4A Ultra-Low-Input-Voltage

LDO Regulators

8

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Applications Information

Output Voltage Selection

The MAX8556/MAX8557 feature an adjustable output

voltage from 0.5V to 3.4V. Set the output voltage using

an external resistor-divider from the output to GND with

FB connected to the center tap as shown in Figures 1

and 2. Choose R3

≤ 1kΩ for light-load stability.

Determine R2 using the following equation:

Capacitor Selection and

Regulator Stability

Capacitors are required at the MAX8556/MAX8557

inputs and outputs for stable operation over the full

temperature range and with load currents up to 4A.

Connect 2 x 10µF capacitors between IN and GND and

2 x 10µF low equivalent-series-resistance (ESR) capac-

lowers the source impedance of the input supply. If the

MAX8556/MAX8557s’ input is close to the output of the

source supply, a smaller input capacitance can be

used. Otherwise, 2 x 10µF ceramic input capacitors are

affects output noise and may affect output stability. Use

output capacitors with an ESR of 0.05

Ω or less to

ensure stability and optimum transient dropout. For

good output transient performance, use the following

formula to select a minimum output capacitance:

Noise, PSRR, and Transient Response

The MAX8556/MAX8557 are designed to operate with

low-dropout voltages and low quiescent currents while

still maintaining low noise, good transient response,

and high AC rejection (see the

Typical Operating

Characteristics for a plot of Power-Supply Rejection

Ratio (PSRR) vs. Frequency). When operating from

noisy sources, improved supply-noise rejection and

transient response can be achieved by increasing the

values of the input and output bypass capacitors and

through passive filtering techniques. The MAX8556/

MAX8557 load-transient response graphs (see the

Typical Operating Characteristics) show two compo-

nents of the output response: a DC shift from the output

impedance due to the load current change, and the

transient response. A typical transient overshoot for a

step change in the load current from 40mA to 4A is

40mV. Use an output capacitance from 20µF to 120µF

to attenuate the overshoot.

RR

x

V

V

OUT

FB

23

1

=−

⎛

⎝⎜

⎞

⎠⎟

IN

7–11

13

14

15

POR

POR

GND

SHUTDOWN

OUT

FB

1–6

12

R1

100k

Ω

R2

2k

Ω

R3

1k

Ω

C1

2 x 10

µF

C2

2 x 10

µF

18

N.C.

U2

EN

ENABLED

1.7V TO 3.6V

1.5V AT 4A

MAX8557

Figure 2. MAX8557 Typical Application Circuit

IN

7–11

13

14

15

POK

POK

GND

SHUTDOWN

OUT

FB

1–6

12

R1

100k

Ω

R2

1.4k

Ω

R3

1k

Ω

C1

2 x 10

µF

C2

2 x 10

µF

18

N.C.

U1

EN

ENABLED

1.425V TO 3.6V

1.2V AT 4A

MAX8556

Figure 1. MAX8556 Typical Application Circuit