Step-Down Converters

6

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Detailed Description

The MAX1556/MAX1557 synchronous step-down con-

verters deliver a guaranteed 1.2A/600mA at output volt-

current-mode control scheme with internal compensation,

allowing for tiny external components and a fast transient

response. At light loads the MAX1556/MAX1557 automat-

ically switch to pulse-skipping mode to keep the quies-

cent supply current as low as 16µA. Figures 2 and 3

show the typical application circuits.

Control Scheme

During PWM operation the converters use a fixed-fre-

quency, current-mode control scheme. The heart of the

current-mode PWM controller is an open-loop, multiple-

input comparator that compares the error-amp voltage

feedback signal against the sum of the amplified cur-

rent-sense signal and the slope-compensation ramp. At

the beginning of each clock cycle, the internal high-side

p-channel MOSFET turns on until the PWM comparator

trips. During this time the current in the inductor ramps

up, sourcing current to the output and storing energy in

the inductor’s magnetic field. When the p-channel turns

off, the internal low-side n-channel MOSFET turns on.

Now the inductor releases the stored energy while the

current ramps down, still providing current to the output.

The output capacitor stores charge when the inductor

current exceeds the load and discharges when the

inductor current is lower than the load. Under overload

conditions, when the inductor current exceeds the cur-

rent limit, the high-side MOSFET is turned off and the

low-side MOSFET remains on until the next clock cycle.

Pin Description

PIN

NAME

FUNCTION

1

IN

Supply Voltage Input. Connect to a 2.6V to 5.5V source.

2

GND

Ground. Connect to PGND.

3SS

discharged through 200

Ω to GND in shutdown.

4

OUT

Output Sense Input. Connect to the output of the regulator. D1 and D2 select the desired output

voltage through an internal feedback resistor-divider. The internal feedback resistor-divider remains

connected in shutdown.

5

SHDN

Shutdown Input. Drive SHDN low to enable low-power shutdown mode. Drive high or connect to IN

for normal operation.

6

D2

OUT Voltage-Select Input. See Table 1.

7

PGND

Power Ground. Connect to GND.

8LX

Inductor Connection. Connected to the drains of the internal power MOSFETs. High impedance in

shutdown mode.

9

INP

Supply Voltage, High-Current Input. Connect to a 2.6V to 5.5V source. Bypass with a 10µF ceramic

capacitor to PGND.

10

D1

OUT Voltage-Select Input. See Table 1.

—EP

Exposed Paddle. Connect to ground plane. EP also functions as a heatsink. Solder to circuit-board

ground plane to maximize thermal dissipation.

D1

D2

MAX1556

MAX1556A

MAX1557

00

Adjustable

from 0.75V to

3.3V

Adjustable

from 0.75V to

0

1

3.3V

1.5V

1.5V

1

0

2.5V

1.2V

1.3V

1

1

1.8V

2.5V

1.0V

Table 1. Output-Voltage-Select Truth Table

A zero represents D_ being driven low or connected to GND.

A 1 represents D_ being driven high or connected to IN.