lm2738xsdx National Semiconductor Corporation, lm2738xsdx Datasheet - Page 10

• Manufacturer Part Number: lm2738xsdx
• Description: Lm2738 550khz/1.6mhz 1.5a Step-down Dc-dc Switching Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
For example, using the X-version let V
V
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 ex-
ceed V
SOFT-START
This function forces V
ing start up. During soft-start, the error amplifier’s reference
voltage ramps from 0V to its nominal value of 0.8V in approx-
imately 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 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 LM2738 from op-
erating until the input voltage exceeds 2.7V (typ).
The UVLO threshold has approximately 400mV of hysteresis,
so the part will operate until V
teresis prevents the part from turning off during power up if
the V
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 ap-
proximately 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 switch must be included to
calculate a more accurate duty cycle. Calculate D by using
the following formula:
D2
= 0.7V, I
I
BOOST
IN
R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11kΩ
IN
ramp-up is non-monotonic.
+ 0.3V.
= 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA
ZENER
= 1mA, and duty cycle D = 50%. Then
OUT
O
to increase at a controlled rate dur-
) to input voltage (V
IN
drops below 2.3V (typ). Hys-
IN
= 10V, V
IN
):
ZENER
= 5V,
10
V
The diode forward drop (V
pending on the quality of the diode. The lower the 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.
One must ensure that the minimum current limit (2.0A) is not
exceeded, so the peak current in the inductor must be calcu-
lated. The peak current (I
In general,
If Δi
be 2.0A. The minimum guaranteed current limit over all op-
erating conditions is 2.0A. One can either reduce Δi
the engineering judgment that zero margin will be safe
enough. The typical current limit is 2.9A.
The LM2738 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
capacitor section for more details on calculating output volt-
age ripple. Now that the ripple current is determined, the
inductance is calculated by:
Where
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Induc-
tor saturation will result in a sudden reduction in inductance
and prevent the regulator from operating correctly. Because
SW
L
can be approximated by:
= 33.3% of 1.50A, the peak current in the inductor will
Δi
FIGURE 6. Inductor Current
L
= 0.1 x (I
V
SW
I
LPK
= I
LPK
D
= I
) can range from 0.3V to 0.7V de-
OUT
OUT
) in the inductor is calculated by:
OUT
)
x R
→
+ Δi
0.2 x (I
DSON
L
OUT
)
L
, or make
D
, the
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