F =

(VIN - VOUT) x ESR

x

(VHYST x D x L) + (VIN x delay x ESR)

VIN

VOUT

-

+

PMOS_drv

Hyst Comp

FB

+

VOUT

PGATE

+

-

Reference

Voltage

VFB = 0.8V

VHYST = 21 mV

PGATE

R1

R2

L

Cff

COUT

VOUT =

R1 + R2

R2

x VFB

LM3475

SNVS239C – OCTOBER 2004 – REVISED OCTOBER 2015

www.ti.com

8.2.2.1 Setting Output Voltage

The output voltage is programmed using a resistor divider between VOUT and GND as shown in Figure 11. The

feedback resistors can be calculated as follows:

where

•

Vfb is 0.8 V typically

(3)

The feedback resistor ratio,

α = (R1+R2) / R2, will also be used below to calculate output ripple and operating

frequency.

Figure 11. Hysteretic Window

8.2.2.2 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 inductance, output capacitance, ESR, VIN,

or Cff is changed, there will be a change in the operating frequency and possibly output ripple. Therefore, care

must be taken to select components 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 process usually involves a few iterations to select appropriate

standard values that will result in the desired frequency and ripple.

Without the feedforward capacitor (Cff), the operating frequency (F) can be approximately calculated using the

formula:

where

•

Delay is the sum of the LM3475 propagation delay time and the PFET delay time

•

The propagation delay is 90ns typically

(4)

Minimum output ripple voltage can be determined using the following equation:

VOUT_PP = VHYST ( R1 + R2 ) / R2

(5)

12

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