SFTB NSC [National Semiconductor], SFTB Datasheet - Page 7

• Manufacturer Part Number: SFTB
• Description: Thin SOT23 1A Load Step-Down DC-DC Regulator
• Manufacturer: NSC [National Semiconductor]

Application Information
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.
IN
FIGURE 5. Boost Voltage Supplied from the Shunt
IN
+ 0.3V.
is non-monotonic.
OUT
Zener on V
to increase at a controlled rate
IN
IN
drops below 2.3V(typ).
(Continued)
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7
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
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:
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:
Note that this is just a guideline.
SW
can be approximated by:
L
) to output current (I
r = 0.387 x I
V
SW
I
LPK
O
= I
) to input voltage (V
= I
D
O
) can range from 0.3V to 0.7V
O
x R
LPK
+ ∆I
OUT
DS(ON)
) in the inductor is calcu-
-0.3667
L
/2
O
) is optimized when it
IN
):
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D
is, the