The power-good, open-drain output for regulator 2

External Reference Input (REFIN)

The MAX8855 has an external reference input. Connect

REFIN to set the FB1 regulation voltage. To use the inter-

nal 0.6V reference, connect REFIN to SS1. When the IC

is shut down, REFIN is pulled to GND through 335Ω.

Startup and Sequencing

The MAX8855 features separate enable inputs (EN1

and EN2) for the two regulators. Driving EN_ high

enables the corresponding regulator; driving EN_ low

turns the regulator off. Driving both EN1 and EN2 low

puts the IC in low-power shutdown mode, reducing the

supply current typically to 30nA. The MAX8855 regula-

tors power up when the following conditions are met

(see Figure 2):

•

EN_ is logic-high.

•

The internal reference is powered.

•

Once these conditions are met, the MAX8855 begins

soft-start. FB2 regulates to the voltage at SS2. During

soft-start, the SS2 capacitor is charged with a constant

8µA current source so that its voltage ramps up for the

soft-start time. See the

Setting the Soft-Start Time sec-

tion to select the SS2 capacitor for the desired soft-start

time. FB1 regulates to the voltage at REFIN. Connect

REFIN to SS1 to use the internal reference with soft-

start time set independently by the SS1 capacitor (see

Figure 3a).

Dual, 5A, 2MHz Step-Down Regulator

10

______________________________________________________________________________________

Figure 2. Startup Control Diagram

UVLO

UVLO

THERM

SHDN

THERM

SHDN

REF

BIAS

GEN

REF

RDY

UVLO

RRUVB

RRUVB

RRUVB

EN1

EN2

REG1 ON

REG2 ON

UVLO

UVLO

TLIM

TLIM

IN1

IN2

Figure 3a. Startup and Sequencing Options—Two Independent Output Startup and Shutdown Waveforms

EN1

SS2

PWRGD1

EN1

OUT1

OUT2

PWRGD2

EN2

SS1

PWRGD2

REFIN

EN2

EN2

PWRGD1

10k

Ω

10k

Ω

EN1