MAX1565ETJ-T Maxim Integrated Products, MAX1565ETJ-T Datasheet - Page 16

• Manufacturer Part Number: MAX1565ETJ-T
• Description: DC/DC Switching Converters 5Ch Digital Camera Power Supply
• Manufacturer: Maxim Integrated Products

The MAX1565 auxiliary PWM controllers have a guaran-
teed maximum duty cycle of 80%. That is to say that 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
such that:
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.
The MAX1565 supports MAX1801 slave PWM con-
trollers that obtain input power, a voltage reference,
and an oscillator signal directly from the MAX1565
master. The master/slave configuration allows channels
to be easily added and minimizes system cost by elimi-
nating redundant circuitry. The slaves also control the
harmonic content of noise since their operating fre-
quency is synchronized to that of the MAX1565 master
converter. A MAX1801 connection to the MAX1565 is
shown in Figure 11.
The MAX1565 has robust fault and overload protection.
After power-up, the device is set to detect an out-of
regulation state that could be caused by an overload or
short. If any DC-to-DC converter channel (step-up,
step-down, or any of the auxiliary controllers) remains
faulted for 100,000 clock cycles, then ALL outputs latch
off until the step-up DC-to-DC converter is reinitialized
by the ONSU pin, or by cycling of input power. The
fault-detection circuitry for any channel is disabled dur-
ing its initial turn-on soft-start sequence.
Note that output of the step-up, or that of any auxiliary
channel set up in boost configuration, does not fall to
0V during shutdown or fault. This is due to the current
path from the battery to the output that remains even
when the channel is off. This path exists through the
boost inductor and the synchronous rectifier body diode.
An auxiliary boost channel falls to the input voltage
minus the rectifier drop during fault and shutdown.
OUTSU falls to the input voltage minus the synchronous
rectifier body diode drop during shutdown, and also
during fault if the input voltage exceeds 2.5V. If the
input voltage is less than 2.5V, OUTSU remains at 2.5V
due to operation of the startup oscillator, but can
source only limited current.
Small, High-Efficiency, Five-Channel
Digital Still Camera Power Supply
16
______________________________________________________________________________________
1 - V
Master/Slave Configurations
IN
/V
OUT
≤ 80%
Maximum Duty Cycle
Fault Protection
The MAX1565 has an internal 1.250V reference.
Connect a 0.1µF ceramic bypass capacitor from REF to
GND within 0.2in (5mm) of the REF pin. REF can source
up to 200µA and is enabled whenever ONSD is high
and OUTSD is above 2.5V. The auxiliary controllers and
MAX1801 slave controllers (if connected) each sink up
to 30µA REF current during startup. If the application
requires that REF be loaded beyond 200µA, it may be
buffered with a unity-gain amplifier or op amp.
All MAX1565 DC-to-DC converter channels employ
fixed-frequency PWM operation. The operating fre-
quency is set by an RC network at the OSC pin. The
range of usable settings is 100kHz to 1MHz. When
MAX1801 slave controllers are added, they operate at
the same frequency set by OSC.
The oscillator uses a comparator, a 300ns one-shot,
and an internal N-FET switch in conjunction with an
external timing resistor and capacitor (Figure 4). When
the switch is open, the capacitor voltage exponentially
approaches the step-up output voltage from zero with a
time constant given by the R
comparator output switches high when the capacitor
voltage reaches V
vates the internal MOSFET switch to discharge the
capacitor within a 300ns interval, and the cycle repeats.
Note that the oscillation frequency changes as the main
output voltage ramps upward following startup. The
oscillation frequency is constant once the main output
is in regulation.
Figure 4. Master Oscillator
OSC
V
OUTSU
R
C
OSC
OSC
REF
(1.25V). In turn, the one-shot acti-
(1.25V)
V
REF
OSC
ONE-SHOT
300ns
C
MAX1565
OSC
product. The
Reference
Oscillator