MP2499– INTEGRATED 100V LOAD DUMP PROTECTION, 2A STEP DOWN REGULATOR

MP2499 Rev. 0.91

www.MonolithicPower.com

10

11/24/2010

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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

300 (1%)

240 (1%)

2.5

300 (1%)

141.1(1%)

3.3

300 (1%)

96 (1%)

5

300 (1%)

57.1 (1%)

Selecting the Inductor

A 33µH to 47µH 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

×

∆

×

−

×

=

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

∆

+

=

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 220µF electrolytic 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 D2 Diode

The D2 Zener diode voltage is a reference

voltage to control the clamp voltage of the OVP

protection during a surge condition to choose

the Zener voltage to be the same as the input

voltage to reduce any OVP protection losses.

Refer to the typical operation curve on page 6

titled “Clamp Voltage vs. VDD-VCTR” to make

sure the VDD voltage doesn’t exceed the

maximum input voltage, when a surge voltage

is applied to the OVP protection.

Selecting Soft Start Capacitor

MP2499 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

0.7 C

T

×

=

×