LM2734YMKX National Semiconductor, LM2734YMKX Datasheet - Page 9

• Manufacturer Part Number: LM2734YMKX
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,BICMOS,TSOP,6PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2734YMKX

• Rohs Compliant: NO

FIGURE 5. Boost Voltage Supplied from the Shunt Zener
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
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 200µs. This forces the regulator output to ramp up in
a more linear and controlled fashion, which helps reduce in-
rush current. Under some circumstances at start-up, an out-
put 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 volt-
age 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 7 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 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 LM2734 from op-
erating until the input voltage exceeds 2.74V(typ).
The UVLO threshold has approximately 440mV of hysteresis,
so the part will operate until V
teresis prevents the part from turning off during power up if
V
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
IN
IN
+ 0.3V.
is non-monotonic.
OUT
to increase at a controlled rate dur-
on V
IN
IN
drops below 2.3V(typ). Hys-
20102348
9
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 must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
V
The diode forward drop (V
pending on the quality of the diode. The lower V
the operating efficiency of the converter.
The inductor value determines the output ripple current. Low-
er 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 (Δi
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
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 empiri-
cally developed for the maximum ripple ratio at any current
below 2A is:
Note that this is just a guideline.
The LM2734 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
SW
can be approximated by:
L
) to output current (I
r = 0.387 x I
V
I
SW
LPK
O
= I
) to input voltage (V
D
= I
) can range from 0.3V to 0.7V de-
O
O
LPK
x R
+ ΔI
O
OUT
) in the inductor is calculated
) is optimized when it is set
DS(ON)
-0.3667
L
/2
IN
D
):
is, the higher
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