Rainbow Electronics MAX1579 - Manual (Page 9)

MAX1578/MAX1579
Complete Bias and White LED Power Supplies
for Small TFT Displays
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Detailed Description
Bias Power and UVLO
The MAX1578/MAX1579 contain an LED driver boost
converter and three charge pumps for LCD bias. The
undervoltage-lockout (UVLO) feature disables the LED
boost converter and the charge pumps when the input
voltage is below 2.35V (typ). Once V
IN rises above
2.35V, and V CTRL and V ONBIAS are high, the boost
converter and charge pumps are enabled, respectively.
Charge-Pump Output Sequencing
The outputs of the MAX1578/MAX1579 charge pumps
are sequenced to turn on and off in a predictable fash-
ion. The turn-on sequence is as follows (Figure 1):
1) When ONBIAS is high, the MAIN regulator (5V) is
enabled.
2) When V MAIN exceeds 4.6V, the NEG charge pump
(-10V) is enabled.
3) When V NEG reaches -8V, the POS charge pump
(+15V) is enabled.
The turn-off sequence is as follows (Figure 2):
1) When ONBIAS is driven low, the NEG charge pump
(-10V) is disabled.
2) Once V NEG is discharged to -0.87V, the POS charge
pump (+15V) is disabled.
3) Once V POS falls to 0.87V, the MAIN regulator (+5V)
is disabled and discharged.
MAIN Charge Pump
The MAX1578/MAX1579 include a charge pump that
uses two external capacitors to provide +5.5V output
(V
DD ) that is used to power the regulated LDO +5V out-
put (MAIN). The control logic configures the pump to
switch automatically between 1.5x and 2x modes to
maximize efficiency. If V
DD exceeds 5.5V, the charge
pump stops switching. When ONBIAS is driven low,
V
DD is connected to IN.
A low-dropout linear regulator regulates the output of
the main charge pump to +5V at MAIN. The MAIN out-
put is capable of sourcing as much as 25mA to an
external load and also supplies the POS and NEG
charge pumps. Drive ONBIAS low to disable the MAIN,
POS, and NEG outputs. During shutdown, MAIN is dis-
charged to GND with an internal 1k
Ω resistor.
POS/NEG Charge Pumps
The MAX1578/MAX1579 include a positive and nega-
tive charge pump for LCD bias. The POS and NEG
charge pumps are powered from V
MAIN . The POS and
NEG charge pumps operate at 15.6kHz with a 50%
duty cycle.
NEG Charge Pump (-10V Supply)
The NEG charge pump uses capacitors at CD1 and
CD2 to generate -10V (-2 x V
MAIN ). Connect 1µF ceram-
ic capacitors and 200 Ω ±5% resistors in series between
CD1 and PMP and between CD2 and PMPB. Drive
ONBIAS high to enable MAIN, NEG, and POS. During
shutdown, the NEG output is discharged to GND with an
internal 1.5k
Ω resistor.
POS Charge Pump (+15V Supply)
The POS charge pump uses capacitors at CU1, CU2,
and CU3 to generate +15V (3 x V
MAIN ). Connect 1µF
ceramic capacitors between CU1 and PMP, between
CU2 and PMPB, and between CU3 and GND. Drive
ONBIAS high to enable MAIN, NEG, and POS. During
shutdown, POS is discharged to GND with an internal
3k
Ω resistor.
LED Backlighting Power
LED power is supplied by an internal MOSFET, 1MHz
boost converter. The boost converter is capable of dri-
ving up to 8 series LEDs at 25mA.
The output of the boost converter is regulated to main-
tain a constant voltage at CS, and therefore a constant
current through the LEDs. Once V
IN is increased above
the UVLO voltage (2.35V) and V CTRL is above 0.17V,
the boost converter enters soft-start and charges the
output to its regulation voltage. An overvoltage-protec-
tion circuit shuts down the boost converter if V
OUT
exceeds 34V.
V MAIN (5V)
V POS (+15V)
V NEG (-10V)
ONBIAS
Figure 2. Charge-Pump Turn-Off Sequence
V MAIN (5V)
V POS (+15V)
V NEG (-10V)
ONBIAS
Figure 1. Charge-Pump Turn-On Sequence