MP8712 – 18V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8712 Rev. 1.2

MonolithicPower.com

20

3/19/2021

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sufficient charge to prevent excessive voltage

ripple at the input.

The input voltage ripple caused by capacitance

can be estimated with Equation (8):

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V







 







(8)

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors. For best results,

use low ESR capacitors to keep the output

voltage ripple low. The output voltage ripple can

be estimated with Equation (9):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

  (9)

Where L1 is the inductor value, and RESR is the

equivalent series resistance (ESR) value of the

output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency, and the capacitance causes the

majority of the output voltage ripple. For

simplification, the output voltage ripple can be

estimated with Equation (10):

OUT

OUT

OUT

2

IN

S1

VV

∆V1

V

8f

L

C2



 



 



(10)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (11):

OUT

OUT

OUT

ESR

IN

S1

VV

∆V1

R

fL

V



 









(11)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP8712 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode can enhance the

efficiency of the regulator given the following

conditions:

 VOUT is 5V or 3.3V

 Duty cycle is high: D > 50%

In these cases, add an external BST diode from

VCC to BST (see Figure 6).

SW

BST

MP8712

C

L

BST

COUT

External BST Diode

VCC

IN4148

Figure 6: Optional External Bootstrap Diode to

Enhance Efficiency

The recommended external BST diode is

1N4148, and the recommended BST capacitor

value is 0.1μF to 1μF.

Connecting VCC to VIN at a Low Input

Voltage

VCC can be connected to VIN directly when

VIN is lower than 3.5V. This helps improve the

MP8712’s

low

input

voltage

efficiency

performance. To use this application set-up, the

VIN spike voltage must be limited below 4V;

otherwise, VCC could be damaged.