LM2734Z_15 Datasheet, PDF (10/32 Pages) Texas Instruments – LM2734Z/LM2734ZQ Thin SOT 1A Load Step-Down DC-DC Regulator

LM2734Z
SNVS334E – JANUARY 2005 – REVISED APRIL 2013
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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):
V_O
D=
V_IN
(13)
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_O + V_D
D=
V_IN + V_D - V_SW
(14)
V_SW can be approximated by:
V_SW = I_O x R_DS(ON)
(15)
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.
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 (Δi_L) to output current (I_O) is optimized when it is set between 0.3 and 0.4 at 1A. The
ratio r is defined as:
'i_L
r = _lO
(16)
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_LPK) in the inductor is calculated by:
I_LPK = I_O + ΔI_L/2
(17)
If r = 0.5 at an output of 1A, the peak current in the inductor will be 1.25A. The minimum specified 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 increased. 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:
r = 0.387 x I_OUT-0.3667
(18)
Note that this is just a guideline.
The LM2734Z operates at frequencies allowing the use of ceramic output capacitors without compromising
transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.
See the OUTPUT CAPACITOR section for more details on calculating output voltage ripple.
Now that the ripple current or ripple ratio is determined, the inductance is calculated by:
V_O + V_D
L=
x (1-D)
I_O x r x f_S
(19)
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