LM2736 datasheet(11/27 Pages) TI1

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r =

'iL

D =

VO + VD

VIN + VD - VSW

D =

VIN

LM2736

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SNVS316E – SEPTEMBER 2004 – REVISED SEPTEMBER 2011

ENABLE PIN / SHUTDOWN MODE

The LM2736 has a shutdown mode that is controlled by the enable pin (EN). When a logic low voltage is applied

to EN, the part is in shutdown mode and its quiescent current drops to typically 30nA. Switch leakage adds

another 40nA from the input supply. The voltage at this pin should never exceed VIN + 0.3V.

SOFT-START

This function forces VOUT to increase at a controlled rate during start up. During soft-start, the error amplifier’s

reference voltage ramps from 0V to its nominal value of 1.25V in approximately 200µs. This forces the regulator

output to ramp up in a more linear and controlled fashion, which helps reduce inrush current.

OUTPUT OVERVOLTAGE PROTECTION

The overvoltage comparator compares the FB pin voltage to a voltage that is 10% higher than the internal

reference Vref. Once the FB pin voltage goes 10% above the internal reference, the internal NMOS control

switch is turned off, which allows the output voltage to decrease toward regulation.

UNDERVOLTAGE LOCKOUT

Undervoltage lockout (UVLO) prevents the LM2736 from operating until the input voltage exceeds 2.74V(typ).

The UVLO threshold has approximately 440mV of hysteresis, so the part will operate until VIN drops below

2.3V(typ). Hysteresis prevents the part from turning off during power up if VIN is non-monotonic.

CURRENT LIMIT

The LM2736 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a

current limit comparator detects if the output switch current exceeds 1.5A (typ), and turns off the switch until the

next switching cycle begins.

THERMAL SHUTDOWN

Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature

exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature

drops to approximately 150°C.

Design Guide

INDUCTOR SELECTION

The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN):

(12)

The catch diode (D1) forward voltage drop and the voltage drop across the internal NMOS must be included to

calculate a more accurate duty cycle. Calculate D by using the following formula:

(13)

VSW can be approximated by:

VSW = IO x RDS(ON)

(14)

The diode forward drop (VD) can range from 0.3V to 0.7V depending on the quality of the diode. The lower VD is,

the higher the operating efficiency of the converter.

The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor,

but increase the output ripple current. An increase in the inductor value will decrease the output ripple current.

The ratio of ripple current (

ΔiL) to output current (IO) is optimized when it is set between 0.3 and 0.4 at 750mA.

The ratio r is defined as:

(15)

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