MP2493 – 2A, 36V, LOW EMI STEP-DOWN WITH PROGRAMMABLE OUTPUT CURRENT LIMIT

MP2493 Rev. 0.9

www.MonolithicPower.com

9

4/23/2010

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see the schematic on front

page). The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor (see Figure 1). Choose

R1 to be around 300kΩ for optimal transient

response. R2 is then given by:

1

V

8

.

0

V

1

R

2

R

=

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

604 (1%)

475 (1%)

2.5

604 (1%)

280 (1%)

3.3

604 (1%)

191 (1%)

5

604 (1%)

115 (1%)

Selecting the Inductor

A 1µH to 15µH inductor with a DC current rating

of at least 25% percent higher than the

maximum load current is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be less than

200mΩ. For most designs, the inductance value

can be derived from the following equation.

SW

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

Choose

inductor

current

ripple

to

be

approximately 30% of the maximum load

current,. The maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, larger

inductance is recommended for improved

efficiency.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input and also the switching

noise from the device. The input capacitor

impedance at the switching frequency should

be less than the input source impedance to

prevent high frequency switching current from

pass to the input. Ceramic capacitors with X5R

or X7R dielectrics are highly recommended

because

of

their

low

ESR

and

small

temperature coefficients. For most applications,

a 10µF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps output voltage small

and ensures regulation loop stability. The

output capacitor impedance should be low at

the switching frequency. Ceramic capacitors

with X5R or X7R dielectrics are recommended.

Selecting Soft Start Capacitor

MP2493 has external soft start function. This

external function reduces the start up current

surge into the output capacitor. This allows the

gradual built up of output voltage to its final set

value. The soft start time is decided by following

equation;

SS

SS

6

C

T0.7

=×

×

PC Board Layout

The high current paths (GND, IN and SW)

should be placed very close to the device with

short, direct and wide traces. The input

capacitor needs to be as close as possible to

the IN and GND pins. The external feedback

resistors should be placed next to the FB pin.

Keep the switching node SW short and away

from the feedback network. ISN, ISP are

sensitive nodes. Put the sensing components

as close to the device as possible and keep

them away from the high current and noisy

paths such as GND, IN, SW). Match the trace

and components on ISN, ISP paths as good as

possible.