Datasheet

LM2738

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SNVS556B –APRIL 2008–REVISED APRIL 2013

For example, using the X-version let V

= 10V, V

ZENER

= 5V, V

= 0.7V, I

ZENER

= 1mA, and duty cycle D = 50%.

Then

BOOST

= 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA (10)

R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11kΩ (11)

ENABLE PIN / SHUTDOWN MODE

The LM2738 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 400nA. The voltage at this pin

should never exceed V

+ 0.3V.

SOFT-START

This function forces V

OUT

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 0.8V in approximately 600µs. This forces the regulator

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

OUTPUT OVERVOLTAGE PROTECTION

The overvoltage comparator compares the FB pin voltage to a voltage that is 16% higher than the internal

reference Vref. Once the FB pin voltage goes 16% 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 LM2738 from operating until the input voltage exceeds 2.7V (typ).

The UVLO threshold has approximately 400mV of hysteresis, so the part will operate until V

drops below 2.3V

(typ). Hysteresis prevents the part from turning off during power up if the V

ramp-up is non-monotonic.

CURRENT LIMIT

The LM2738 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 2.9A (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.

Product Folder Links: LM2738