F =

'I

2

3

L =

'I

x

F

D

LM3475

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SNVS239C – OCTOBER 2004 – REVISED OCTOBER 2015

8.2.2.3 Using a Feed-forward Capacitor

The operating frequency and output ripple voltage can also be significantly influenced using a speed up

output ripple causes a current to be sourced or sunk through this capacitor. This current is essentially a square

wave. Since the input to the feedback pin (FB) is a high impedance node, the bulk of the current flows through

R2. This superimposes a square wave ripple voltage on the FB node. The end result is a reduction in output

100kHz decreasing linearly with increased operating frequency. Also note that as the output voltage is

8.2.2.4 Inductor Selection

The most important parameters for the inductor are the inductance and the current rating. The LM3475 operates

over a wide frequency range and can use a wide range of inductance values. Minimum inductance can be

calculated using the following equation:

where

•

•

ΔI is the allowable inductor ripple current

(6)

below:

(7)

(8)

The inductance value and the resulting ripple is one of the key parameters controlling operating frequency.

8.2.2.5 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 12. 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.

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