MP2234S – SYNCHRONOUS STEP-DOWN CONVERTER

MP2234S Rev.1.0

www.MonolithicPower.com

15

4/3/2015

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voltage ripple caused by capacitance can be

estimated as:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

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 as:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝

⎠

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 as:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated as:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP2234S can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

In particular conditions, BST voltage may

become insufficient (see equations below).

During these conditions an external bootstrap

diode can enhance the efficiency of the

regulator and avoid insufficient BST voltage at

light-load PFM operation. Insufficient BST

voltage is more likely to occur during either of

the following conditions:

Duty cycle is high: D=

IN

OUT

V

V

>65%

If the BST voltage is insufficient, the output-

ripple voltage may become extremely large

during a light-load condition. If this occurs, add

an external BST diode from VCC to BST (see

Figure 8).

Figure 8. Optional External Bootstrap Diode to

Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST capacitor value is 0.1µF

to 1μF.

PCB layout is very important to achieve stable

operation especially for VCC capacitor and

input capacitor placement. For best results,

refer to Figure 9 and the guidelines below:

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.

4. Route SW, BST away from sensitive

analog areas such as FB.

5. Place the T-type feedback resistor R5 close

to chip to ensure the trace which connects

to FB pin as short as possible.

Notes:

9)

The recommended layout is based on the Figure 10 Typical

Application circuit on page 17.

MP2234S