Rainbow Electronics MAX8728 - Manual (Page 20)

MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
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turns off and Q2 turns on, connecting GON to DRN.
GON can then be discharged through a resistor con-
nected between DRN and GND or AV
DD . Q2 turns off
and stops discharging GON when V GON reaches 10
times the voltage on THR.
When V MODE is less than 0.9 x V REF , the switch control
block works in the second mode. The rising edge of
V
CTL turns on Q1 and turns off Q2, connecting GON to
SRC. An internal n-channel MOSFET Q3 between
MODE and GND is also turned on to discharge an
external capacitor between MODE and GND. The
falling edge of VCTL turns off Q3, and an internal 50µA
current source starts charging the MODE capacitor.
Once V
MODE exceeds 0.5 x V REF , the switch control
block turns off Q1 and turns on Q2, connecting GON to
DRN. GON can then be discharged through a resistor
connected between DRN and GND or AV
DD . Q2 turns
off and stops discharging GON when V GON reaches 10
times the voltage on THR.
When the LCD is shut down or in a fault state, the
switch control block is disabled, DZL is held low, and
GON is discharged to GND through an internal 4mA
current source. If the DRN resistor connects DRN to
AV
DD
or another voltage above ground, the Q2 body
diode conducts. To prevent the body diode conduc-
tion, an external diode must be added in series with the
DRN resistor (D6 in Figure 1). During startup, the 4mA
current source and Q4 are released when GATE reach-
es the GATE DONE threshold.
Linear Regulator (VL)
The MAX8728 includes an internal linear regulator. INL
is the input of the linear regulator. The input voltage
range is between 7V and 13.2V. The output voltage is
set to 5V. The regulator powers the internal MOSFET
drivers, PWM controllers, charge-pump regulators, and
logic circuitry. The total external load capability is
25mA. Bypass VL to GND with a minimum 1µF ceramic
capacitor.
Reference Voltage (REF)
The reference output is nominally 2V, and can
source at least 50µA (see the Typical Operating
Characteristics section). V
CC
is the input of the internal
reference block. Bypass REF with a 0.22µF ceramic
capacitor connected between REF and GND.
Frequency Selection (FSEL)
The step-down regulator and step-up regulator use the
same internal oscillator. The FSEL input selects the
switching frequency. Table 3 shows the switching fre-
quency based on the FSEL connection. High-frequency
(1.5MHz) operation optimizes the application for the
smallest component size, trading off efficiency due to
higher switching losses. Low-frequency (500kHz) oper-
ation offers the best overall efficiency at the expense of
component size and board space.
To reduce the input RMS current, the step-down regu-
lator and the step-up regulator operate 180° out of
phase from each other. The feature allows the use of
less input capacitance.
Power-Up Sequence
The step-down regulator starts up when the MAX8728’s
internal reference voltage (REF) is above its undervolt-
age lockout (UVLO) threshold and
SHDN is logic high.
The FB1 fault-detection circuit is enabled after the step-
down regulator reaches regulation. The negative
charge-pump regulator starts up when both EN and
SHDN are logic high and REF is above its UVLO thresh-
old. Once the negative charge-pump regulator output is
in regulation, the MAX8728 enables the FBN fault-detec-
tion circuit and the input-switch control block, which starts
pulling down GATE with a 11µA internal current source.
The external p-channel MOSFET turns on and connects
the input supply to the step-up regulator when V
GATE
falls below the turn-on threshold of the MOSFET.
When V
GATE
reaches the GATE DONE threshold, the
MAX8728 enables the step-up regulator and the posi-
tive charge-pump adjustable delay block. The FB2
fault-detection circuit is enabled after the step-up regu-
lator reaches regulation. The delay block charges the
DEL capacitor with an internal 5µA current source and
V
DEL
rises linearly. When V
DEL
exceeds 1V (typ), the
MAX8728 enables the positive charge-pump regulator
and the high-voltage switch control block. The FBP fault
detection is enabled after the positive charge-pump
regulator reaches regulation.
Power-Down Control
The MAX8728 disables the step-up regulator, positive
charge-pump regulator, negative charge-pump regula-
tor, input switch control block, delay block, and high-
voltage switch control block when EN or
SHDN is logic
low, or when any fault latch is set. The step-down regu-
lator is disabled only when
SHDN is logic low, the step-
down fault latch is set, or during thermal overload.
Table 3. Frequency Selection
FSEL SWITCHING FREQUENCY (kHz)
GND 1500
VCC 1000
REF 500