MAX1566-MAX1567 Datasheet, PDF (20/35 Pages) Maxim Integrated Products – Six-Channel, High-Efficiency, Digital Camera Power Supplies

Six-Channel, High-Efficiency, Digital
Camera Power Supplies
Core Step-Down DC-to-DC Converter
The step-down DC-to-DC is optimized for generating low
output voltages (down to 1.25V) at high efficiency. The
step-down runs from the voltage at PVSD. This pin can
be connected directly to the battery if sufficient head-
room exists to avoid dropout; otherwise, PVSD can be
powered from the output of another converter. The step-
down can also operate with the step-up, or the main con-
verter in step-up mode, for boost-buck operation.
Under moderate to heavy loading, the converter oper-
ates in a low-noise PWM mode with constant frequency
and modulated pulse width. Efficiency is enhanced
under light (<75mA typ) loading by assuming an Idle
Mode during which the step-down switches only as
needed to service the load. In this mode, the maximum
inductor current is 100mA for each pulse. The step-
down DC-to-DC is inactive until the step-up DC-to-DC
is in regulation.
The step-down also features an open-drain SDOK out-
put that goes low when the step-down output is in regu-
lation. SDOK can be used to drive an external MOSFET
switch that gates 3.3V power to the processor after the
core voltage is in regulation. This connection is shown
in Figure 15.
AUX1, AUX2, and AUX3 DC-to-DC
Controllers
The three auxiliary controllers operate as fixed-frequen-
cy voltage-mode PWM controllers. They do not have
internal MOSFETs, so output power is determined by
external components. The controllers regulate output
voltage by modulating the pulse width of the DL_ drive
signal to an external MOSFET switch.
On the MAX1566, AUX1 and AUX2 are boost/flyback
PWM controllers. On the MAX1567, AUX1 is a boost/fly-
back PWM controller, but AUX2 is an inverting PWM
controller. On both devices, AUX3 is a boost/flyback
controller that can be connected to regulate output volt-
age and/or current (for white-LED drive).
Figure 5 shows a functional diagram of an AUX boost
controller channel. A sawtooth oscillator signal at OSC
governs timing. At the start of each cycle, DL_ goes high,
turning on the external NFET switch. The switch then
turns off when the internally level-shifted sawtooth rises
above CC_ or when the maximum duty cycle is exceed-
ed. The switch remains off until the start of the next cycle.
A transconductance error amplifier forms an integrator at
CC_ to maintain high DC loop gain and accuracy.
The auxiliary controllers do not start until 1024 OSC
cycles after the step-up DC-to-DC output is in regula-
tion. If the auxiliary controller remains faulted for
100,000 OSC cycles (200ms at 500kHz), then all
MAX1566/MAX1567 channels latch off.
Maximum Duty Cycle
The AUX PWM controllers have a guaranteed maximum
duty cycle of 80%: all controllers can achieve at least
80% and typically reach 85%. In boost designs that
employ continuous current, the maximum duty cycle
limits the boost ratio so:
1 - V_IN / V_OUT < 80%
With discontinuous inductor current, no such limit exists
for the input/output ratio since the inductor has time to
fully discharge before the next cycle begins.
AUX1
AUX1 can be used for conventional DC-to-DC boost
and flyback designs (Figures 8 and 9). Its output (DL1)
is designed to drive an N-channel MOSFET. Its feed-
back (FB1) threshold is 1.25V.
AUX2
In the MAX1566, AUX2 is identical to AUX1. In the
MAX1567, AUX2 is an inverting controller that gener-
ates a regulated negative output voltage, typically for
CCD and LCD bias. This is useful in height-limited
designs where transformers may not be desired.
The AUX2 MOSFET driver (DL2) in the MAX1567 is
designed to drive P-channel MOSFETs. INDL2 biases
the driver so V_INDL2 is the high output level of DL2.
INDL2 should be connected to the P-channel MOSFET
source to ensure the MOSFET turns completely off when
DL2 is high. See Figure 10 for a typical inverter circuit.
AUX3 DC-to-DC Controller, LED Driver
The AUX3 step-up DC-to-DC controller has two feed-
back inputs, FB3L and FB3H, with feedback thresholds
of 0.2V (FB3L) and 1.25V (FB3H). If used as a conven-
tional voltage-output step-up, FB3L is grounded and
FB3H is used as the feedback input. In that case, AUX3
behaves exactly like AUX1.
If AUX3 is used as a switch-mode boost current source
for white LEDs, FB3L provides current-sensing feed-
back, while FB3H provides (optional) open-LED over-
voltage protection (Figure 7).
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