5V, Step-Down,
Current-Mode PWM DC-DC Converters
MAX730A/MAX738A/MAX744A
_________________Detailed Description
The MAX730A/MAX738A/MAX744A switch-mode regu-
lators use a current-mode pulse-width-modulation
(PWM) control system coupled with a simple step-down
(buck) regulator topography. They convert an unregu-
lated DC voltage from 5.2V to 11V for the MAX730A,
and from 6V to 16V for the MAX738A/MAX744A. The
current-mode PWM architecture provides cycle-by-
cycle current limiting, improved load-transient response
characteristics, and simpler outer-loop design.
The controller consists of two feedback loops: an inner
(current) loop that monitors the switch current via the cur-
rent-sense resistor and amplifier, and an outer (voltage)
loop that monitors the output voltage through the error
amplifier (Figure 1). The inner loop performs cycle-by-
cycle current limiting, truncating the power transistor on-
time when the switch current reaches a predetermined
threshold. This threshold is determined by the outer loop.
For example, a sagging output voltage produces an error
signal that raises the threshold, allowing the circuit to
store and transfer more energy during each cycle.
for overcurrent protection to function properly. A typical
value is 0.1µF.
Undervoltage Lockout
The MAX738A/MAX744A’s undervoltage-lockout fea-
ture monitors the supply voltage at V+, and allows
operation to start when V+ rises above 5.7V (6V guar-
anteed). When V+ falls, operation continues until the
supply voltage falls below 5.45V (see the
MAX738A/MAX744A Quiescent Supply Current vs.
Supply Voltage graph in the
Typical Operating
Characteristics).
The MAX730A is similar, starting oper-
ation at V+ > 4.7V and continuing to operate down to
4.45V. When an undervoltage condition is detected,
control logic turns off the output power FET and dis-
charges the SS capacitor to ground. This prevents par-
tial turn-on of the power MOSFET and avoids excessive
power dissipation. The control logic holds the output
power FET off until the supply voltage rises above
approximately 4.7V (MAX730A) or 5.7V (MAX738A/
MAX744A), at which time an SS cycle begins.
Shutdown Mode
The MAX730A/MAX738A/MAX744A are shut down by
keeping
SHDN
at ground. In shutdown mode, the output
drops to 0V and the output power FET is held in an off
state. The internal reference also turns off, which causes
the SS capacitor to discharge. Typical standby current in
shutdown mode is 6µA. The actual design limit for stand-
by current is much less than the 100µA specified in the
Electrical Characteristics
(see Standby Current vs.
Temperature in the
Typical Operating Characteristics).
However, testing to tighter limits is prohibitive because
the current takes several seconds to settle to a final value.
For normal operation, connect
SHDN
to V+. Note that
coming out of shutdown mode initiates an SS cycle.
Programmable Soft-Start
Figures 1 and 2 show a capacitor and a resistor con-
nected to the soft-start (SS) pin to ensure an orderly
power-up. Typical values are 0.1µF and 510kΩ. SS con-
trols both the SS timing and the maximum output current
that can be delivered while maintaining regulation.
The charging capacitor slowly raises the clamp on the
error-amplifier output voltage, limiting surge currents at
power-up by slowly increasing the cycle-by-cycle cur-
rent-limit threshold. The 510kΩ resistor sets the SS
clamp at a value high enough to maintain regulation,
even at currents exceeding 1A. This resistor is not nec-
essary for lower-current loads. Refer to the Maximum
Output Current vs. Supply Voltage graph in the
Typical
Operating Characteristics.
Table 1 lists timing charac-
teristics for selected capacitor values and circuit condi-
tions.
The overcurrent comparator trips when the load
exceeds approximately 1.5A. An SS cycle begins when
either an undervoltage or overcurrent fault condition
triggers an internal transistor to momentarily discharge
the SS capacitor to ground. An SS cycle also begins at
power-up and when coming out of shutdown mode.
Continuous-/Discontinuous-
Conduction Modes
The input voltage, output voltage, load current, and induc-
tor value determine whether the IC operates in continuous
or discontinuous mode. As the inductor value or load cur-
rent decreases, or the input voltage increases, the
MAX730A/MAX738A/MAX744A tend to operate in discon-
tinuous-conduction mode (DCM). In DCM, the inductor
current slope is steep enough so it decays to zero before
the end of the transistor off-time. In continuous-conduc-
tion mode (CCM), the inductor current never decays to
zero, which is typically more efficient than DCM. CCM
allows the MAX730A/ MAX738A/MAX744A to deliver maxi-
mum load current, and is also slightly less noisy than
DCM, because the peak-to-average inductor current ratio
is reduced.
Overcurrent Limiting
The overcurrent comparator triggers when the load cur-
rent exceeds approximately 1.5A. On each clock cycle,
the output FET turns on and attempts to deliver current
until cycle-by-cycle or overcurrent limits are exceeded.
Note that the SS capacitor must be greater than 0.01µF
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