limit. Following the termination of the on-time, a non-

resetable extended off timer is initiated. The length of the

off-time is proportional to the feedback voltage. When FB =

0V the off-time is preset to 20 µs. This condition is often a

result of in short circuit operation when a maximum amount

of off-time is required. This amount of time ensures safe

short circuit operation up to the maximum input voltage of

24V.

the current limit off-time is reduced. Reduction of the off-time

during smaller overloads reduces the amount of fold back.

This also reduces the initial startup time.

N-CHANNEL HIGH SIDE SWITCH AND DRIVER

The LM2696 utilizes an integrated N-Channel high side

switch and associated floating high voltage gate driver. This

gate driver circuit works in conjunction with an external

bootstrap capacitor and an internal diode. The minimum

off-time (165 ns) is set to ensure that the bootstrap capacitor

has sufficient time to charge.

THERMAL SHUTDOWN

An internal thermal sensor is incorporated to monitor the die

will trip causing the part to stop switching. Soft-start will

COMPONENT SELECTION

As with any DC-DC converter, numerous trade-offs are

present that allow the designer to optimize a design for

efficiency, size and performance. These trade-offs are taken

into consideration throughout this section.

The first calculation for any buck converter is duty cycle.

Ignoring voltage drops associated with parasitic resistances

and non-ideal components, the duty cycle may be expressed

as:

A duty cycle relationship that considers the voltage drop

across the internal FET and voltage drop across the external

catch diode may be expressed as:

Where V

(D

FET.

FREQUENCY SELECTION

Switching frequency affects the selection of the output induc-

tor, capacitor, and overall efficiency. The trade-offs in fre-

quency selection may be summarized as; higher switching

frequencies permit use of smaller inductors possibly saving

board space at the trade-off of lower efficiency. It is recom-

mended that a nominal frequency of 300 kHz should be used

in the initial stages of design and iterated if necessary.

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 imped-

ance node to prevent current flow.

The on time may be exoressed as:

Where V

(typically PV

pin (0.65V typical), R

Ω, and k

set internally (66 µA

re-arranged such that R

quency:

Where f

In CCM the frequency may be determined using the relation-

ship:

(T

Which is typically used to set the switching frequency.

Under no condition should a bypass capacitor be connected

to the R

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 (

∆I

Therefore, L can be initially set to the following by applying

the 30% guideline:

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.

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