MP1499 – 16V/5A PEAK SYNCHRONOUS STEP-DOWN CONVERTER

MP1499 Rev. 1.0

www.MonolithicPower.com

14

7/26/2012

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The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors,

a

small,

high

quality

ceramic

capacitor, i.e. 0.1μF, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V







 







(7)

Selecting the Output Capacitor

The output capacitor (C2) is required to maintain

the DC output voltage. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred to keep the

output voltage ripple low. The output voltage

ripple can be estimated by:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2







 









 

 

(8)

equivalent series resistance (ESR) value of the

output capacitor.

In the case of ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated by:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2



 



 



(9)

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the output

ripple can be approximated to:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V



 









(10)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP1499 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator, the applicable

conditions of external BST diode are:





Duty cycle is high: D=

IN

OUT

V

V

>65%

In these cases, an external BST diode is

recommended from the VCC pin to BST pin, as

shown in Figure 8.

Figure 8—Add Optional External Bootstrap

Diode to Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST cap is 0.1-1uF.

PCB layout is very important to achieve stable

operation especially for VCC capacitor and input

capacitor placement. For best results, follow

these guidelines:

1) Use large ground plane directly connect to

GND pin. Add vias near the GND pin if

bottom layer is ground plane.

2) Place the VCC capacitor to VCC pin and

GND pin as close as possible. Make the

trace length of VCC pin-VCC capacitor

anode-VCC capacitor cathode-chip GND pin

as short as possible.

3) Place the ceramic input capacitor close to IN

and GND pins. Keep the connection of input

capacitor and IN pin as short and wide as

possible.