Design Information

SETTING OUTPUT VOLTAGE

The output voltage is programmed using a resistor divider

between V

resistors can be calculated as follows:

Where Vfb is 0.8V typically.

The feedback resistor ratio,

α = (R1+R2) / R2, will also be

used below to calculate output ripple and operating fre-

quency.

SETTING OPERATING FREQUENCY AND OUTPUT

RIPPLE

Although hysteretic control is a simple control scheme, the

operating frequency and other performance characteristics

depend on external conditions and components. If the induc-

tance, output capacitance, ESR, V

will be a change in the operating frequency and possibly

output ripple. Therefore, care must be taken to select com-

ponents which will provide the desired operating range. The

best approach is to determine what operating frequency is

desirable in the application and then begin with the selection

of the inductor and output capacitor ESR. The design pro-

cess usually involves a few iterations to select appropriate

standard values that will result in the desired frequency and

ripple.

Without the feedforward capacitor (C

quency (F) can be approximately calculated using the for-

mula:

Where delay is the sum of the LM3475 propagation delay

time and the PFET delay time. The propagation delay is

90ns typically.

Minimum output ripple voltage can be determined using the

following equation:

V

USING A FEED-FORWARD CAPACITOR

The operating frequency and output ripple voltage can also

be significantly influenced using a speed up capacitor, C

shown in Figure 2.C

side feedback resistor, R1. The output ripple causes a cur-

rent to be sourced or sunk through this capacitor. This cur-

rent 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 ripple and an increase in operating frequency.

When adding C

α= 1. The

value of C

the value of R2. A good starting point is 1nF ceramic at

100kHz decreasing linearly with increased operating fre-

quency. Also note that as the output voltage is programmed

below 1.6V, the effect of C

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 D is the duty cycle, defined as V

∆Iisthe

allowable inductor ripple current.

Maximum allowable inductor ripple current should be calcu-

lated as a function of output current (I

∆I

The inductor must also be rated to handle the peak current

(I

I

∆I/2) x 1.1

The inductance value and the resulting ripple is one of the

key parameters controlling operating frequency.

OUTPUT CAPACITOR SELECTION

Once the desired operating frequency and inductance value

are selected, ESR must be selected based on the equation

in the Setting Operating Frequency and Output Ripple. 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. How-

ever, the comparator hysteresis sets the first order value of

this ripple. Therefore, as ESR and ripple current vary, oper-

ating frequency must also vary to keep the output ripple

20070115

FIGURE 2. Hysteretic Window

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