
(c) Zetex plc 2003
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