Rainbow Electronics MAX1585 - Manual (Page 15)

MAX1584/MAX1585
5-Channel Slim DSC Power Supplies
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Step-Down DC-DC Converter
The step-down DC-DC converter is optimized for gen-
erating low output voltages (down to 1.25V) at high effi-
ciency. Output voltages lower than 1V can be set by
adding an additional resistor (see the Applications
Information section). The step-down runs from the volt-
age at PVSD. This pin can be connected directly to the
battery if sufficient headroom 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 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-DC is inactive until the step-up DC-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 13.
Boost-Buck Operation
The step-down input can be powered from the output
of the step-up. By cascading these two channels, the
step-down output can maintain regulation even as the
battery voltage falls below the step-down output volt-
age. This is especially useful when trying to generate
3.3V from 1-cell Li+ inputs, or 2.5V from 2-cell alkaline
or NiMH inputs, or when designing a power supply that
must operate from both Li+ and alkaline/NiMH inputs.
Compound efficiencies of up to 90% can be achieved
when the step-up and step-down are operated in
series.
Note that the step-up output supplies both the step-up
load and the step-down input current when the step-
down is powered from the step-up. The step-down
input current reduces the available step-up output cur-
rent for other loads.
Direct Battery Step-Down Operation
The step-down converter can also be operated directly
from the battery as long as the voltage at PVSD does
not exceed PVSU by more than a Schottky diode for-
ward voltage. When using this connection, connect an
external Schottky diode from the battery input to PVSU.
On the MAX1584/MAX1585, there is an internal 10k
Ω
resistance from PVSU to PVSD. This adds a small addi-
tional current drain (of approximately (V PVSU - V PVSD ) /
10k Ω) from PVSU when PVSD is not connected directly
to PVSU.
Step-down direct battery operation improves efficiency
for the step-down output (up to 95%), but restricts the
upper limit of the output voltage to 200mV less than the
minimum battery voltage. In 1-cell Li+ designs (with a
2.7V min), the output can be set up to 2.5V. In 2-cell
alkaline or NiMH designs, the output can be limited to
1.5V or 1.8V, depending on the minimum-allowed cell
voltage.
The step-down can only be briefly operated in dropout
since the MAX1584/MAX1585 fault protection detects
the out-of-regulation condition and activates after
100,000 OSC cycles (200ms at f
OSC
= 500kHz). At that
point, all MAX1584/MAX1585 channels shut down.
AUX1, AUX2, and AUX3 DC-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. The MAX1584
contains two step-up/flyback controllers (AUX1 and
AUX2) and one step-down controller (AUX3). The
MAX1585 contains one step-up controller (AUX1), one
inverting controller (AUX2), and one step-down con-
troller (AUX3).
Figure 3 shows a functional diagram of the AUX con-
trollers. The inverting and step-down controllers differ
from the step-up controllers only in the gate-drive logic
and FB polarity and threshold. The sawtooth oscillator
signal at OSC governs timing. At the start of each
cycle, DL_ turns on the external MOSFET switch. For
step-up controllers, DL_ goes high, while for inverting
and step-down controllers, DL_ goes low (to turn on
PFETs). The external MOSFET then turns off when the
internally level-shifted sawtooth rises above CC_ or
when the maximum duty cycle is exceeded. The switch
remains off until the start of the next cycle. A transcon-
ductance error amplifier forms an integrator at CC_ so
that high DC loop gain and accuracy can be main-
tained. In step-up and step-down controllers, the FB_
threshold is 1.25V, and higher FB_ voltages reduce the
MOSFET duty cycle. In inverting controllers, the FB_
threshold is 0V, and lower (more negative) FB_ volt-
ages reduce the MOSFET duty cycle.
Auxiliary controllers do not start until the step-up DC-DC
output is in regulation. If the step-up, step-down, or any
of the auxiliary controllers remains faulted for 100,000