EXT Drive Voltage Range

EXT swings from V+ to GND and provides the gate

drive for an external P-channel power MOSFET. A high-

er supply voltage increases the gate drive to the

External Switching Transistor section.

Quiescent Current

The device’s typical quiescent current is 70µA.

However, actual applications draw additional current to

supply MOSFET switching currents, OUT pin current, or

external feedback resistors (if used), and both the diode

and capacitor leakage currents. For example, in the cir-

no-load supply current for the entire circuit is 84µA.

When designing a circuit for high-temperature opera-

tion, select a Schottky diode with low reverse leakage.

Shutdown Mode

When SHDN is high, the device enters shutdown mode.

In this mode, the feedback and control circuit, reference,

and internal biasing circuitry are turned off. EXT goes

high, turning off the external MOSFET. The shutdown

supply current drops to less than 1µA. SHDN is a logic-

level input. Connect SHDN to GND for normal operation.

Reference

The 1.3V reference is suitable for driving external loads,

such as an analog-to-digital converter. It has a guaran-

teed 10mV maximum load regulation while sourcing load

currents up to 100µA. The reference is turned off during

shutdown. Bypass the reference with 0.1µF for normal

operation. Place the bypass capacitor within 0.2 inches

(5mm) of REF, with a direct trace to GND (Figure 7).

Soft-Start

Soft-start reduces stress and transient voltage slumps

on the power source. When the output voltage is near

ground, the minimum off-time is lengthened to limit peak

switching current. This compensates for reduced nega-

tive inductor current slope due to low output voltages.

________________Design Information

Setting the Output Voltage

The MAX1626’s output voltage can be selected to 3.3V

or 5V under logic control by using the

3/5 pin. The 3/5

pin requires less than 0.5V to ensure a 3.3V output, or

more than (V+ - 0.5)V to guarantee a 5V output. The

voltage sense pin (OUT) must be connected to the out-

put for the MAX1626.

The MAX1627’s output voltage is set using two resis-

tors, R2 and R3 (Figure 5), which form a voltage divider

between the output and GND. R2 is given by:

has a maximum value of 50nA, large values (10k

Ω to

200k

Ω) can be used for R3 with no significant accuracy

loss. For 1% error, the current through R2 should be at

R2 = R3 x

V

V

OUT

REF

−









1

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

8

_______________________________________________________________________________________

10

µs/div

A: MOSFET DRAIN, 5V/div

B: OUT, 50mV/div, 5V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

B

0A

C

A

Figure 3. Discontinuous-Conduction Mode, Light-Load-Current

Waveform

10

µs/div

A: MOSFET DRAIN, 5V/div

B: OUT, 50mV/div, 5V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

B

0A

C

A

Figure 4. Continuous-Conduction Mode, Heavy-Load-Current

Waveform