MP4431

– 36V, 1A, LOW IQ, SYNC, STEP-DOWN CONVERTER

MP4431 Rev. 1.01

www.MonolithicPower.com

21

1/12/2018

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For simplification, the output voltage ripple can

be estimated with Equation (8):





 



 

OUT

OUT

OUT

2

SW

OUT

IN

VV

V

(1

)

8 f

L C

V

(8)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (9):





 





OUT

OUT

OUT

ESR

SW

IN

VV

V

(1

) R

f

L

V

(9)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP4431 can be optimized for a wide range of

capacitance and ESR values.

Selecting the Inductor

A 1µH to 10µH inductor with a DC current rating

at least 25% higher than the maximum load

current is recommended for most applications.

For higher efficiency, choose an inductor with a

lower DC resistance. A larger-value inductor

results in less ripple current and a lower output

ripple voltage, but also has a larger physical

size,

higher

series

resistance,

and

lower

saturation current. A good rule for determining

the inductor value is to allow the inductor ripple

current

to

be

approximately

30%

of

the

maximum load current. The inductance value

can then be calculated with Equation (10):



 



OUT

OUT

SW

L

IN

VV

L

(1

)

f

I

V

(10)

Where

∆I

current.

Choose the inductor ripple current to be

approximately 30%

of the maximum load

current. The maximum inductor peak current

can be calculated with Equation (11):





 



OUT

OUT

LP

LOAD

SW

IN

VV

I

I

(1

)

2f

L

V

(11)

VIN UVLO Setting

The MP4431 has an internal, fixed, under-

voltage lockout (UVLO) threshold. The rising

threshold is 2.8V, while the falling threshold is

about 2.65V. For applications that need a

higher UVLO point, an external resistor divider

between VIN and EN can be used to achieve a

higher equivalent UVLO threshold (see Figure

6).

Figure 6: Adjustable UVLO Using EN Divider

The UVLO threshold can be calculated with

Equation (12) and Equation (13):

EN_RISING

DOWN

UP

RISING

V

R

R

(1

INUV







)

(12)

EN_FALLING

DOWN

UP

FALLING

V

R

R

(1

INUV







)

(13)

0.93V.

External BST Diode and Resistor

An external BST diode can enhance the

efficiency of the regulator when the duty cycle is

high. A power supply between 2.5V and 5V can

be used to power the external bootstrap diode.

supply in the circuit (see Figure 7).

L

BST

SW

External BST diode

IN4148

VCC

Figure 7: External Bootstrap Diode and Resistor

The recommended external BST diode is

IN4148, and the recommended BST capacitor

value is 0.1µF to 1μF. A resistor in series with

rising rate and voltage spikes. This helps

enhance EMI performance and reduce voltage

stress at a high VIN. A higher resistance is

better for SW spike reduction but compromises

efficiency. To make a tradeoff between EMI and

efficiency, a

≤20Ω R

VIN

EN

VIN

R

DOWN