Datasheet

LM2734
V
IN
BOOST
SW
GND
L
D1
D2
D3
R3
C4
V
BOOST
C
BOOST
V
Z
V
IN
C
IN
V
OUT
C
OUT
LM2734Z
www.ti.com SNVS334E –JANUARY 2005–REVISED APRIL 2013
R3 will then be given by
R3 = (V
IN
- V
ZENER
) / (1.25 x I
BOOST
+ I
ZENER
) (10)
For example, let V
IN
= 10V, V
ZENER
= 5V, V
D2
= 0.7V, I
ZENER
= 1mA, and duty cycle D = 50%. Then
I
BOOST
= (0.5 + 0.5) x (5 - 0.7) mA = 4.3mA (11)
R3 = (10V - 5V) / (1.25 x 4.3mA + 1mA) = 787Ω (12)
Figure 12. Boost Voltage Supplied from the Shunt Zener on V
IN
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
IN
+ 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 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 LM2734Z from operating until the input voltage exceeds 2.74V(typ).
The UVLO threshold has approximately 440mV of hysteresis, so the part will operate until V
IN
drops below
2.3V(typ). Hysteresis prevents the part from turning off during power up if V
IN
is non-monotonic.
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.
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