1
ZXLB1600
LCD BIAS BOOST CONVERTER
DESCRIPTION
The ZXLB1600 DC/DC 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 backplane bias. The device contains the
output switch and a second switch to isolate the coil from the input and provide true isolation in
shutdown mode. The output can be adjusted by means of an externally applied dc voltage, or PWM
control signal 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.
The voltage control range can be modified, if required, by external resistors. A low battery comparator
is provided to indicate when the input voltage has fallen to within
2% of a preset threshold.
FEATURES
Wide input voltage range: 1.6 to 5.5V
Adjustable output voltage up to 28V using PWM or analog control voltage
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 (PFM)
85%
efficiency
Low battery flag
Small MSOP10 package
Low external component count
APPLICATIONS
PDAs
Mobile
Phones
Digital
Cameras
Portable
Internet
Appliances
Notebook and Palmtop computers
ORDERING INFORMATION
DEVICE
DEVICE DESCRIPTION
TEMPERATURE RANGE
PART MARK
TAPING
OPTIONS
ZXLB1600X10
Boost converter for LCD
bias in MSOP10
0
C to 70
C
ZXLB1600
TA, TC
TA reels 500 devices, TC reels 2500 devices
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ZXLB1600
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ZXLB1600
ABSOLUTE MAXIMUM RATINGS
(Voltages to GND unless otherwise stated)
Output Voltage
30V
Input Voltage
7V
Switch output current
500mA
Power Dissipation
800mW
Operating Temperature
0 to 70
C
Storage Temperature -55 to 125
C
Junction Temperature
150
C
ELECTRICAL CHARACTERISTICS:
Test conditions unless otherwise stated: V
IN
=3.0V, T
AMB
=25
C
Symbol Parameter
Conditions
Min Typ Max Units
General
V
IN
Input voltage
1.6
3
5.5
V
I
IN
Supply current
Shutdown
Quiescent
V
EN
= 0V, V
IN
=5.5V (Note 1)
V
EN
= V
IN
, I
OUT
= 0V
3
5
75
A
A
Isolating switch
R
ISO
`On' resistance
V
EN
= V
IN
0.8
2
I
ISO
Leakage current
V
EN
= 0V
1
A
LX Switch
I
LX
Switch peak current limit
0.2
0.3
0.5
A
R
LX
`On' resistance
1.0
2
I
LX(leak)
Switch leakage current
1
A
Controller output
V
OUT
Output voltage range
V
IN
29
V
I
OUT
Maximum output current
Lx = 22H, V
OUT
= 28V
5
10
mA
V
LNR
Line regulation
I
OUT
= 1mA, 2V < V
IN
<5.5V
0.1
%/V
V
LDR
Load regulation
V
OUT
= 28V, 100uA < I
OUT
< 5mA
0.01
%/mA
f
LX
Operating frequency
200
500
kHz
T
ON
Output `ON' time
LX output low
10
s
T
OFF
Output `OFF' time
LX output off
1
s
Efficiency
V
OUT
= 28V, I
OUT
= 1mA, L=22H
85
%
Note:1 Shutdown current includes 3
A (max) for the battery low comparator, which remains active in shutdown
mode
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Test conditions unless otherwise stated: V
IN
=3.0V, T
AMB
=25
C
Symbol Parameter
Conditions
Min Typ Max Units
Output Voltage control by dc voltage applied to `ADJ' pin
V
ADJ (NOM)
Internal reference voltage
`ADJ' pin floating, `EN'= V
IN
1.23
V
V
ADJ
External overdrive voltage range
on `ADJ' pin for output voltage
control
0.5
V
ADJ
(NOM)
V
I
ADJ
Input current
0.5<V
ADJ <
V
ADJ (NOM)
-10
A
V
SENSE
Output voltage
`ADJ' pin floating, `EN'= V
IN
27
29
V
Output Voltage control by PWM signal applied to `EN' input
T/T
PWM duty cycle range at `EN'
input
10kHz < f < 100kHz, V
ENH
=V
IN
40
100
%
f
LPF
Internal low pass filter cut-off
frequency
4
kHz
A
LPF
Filter attenuation
F=30kHz
52.5
dB
Enable input
V
ENL
Low level Input voltage
Device in shutdown
0.4
V
V
ENH
High level Input voltage
Device active
1.4
Vin
V
I
ENL
Low level input current
V
EN
=0V
-100
nA
I
ENH
High level input current
V
EN
=VIN
100
nA
Battery low comparator
I
BLQ
Quiescent current
V
IN
=5.5V
3
A
V
BLT
Detection threshold
V
IN
falling
2
V
V
BTOL
Threshold tolerance
+/-2
%
V
BLHYS
Hysteresis
V
IN
rising
20
mV
V
LBT
Threshold voltage on LBT pin
LBT floating
1.21
V
V
LBT (max)
Maximum voltage on LBT pin
during normal device operation
Vin-
0.5
V
I
BLOL
Low level output current
Output `on'
1
mA
V
BLOL
Low level output voltage
I
BLOL
= 1mA
0.4
V
I
BLOH
High level output current
Output `off'
2
A
V
BLOH
High level output voltage
Output `off'
29
V
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PIN DESCRIPTION
Pin #
Name
Description
1
ADJ
Internal or external reference voltage
2
EN
Enable input (active high)
Also used to adjust output voltage by PWM signal
3
VIN
Input voltage
4
SW
Output of high side PMOS isolation switch
5
SENSE
Output voltage sense
6
LX
Output of NMOS switch
7
LBF
Low Battery Flag output: open drain
(active low for low battery voltage)
Remains active when EN is high
8
GND
Ground
9
FB
Voltage Feedback pin for output (threshold 1.23V)
10
LBT
Low Battery flag Threshold adjust input
(threshold 1.21V)
BLOCK DIAGRAM
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Device Description
The device is a flyback boost regulator, working in discontinuous mode.
Device Operation
With reference to the chip block diagram and typical application circuit:
Control loop operation
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 a NMOS transistor (MN).
The current in the coil is allowed to build up to an internally defined level (nominally 300mA) before
MN is turned off. The energy stored in the coil is then transferred to the output capacitor (C1) via a
Schottky diode (SD). 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 fixed pulses
of 10us maximum width, at a frequency that varies in proportion to the output load current. This mode
of operation is known as Pulse Frequency Modulation (PFM)
Setting output 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
V
ADJ(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, in order to adjust the output voltage to a value
lower than the default value.
The reference voltage on the ADJ pin is switched into a low pass filter circuit when the EN pin is high
and the output of this filter drives the comparator within the control loop. By varying the duty cycle of
the EN signal, the control loop will see a voltage, which has an average value equal to the duty cycle
multiplied by the voltage present on the ADJ pin. This provides an alternative means of adjusting the
output voltage. It also allows the device to be both turned on and adjusted with a single signal at the
`EN' pin. The output voltage during this mode of operation will be V
ADJ
x (R1+R2)/R1 x duty cycle.
To reduce standby current consumption, the internal reference is turned off when no signal is present
on the `EN' pin
Further control over output voltage during either mode of operation is possible by shunting the internal
resistor network comprising R1 and R2 by means of external resistors connected to the SENSE and
FB pins.
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 internally to 2V nominal, 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. A small amount (nominal 20mV) of hysteresis is provided to aid clean switching. The low
battery detect circuit remains active when `EN' is low.
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ZXLB1600
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