D =

D =

LM2696

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SNVS375B – OCTOBER 2005 – REVISED APRIL 2013

INTERNAL OPERATION UNDER-VOLTAGE COMPARATOR

An internal comparator is used to monitor the feedback pin for sensing under-voltage output events. If the output

voltage drops below the UVP threshold the power-good flag will fall.

ON-TIME GENERATOR SHUTDOWN

The on-time for the LM2696 is inversely proportional to the input voltage. This scheme of on-time control

maintains a constant frequency over the input voltage range. The on-time can be adjusted by using an external

resistor connected between the PVIN and RON pins.

CURRENT LIMIT

The LM2696 contains an intelligent current limit off-timer. If the peak current in the internal FET exceeds 4.9A the

present on-time is terminated; this is a cycle-by-cycle current 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.

In cases of overload (not complete short circuit, FB > 0V) 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 temperature. If the die temp exceeds 165ºC then the

sensor will trip causing the part to stop switching. Soft-start will restart after the temperature falls below 155ºC.

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:

(1)

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

•

•

(2)

FREQUENCY SELECTION

Switching frequency affects the selection of the output inductor, capacitor, and overall efficiency. The trade-offs

in frequency 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 recommended that a nominal frequency of

300 kHz should be used in the initial stages of design and iterated if necessary.

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