D =
V
O
+ V
D
V
IN
+ V
D
- V
SW
D =
V
O
V
IN
LM2734
SNVS288I – SEPTEMBER 2004 – REVISED APRIIL 2013
www.ti.com
ENABLE PIN / SHUTDOWN MODE
The LM2734 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. Under some
circumstances at start-up, an output voltage overshoot may still be observed. This may be due to a large output
load applied during start up. Large amounts of output external capacitance can also increase output voltage
overshoot. A simple solution is to add a feed forward capacitor with a value between 470pf and 1000pf across
the top feedback resistor (R1). See
Figure 28
for further detail.
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 LM2734 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 LM2734 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
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 (V
O
) to input voltage (V
IN
):
(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)
V
SW
can be approximated by:
V
SW
= I
O
x R
DS(ON)
(14)
The diode forward drop (V
D
) can range from 0.3V to 0.7V depending on the quality of the diode. The lower V
D
is,
the higher the operating efficiency of the converter.
12
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