Thermal-Shutdown Protection

Thermal-shutdown protection limits total power dissipation

in the device. When the junction temperature of the device

exceeds +165ºC, an on-chip thermal sensor shuts down

the device, allowing it to cool. The thermal sensor turns

the device on again after the junction temperature cools by

10ºC. Soft-start resets during thermal shutdown. Carefully

evaluate the total power dissipation (see the Power

Dissipation section) to avoid unwanted triggering of the

thermal shutdown in normal operation.

Applications Information

Input Capacitor Selection

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

defined by the following equation:

×

=

×

OUT

IN

OUT

RMS

OUT(MAX)

IN

V

(V - V

)

II

V

Choose an input capacitor that exhibits less than +10ºC

temperature rise at the RMS input current for optimal

long-term reliability. Use low-ESR ceramic capacitors with

high-ripple-current capability at the input. X7R capacitors

are recommended in industrial applications for their

temperature stability. Calculate the input capacitance

using the following equation:

××

=

η×

× ∆

OUT(MAX)

IN

SW

IN

I

D (1- D)

C

fV

voltage ripple, and η is the efficiency.

In applications where the source is located some distance

from the device input, an electrolytic capacitor should

be added in parallel to the ceramic capacitor to provide

necessary damping for potential oscillations caused by

the inductance of the longer input power path and input

ceramic capacitor.

Inductor Selection

Three key inductor parameters must be specified for

operation with the device: inductance value (L), inductor

switching frequency and output voltage determine the

inductor value as follows:

OUT

SW

V

L

f

=

Select a low-loss inductor closest to the calculated

value with acceptable dimensions and having the lowest

possible DC resistance. The saturation current rating

saturation can occur only above the peak current-limit

value of 3.7A.

Output Capacitor Selection

X7R ceramic output capacitors are preferred due to their

stability over temperature in industrial applications. The

output capacitors are usually sized to support a step load

of 50% of the maximum output current in the application,

such that output voltage deviation is contained to 3% of

nominal output voltage. The minimum required output

capacitance can be calculated as follows:

(

)

OUT

C

OUT

5.5

C

fV

=

×

the switching frequency is less than or equal to 500kHz.

to be 55kHz.

Derating of ceramic capacitors with DC-voltage must be

considered while selecting the output capacitor. Derating

curves are available from all major ceramic capacitor

vendors.

Soft-Start Capacitor Selection

The device implements adjustable soft-start operation to

reduce inrush current. A capacitor connected from the SS pin

to SGND programs the soft-start time. The selected output

the minimum required soft-start capacitor as follows:

-6

SS

SEL

OUT

C

28 10

C

V

≥×

×

×

SS

SS

-6

C

t

5.55 10

=

×

For example, to program a 1ms soft-start time, a 5.6nF

capacitor should be connected from the SS pin to SGND.

MAX17543

4.5V–42V, 2.5A, High-Efficiency,

Synchronous Step-Down DC-DC Converter

with Internal Compensation

www.maximintegrated.com

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