Rainbow Electronics MAX17075 - Manual (Page 14)

MAX17075
Boost Regulator with Integrated Charge Pumps,
Switch Control, and High-Current Op Amp
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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
current 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.
Positive Charge-Pump Regulator
The positive charge-pump regulator is typically used to
generate the positive supply rail for the TFT LCD gate-
driver ICs. The output voltage is set with an external
resistive voltage-divider from its output to GND with the
midpoint connected to FBP. The number of charge-
pump stages and the setting of the feedback divider
determine the output voltage of the positive charge-
pump regulator. The charge pump includes a high-side
p-channel MOSFET (P1) and a low-side n-channel
MOSFET (N1) to control the power transfer as shown in
Figure 4.
The error amplifier compares the feedback signal (FBP)
with a 1.25V internal reference. If the feedback signal is
below the reference, the charge-pump regulator turns
on P1 and turns off N1 when the rising edge of the
oscillator clock arrives, level shifting C15 and C17 by
V
SUP volts. If the voltage across C POUT plus a diode
drop (V
POUT
+ V
DIODE
) is smaller than the level-shifted
flying capacitor voltage (V C17 + V SUP ), charge flows
from C17 to C POUT until diode D3-1 turns off. Similarly,
if the voltage across C16 plus a diode drop (V C16 +
V DIODE ) is smaller than the level-shifted flying capacitor
voltage (V C15 + V SUP ), charge flows from C15 to C16
until diode D2-1 turns off. The falling edge of the oscil-
lator clock turns off P1 and turns on N1, allowing V
SUP
to charge up the flying capacitor C15 through D2-2 and
C16 to charge C17 through diode D3-2. If the feedback
signal is above the reference when the rising edge of
the oscillator comes, the regulator ignores this clock
edge and keeps N1 on and P1 off.
The MAX17075 also monitors the FBP voltage for
undervoltage conditions. If the V
FBP is continuously
below 80% of the nominal regulation voltage for
approximately 50ms, the MAX17075 sets a fault latch,
shutting down all outputs except REF. Once the fault
condition is removed, cycle the input voltage (below the
UVLO falling threshold) to clear the fault latch and reac-
tivate the device.
OSC
REF
1.25V
ERROR
AMPLIFIER
POSITIVE CHARGE-PUMP REGULATOR
MAX17075
P1
N1
SUP
INPUT
SUPPLY
POUT
C14
C16
C15
C6
D2-2
D2-1
C17 D3-2
D3-1
DRVP
FBP
Figure 4. Positive Charge-Pump Regulator Block Diagram