tors’ leakage currents rises sharply. The actual design

limit for standby current is much less than the 100µA

specified in the

Electrical Characteristics (see the

Standby Current vs. Temperature graph 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+. Coming out of shutdown mode

initiates a soft-start cycle.

Continuous-/Discontinuous-

Conduction Modes

The input voltage, output voltage, load current, and

inductor value determine whether the IC operates in

continuous or discontinuous mode. As the inductor

value or load current decreases, or the input voltage

increases, the MAX750A/MAX758A tend to operate in

discontinuous-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 contin-

uous-conduction mode (CCM), the inductor current

never decays to zero, which is typically more efficient

than DCM. CCM allows the MAX750A/MAX758A to

deliver maximum load current, and is also slightly less

noisy than DCM because the ripple current in the out-

put capacitor is smaller.

Internal Reference

The +1.23V bandgap reference supplies up to 100µA

at REF. Connect a 0.01µF bypass capacitor from REF

to GND.

Oscillator

The internal oscillator of the MAX750A typically oper-

ates at 170kHz (160kHz for the MAX758A). The

Typical

Operating Characteristics indicate stability of the oscil-

lator frequency over temperature and supply voltage.

__________Applications Information

Figure 3 shows the MAX750A/MAX758A configured for a

standard 5V step-down application. Table 2 lists the com-

ponents for the desired operating temperature range.

These circuits are useful in systems that require high cur-

rent at high efficiency and are powered by an unregulated

supply, such as a battery or wall-plug AC-DC transformer.

They will operate over the entire line, load, and tempera-

ture ranges using the single set of component values

shown in Figure 3 and listed in Table 2.

Inductor Selection

The MAX750A/MAX758A require no inductor design

because they are tested in-circuit, and are guaranteed

to deliver the power specified in the

Electrical

Characteristics with high efficiency using a single

100µH (MAX75_AC) or 33µH (MAX75_AE/AM) inductor

The inductor’s incremental saturation-current rating

should be greater than 1A, and its DC resistance

should be less than 0.8

Ω. Table 2 lists inductor types

and suppliers for various applications. The surface-

mount inductors and the larger-size through-hole

inductors have nearly equivalent efficiencies.

Adjusting the Output Voltage

The MAX750A/MAX758A have outputs adjustable from

1.25V to the input voltage. To set the output voltage,

connect a voltage divider to the feedback input pin

(CC) as shown in Figure 3. The output voltage is set by

R2 and R3 as follows:

Let R3 be any resistance in the 10k

Ω to 20kΩ range

(typically 10k

Ω), then

Output tolerance over temperature is ±4.5% plus exter-

nal resistor tolerances.

Output Filter Capacitor Selection

The primary criterion for selecting the output filter

capacitor is low equivalent series resistance (ESR).

The product of the inductor-current variation and the

ESR of the output capacitor determines the amplitude

of the sawtooth ripple seen on the output voltage. In

addition, the ESR of the output filter capacitor should

be minimized to maintain AC stability. The ESR of the

capacitor should be less than 0.25

Ω to keep the output

ripple less than 50mVp-p over the entire current range

(using a 100µH inductor). Capacitor ESR ususally rises

Adjustable, Step-Down,

Current-Mode PWM Regulators

_______________________________________________________________________________________

9

SS

MAX750A

MAX758A

SS CLAMP

1M

±35%

1.23V

R1

510k

C1

FROM SHDN

Figure 2. Block Diagram of Soft-Start Circuitry