Rainbow Electronics MAX1800 - Manual (Page 11)

MAX1800
Digital Camera Step-Up
Power Supply
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the output voltage. The controller forces the inductor cur-
rent to rise above the 300mA Idle Mode threshold to
ensure pulse skipping and improved efficiency at light
loads.
Auxiliary DC-DC Controllers
The MAX1800 auxiliary controllers operate in a low-noise,
fixed-frequency, PWM mode, with output power limited by
the external components. The controllers regulate their out-
put voltages by modulating the pulse width of the drive sig-
nal for an external N-channel MOSFET switch. The auxiliary
controllers are inactive until the main output has started.
Figure 3 shows a block diagram for a MAX1800 auxil-
iary PWM controller. A sawtooth oscillator signal at OSC
governs the internal timing. At the beginning of each
cycle, DL_ goes high to turn on the external MOSFET
switch. The MOSFET switch turns off when the internally
level-shifted sawtooth rises above COMP_ or when the
maximum duty cycle is exceeded. The switch remains
off until the beginning of the next cycle. An internal
transconductance amplifier establishes an integrated
error voltage at COMP_, thereby increasing the loop
gain for improved regulation accuracy.
D Q 1
Q 2
EN
EN Q
FB
EN
EN
ON
REF
V REF
0.9V REF
RDY
OSC
COMP
AUX1
AUX2
AUX3
IC POWER
2.35V
1.25V
POUT
POUT
LX
LX
PGND
PGND
STARTUP
OSCILLATOR
MAIN PWM
CONTROLLER
REFERENCE
GAIN
BLOCK
DCON1
ONM
REF
GND
OSC
FBM
COMPM
FB1
COMP1
OUT
FB2
COMP2
DCON2
FB3
COMP3
DCON3
AI
ON1
ON2
ON3
ONA
OSCILLATOR
UNDERVOLTAGE LOCKOUT
OSC
EN
Q
COMP
DUTY
FB
OSC
EN
COMP
DUTY
FB
OSC
EN
EN
COMP
DUTY
FB
Q
Q
N
P
N
DL1
DL2
DL3
AO
RDYM
MAX1800
Figure 2. Simplified Functional Diagram