Step-Down Converters

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9

for a few recommended inductors is listed in Table 2.

Soft-Start

The MAX1556/MAX1556A/MAX1557 use soft-start to

eliminate inrush current during startup, reducing tran-

sients at the input source. Soft-start is particularly use-

ful for higher-impedance input sources such as Li+ and

alkaline cells. Connect the required soft-start capacitor

from SS to GND. For most applications using a 22µF

output capacitor, connect a 1000pF capacitor from SS

to GND. If a larger output capacitor is used, then use

the following formula to find the value of the soft-start

capacitor:

Soft-start is implemented by exponentially ramping up

Ω (see the Typical

Operating Characteristics). Assuming three time con-

stants to full output voltage, use the following formula to

calculate the soft-start time:

Shutdown Mode

Connecting SHDN to GND or logic low places the

MAX1556/MAX1556A/MAX1557 in shutdown mode and

reduces supply current to 0.1µA. In shutdown, the con-

trol circuitry and the internal p-channel and n-channel

MOSFETs turn off and LX becomes high impedance.

Connect SHDN to IN or logic high for normal operation.

Thermal Shutdown

As soon as the junction temperature of the

MAX1556/MAX1556A/MAX1557 exceeds +160°C, the

ICs go into thermal shutdown. In this mode the internal

p-channel switch and the internal n-channel synchro-

nous rectifier are turned off. The device resumes nor-

mal operation when the junction temperature falls

below +145°C.

Applications Information

The MAX1556/MAX1556A/MAX1557 are optimized for

use with small external components. The correct selec-

tion of inductors and input and output capacitors

ensures high efficiency, low output ripple, and fast tran-

sient response.

Adjusting the Output Voltage

The MAX1556/MAX1556A/MAX1557 offer preset output

voltages of 1.0V, 1.2V, 1.3V, 1.5V, 1.8V, 2.5V, and 3.3V

as well as an adjustable output using external resistors.

Whenever possible, the preset outputs (set by D1 and

D2) should be used. With external resistor feedback,

noise coupling to FB can cause alternate LX pulse to

terminate early resulting in an inductor current wave-

form with alternate large and small current pulses. See

the

External Feedback Switching Waveforms graph in

Typical Operating Characteristics section). Note that

external feedback and the alternating large-small pulse

waveform do not impact loop stability and have no

harmful effect on regulation or reliability.

The adjustable output is selected when D1 = D2 = 0

and an external resistor-divider is used to set the output

voltage (see Figure 6). The MAX1556/MAX1557 have a

defined line- and load-regulation slope. The load regu-

lation is for both preset and adjustable outputs and is

described in the

Electrical Characteristics table and

Figures 4 and 5. The impact of the line-regulation slope

tx

x C

SS

SS

= 600

10

3

C

C

SS

OUT

=

22000

Table 2. Inductor Selection

MANUFACTURER

PART

VALUE (µH)

DCR (m

SIZE (mm)

SHIELDED

Taiyo Yuden

LMNP04SB3R3N

3.3

36

1300

5 x 5 x 2.0

Yes

Taiyo Yuden

LMNP04SB4R7N

4.7

50

1200

5 x 5 x 2.0

Yes

TOKO

D52LC

3.5

73

1340

5 x 5 x 2.0

Yes

TOKO

D52LC

4.7

87

1140

5 x 5 x 2.0

Yes

Sumida

CDRH3D16

4.7

50

1200

3.8 x 3.8 x 1.8

Yes

TOKO

D412F

4.7

100*

1200*

4.8 x 4.8 x 1.2

Yes

Murata

LQH32CN

4.7

97

790

2.5 x 3.2 x 2.0

No

Sumitomo

CXL180

4.7

70*

1000*

3.0 x 3.2 x 1.7

No

Sumitomo

CXLD140

4.7

100*

800*

2.8 x 3.2 x 1.5

No

*

Estimated based upon similar-valued prototype inductors.