Rainbow Electronics MAX8728 - Manual (Page 17)

MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
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Current Limiting and Lossless Current Sensing
The current-limit circuit turns off the high-side MOSFET
switch whenever the voltage across the high-side
MOSFET exceeds an internal threshold corresponding to
the actual current limit of 2.8A ±10%.
For current-mode control, an internal lossless sense
network derives a current-sense signal from the induc-
tor DC resistance. The time constant of the current-
sense network is not required to match the time
constant of the inductor and has been chosen to pro-
vide sufficient current-ramp signal for stable operation
at each operating frequency. The current-sense signal
is AC-coupled into the PWM comparator, eliminating
most DC output voltage variation with load current.
Low-Frequency Operation
The step-down regulator of the MAX8728 enters into
low-frequency operating mode if the voltage on OUT1
is below 1.3V. In the low-frequency mode, the switching
frequency of the step-down regulator is 1/6 the oscilla-
tor frequency. This feature prevents potentially uncon-
trolled inductor current if OUT1 is overloaded or
shorted to ground.
Soft-Start and Fault Protection
The step-down regulator includes a 7-bit soft-start DAC
that steps the internal reference voltage from zero to 2V
in 128 steps. The soft-start period is 3ms (typ) and FB1
fault detection is disabled during this period. The soft-
start feature effectively limits the inrush current during
startup (see the Step-Down Regulator Soft-Start
Waveforms in the Typical Operating Characteristics).
The MAX8728 monitors OUT1 (fixed-output mode) or
FB1 (adjustable-output mode) for undervoltage condi-
tions. If the voltage is continuously below 80% (typ) of
the nominal regulation point for approximately 50ms,
the MAX8728 sets a fault latch, shutting down all out-
puts except VL and REF.
Step-Up Regulator
The step-up regulator employs a current-mode, fixed-
frequency PWM architecture to maximize loop band-
width and provide fast transient response to pulsed
loads typical of TFT LCD panel source drivers. The inte-
grated MOSFET and the built-in digital soft-start function
reduce the number of external components required
while controlling inrush currents. The output voltage can
be set from V
IN to 28V with an external resistive voltage-
divider. The regulator controls the output voltage and
the power delivered to the output by modulating the
duty cycle of the internal power MOSFET in each
switching cycle.
PWM Controller Block
An error amplifier compares the signal at FB2 to 2.0V
and changes the COMP output. The voltage at COMP
sets the peak inductor current. As the load varies, the
error amplifier sources or sinks current to the COMP
output accordingly to produce the inductor peak cur-
rent necessary to service the load. To maintain stability
at high duty cycles, a slope-compensation signal is
summed with the current-sense signal.
On the rising edge of the internal clock, the controller
sets a flip-flop, turning on the n-channel MOSFET and
applying the input voltage across the inductor. The cur-
rent through the inductor ramps up linearly, storing
energy in its magnetic field. Once the sum of the cur-
rent-feedback signal and the slope compensation
exceed the COMP voltage, the controller resets the flip-
flop and turns off the MOSFET. Since the inductor cur-
rent is continuous, a transverse potential develops
across the inductor that turns on the diode (D1). The
voltage across the inductor then becomes the differ-
ence between the output voltage and the input voltage.
This discharge condition forces the current through the
inductor to ramp back down, transferring the energy
stored in the magnetic field to the output capacitor and
the load. The MOSFET remains off for the rest of the
clock cycle.
Input Switch Control
The GATE pin of the MAX8728 controls an optional
external p-channel MOSFET between the input supply
and the inductor of the step-up regulator. This function
disconnects the step-up regulator from the input supply
and allows the regulator output to discharge to ground
when the step-up regulator is disabled. When EN is low,
GATE is internally pulled up to the input supply through
a 1k
Ω resistor. Once EN and SHDN are high and the
negative charge-pump regulator is in regulation, the
MAX8728 starts pulling down GATE with an 11µA inter-
nal current source. The external p-channel MOSFET
turns on and connects the input supply to the step-regu-
lator when V
GATE falls below the turn-on threshold of the
MOSFET. When V GATE reaches V IN - 4V, the step-up
regulator is enabled and initiates a soft-start routine.
V
GATE
continues to fall until it reaches V
IN
- 5V.
Soft-Start and Fault Protection
The step-up regulator achieves soft-start by linearly
ramping up its internal current limit. The soft-start termi-
nates when the output reaches regulation or the full
current limit has been reached. The current limit rises
from zero to the full current limit in approximately 3ms.