L1

D1

Q1

10 PF

100 PF

10 PH

10k

1.27k

Si2343

LM3475

EN

GND

PGATE

FB

1

2

3

4

5

C2

390 pF

C1

3.9 nF

R1

200k

F =

LM3475

www.ti.com

SNVS239A – OCTOBER 2004 – REVISED JANUARY 2005

OUTPUT CAPACITOR SELECTION

Once the desired operating frequency and inductance value are selected, ESR must be selected based on

Equation 4. This process may involve a few iterations to select standard ESR and inductance values.

In general, the ESR of the output capacitor and the inductor ripple current create the output ripple of the

regulator. However, the comparator hysteresis sets the first order value of this ripple. Therefore, as ESR and

ripple current vary, operating frequency must also vary to keep the output ripple voltage regulated. The hysteretic

control topology is well suited to using ceramic output capacitors. However, ceramic capacitors have a very low

ESR, resulting in a 90° phase shift of the output voltage ripple. This results in low operating frequency and

increased output ripple. To fix this problem a low value resistor could be added in series with the ceramic output

capacitor. Although counter intuitive, this combination of a ceramic capacitor and external series resistance

provide highly accurate control over the output voltage ripple. Another method is to add an external ramp at the

FB pin as shown in Figure 14. By proper selection of R1 and C2, the FB pin sees faster voltage change than the

output ripple can cause. As a result, the switching frequency is higher while the output ripple becomes lower. The

switching frequency is approximately:

(9)

Other types of capacitor, such as Sanyo POSCAP, OS-CON, and Nichicon ’NA’ series are also recommended

and may be used without additional series resistance. For all practical purposes, any type of output capacitor

may be used with proper circuit verification.

Capacitors with high ESL (equivalent series inductance) values should not be used. As shown in Figure 12, the

output ripple voltage contains a small step at both the high and low peaks. This step is caused by and is directly

proportional to the output capacitor’s ESL. A large ESL, such as in an electrolytic capacitor, can create a step

large enough to cause abnormal switching behavior.

INPUT CAPACITOR SELECTION

The input capacitor prevents large voltage transients at the input and provides the instantaneous current when

the PFET turns on. The important parameters for the input capacitor are the voltage rating and the RMS current

rating. Follow the manufacturer’s recommended voltage de-rating. RMS current and power dissipation (PD) can

be calculated with the equations below:

(10)

Figure 14. External Ramp

Copyright © 2004–2005, Texas Instruments Incorporated

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