_________________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.

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 510k

Ω. 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 510k

Ω 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.

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

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

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.

coming out of shutdown mode initiates an SS cycle.

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.

5V, Step-Down,

Current-Mode PWM DC-DC Converters

8

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