ENABLE PIN / SHUTDOWN MODE

The LM2734Z 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

V

SOFT-START

This function forces V

during start up. During soft-start, the error amplifier’s refer-

ence voltage ramps from 0V to its nominal value of 0.8V 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 refer-

ence, the internal NMOS control switch is turned off, which

allows the output voltage to decrease toward regulation.

UNDERVOLTAGE LOCKOUT

Undervoltage lockout (UVLO) prevents the LM2734Z from

operating until the input voltage exceeds 2.74V(typ).

The UVLO threshold has approximately 440mV of hyster-

esis, so the part will operate until V

Hysteresis prevents the part from turning off during power up

if V

CURRENT LIMIT

The LM2734Z 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 1.7A

(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 ex-

ceeds 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 (V

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 follow-

ing formula:

V

V

The diode forward drop (V

depending on the quality of the diode. The lower V

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 (

∆i

is set between 0.3 and 0.4 at 1A. The ratio r is defined as:

One must also ensure that the minimum current limit (1.2A)

is not exceeded, so the peak current in the inductor must be

calculated. The peak current (I

lated by:

I

∆I

If r = 0.5 at an output of 1A, the peak current in the inductor

will be 1.25A. The minimum guaranteed current limit over all

operating conditions is 1.2A. One can either reduce r to 0.4

resulting in a 1.2A peak current, or make the engineering

judgement that 50mA over will be safe enough with a 1.7A

typical current limit and 6 sigma limits. When the designed

maximum output current is reduced, the ratio r can be in-

creased. At a current of 0.1A, r can be made as high as 0.9.

The ripple ratio can be increased at lighter loads because

the net ripple is actually quite low, and if r remains constant

the inductor value can be made quite large. An equation

empirically developed for the maximum ripple ratio at any

current below 2A is:

r = 0.387 x I

OUT

-0.3667

Note that this is just a guideline.

20130348

FIGURE 5. Boost Voltage Supplied from the Shunt

Zener on V

IN

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