L =

D

10

‡ '

10

6

k

10

-3

Ps

LM2696

SNVS375B – OCTOBER 2005 – REVISED APRIL 2013

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The switching frequency of the LM2696 is set by the resistor connected to the RON pin. This resistor controls the

current flowing into the RON pin and is directly related to the on-time pulse. Connecting a resistor from this pin to

PVIN allows the switching frequency to remain constant as the input voltage changes. In normal operation this

pin is approximately 0.65V above GND. In shutdown, this pin becomes a high impedance node to prevent current

flow.

The ON time may be expressed as:

where

•

•

•

•

(3)

where

•

(4)

In CCM the frequency may be determined using the relationship:

(5)

Which is typically used to set the switching frequency.

perturbations into the pin and prevents proper operation.

INDUCTOR SELECTION

Selecting an inductor is a process that may require several iterations. The reason for this is that the size of the

inductor influences the amount of ripple present at the output that is critical to the stability of an adaptive on-time

circuit. Typically, an inductor is selected such that the maximum peak-to-peak ripple current is equal to 30% of

the maximum load current. The inductor current ripple (

(6)

Therefore, L can be initially set to the following by applying the 30% guideline:

(7)

The other features of the inductor that should be taken into account are saturation current and core material. A

shielded inductor or low profile unshielded inductor is recommended to reduce EMI.

OUTPUT CAPACITOR

The output capacitor size and ESR have a direct affect on the stability of the loop. This is because the adaptive

on-time control scheme works by sensing the output voltage ripple and switching appropriately. The output

voltage ripple on a buck converter can be approximated by assuming that the AC inductor ripple current flows

entirely into the output capacitor and the ESR of the capacitor creates the voltage ripple. This is expressed as:

(8)

10

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