MP2235S –3 A, 16 V, 800 kHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2235S Rev.1.0

www.MonolithicPower.com

15

4/15/2015

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2015 MPS. All Rights Reserved.

make sure that they have enough capacitance

to provide sufficient charge in order to prevent

excessive voltage ripple at the input. The input

voltage ripple caused by capacitance can be

estimated using Equation (7):

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

(7)

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 (8):

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

(8)

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 (9):

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

(9)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (10):

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

(10)

The characteristics of the output capacitor

affect the stability of the regulation system. The

MP2235S can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

In particular conditions, the BST voltage may

become insufficient. 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:

the 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).

MP2235S

Figure 8—Optional external bootstrap diode to

enhance efficiency

The recommended external BST diode is

IN4148, and the BST capacitor value is 0.1 µF

to 1 μF.

Efficient PCB layout is critical to achieve stable

operation, especially for VCC capacitor and

input capacitor placement. For best results,

refer to Figure 9 and follow the guidelines below:

1. Use

a

large

ground

plane

directly

connected to GND. Add vias near GND if

the bottom layer is ground plane.

2. Place the VCC capacitor as close as

possible to the chip VCC and GND. Make

the trace length of VCC pin to the VCC

capacitor anode to the VCC capacitor

cathode to the chip GND as short as

possible.

3. Place the ceramic input capacitor close to

IN and GND. Keep the connection of the

input capacitor and IN as short and wide as

possible.

4. Route SW and BST away from sensitive

analog areas such as FB.

5. Place the T-type feedback resistor (R5)

close to the chip to ensure the trace (which

connects to FB) is as short as possible.

NOTES:

9) The recommended layout is based on Figure 10 on page 17.