Datasheet

LMZ14202H
SNVS691E –JANUARY 2011–REVISED OCTOBER 2013 www.ti.com
V
O
= 0.8V x (1 + R
FBT
/ R
FBB
) (2)
Rearranging terms; the ratio of the feedback resistors for a desired output voltage is:
R
FBT
/ R
FBB
= (V
O
/ 0.8V) - 1 (3)
These resistors should be chosen from values in the range of 1 kΩ to 50 kΩ.
A feed-forward capacitor is placed in parallel with R
FBT
to improve load step transient response. Its value is
usually determined experimentally by load stepping between DCM and CCM conduction modes and adjusting for
best transient response and minimum output ripple.
A table of values for R
FBT
, R
FBB
, and R
ON
is included in the simplified applications schematic.
SOFT-START CAPACITOR, C
SS
, SELECTION
Programmable soft-start permits the regulator to slowly ramp to its steady state operating point after being
enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time to
prevent overshoot.
Upon turn-on, after all UVLO conditions have been passed, an internal 8uA current source begins charging the
external soft-start capacitor. The soft-start time duration to reach steady state operation is given by the formula:
t
SS
= V
REF
x C
SS
/ Iss = 0.8V x C
SS
/ 8uA (4)
This equation can be rearranged as follows:
C
SS
= t
SS
x 8 μA / 0.8V (5)
Use of a 4700pF capacitor results in 0.5ms soft-start duration. This is a recommended value. Note that high
values of C
SS
capacitance will cause more output voltage droop when a load transient goes across the DCM-
CCM boundary. Use Equation 18 below to find the DCM-CCM boundary load current for the specific operating
condition. If a fast load transient response is desired for steps between DCM and CCM mode the softstart
capacitor value should be less than 0.018µF.
As the soft-start input exceeds 0.8V the output of the power stage will be in regulation. Note that the following
conditions will reset the soft-start capacitor by discharging the SS input to ground with an internal 200 μA current
sink:
• The enable input being “pulled low”
• Thermal shutdown condition
• Over-current fault
• Internal V
IN
UVLO
OUTPUT CAPACITOR, C
O
, SELECTION
None of the required output capacitance is contained within the module. At a minimum, the output capacitor must
meet the worst case RMS current rating of 0.5 x I
LR P-P
, as calculated in Equation 19. Beyond that, additional
capacitance will reduce output ripple so long as the ESR is low enough to permit it. A minimum value of 10 μF is
generally required. Experimentation will be required if attempting to operate with a minimum value. Low ESR
capacitors, such as ceramic and polymer electrolytic capacitors are recommended.
CAPACITANCE:
Equation 6 provides a good first pass approximation of C
O
for load transient requirements:
C
O
≥I
STEP
x V
FB
x L x V
IN
/ (4 x V
O
x (V
IN
— V
O
) x V
OUT-TRAN
) (6)
As an example, for 2A load step, V
IN
= 24V, V
OUT
= 12V, V
OUT-TRAN
= 50mV:
C
O
≥ 2A x 0.8V x 15μH x 24V / (4 x 12V x ( 24V — 12V) x 50mV)
C
O
≥ 20μF
ESR:
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