LMZ14203DEMO/NOPB National Semiconductor, LMZ14203DEMO/NOPB Datasheet - Page 10

• Manufacturer Part Number: LMZ14203DEMO/NOPB
• Description: BOARD DEV PWR MODULE LMZ14203
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r

Specifications of LMZ14203DEMO/NOPB

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 6 ~ 42 V
• Regulator Topology: Buck
• Frequency - Switching: Up to 1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LMZ14203
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LMZ14203DEMO

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These resistors should be chosen from values in the range of
1.0 kohm to 10.0 kohm.
For V
rectly so long as an output preload resistor remains that draws
more than 20uA. Converter operation requires this minimum
load to create a small inductor ripple current and maintain
proper regulation when no load is present.
A feed-forward capacitor is placed in parallel with R
prove load step transient response. Its value is usually deter-
mined experimentally by load stepping between DCM and
CCM conduction modes and adjusting for best transient re-
sponse and minimum output ripple.
A table of values for R
the applications schematic.
SOFT-START CAPACITOR 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
This equation can be rearranged as follows:
C
Use of a 0.022μF capacitor results in 2.2 msec soft-start du-
ration. This is recommended as a minimum value.
As the soft-start input exceeds 0.8V the output of the power
stage will be in regulation. The soft-start capacitor continues
charging until it reaches approximately 3.8V on the SS pin.
Voltage levels between 0.8V and 3.8V have no effect on other
circuit operation. 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 Vcc UVLO (Approx 4V input to V
C
None of the required C
in the module. At a minimum, the output capacitor must meet
the worst case minimum ripple current rating of 0.5 * I
as calculated in equation (19) below. 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 op-
erate with a minimum value. Ceramic capacitors or other low
ESR types are recommended. See AN-2024 for more detail.
The following equation provides a good first pass approxima-
tion of C
C
Solving:
C
≥
The LMZ14203 demonstration and evaluation boards are
populated with a 100 uF 6.3V X5R output capacitor. Locations
for extra output capacitors are provided.
SS
SS
O
O
O
43μF (7)
≥
≥
= V
SELECTION
= t
I
STEP
3A*0.8V*6.8μH*24V / (4*3.3V*( 24V — 3.3V)*33mV)
O
REF
SS
= 0.8V the FB pin can be connected to the output di-
O
*V
* 8 μA / 0.8V (5)
for load transient requirements:
* C
FB
*L*V
SS
/ Iss = 0.8V * C
IN
/ (4*V
FBT
O
output capacitance is contained with-
O
, R
*(V
FBB
IN
—V
SS
, C
/ 8uA (4)
FF
O
)*V
and R
OUT-TRAN
IN
)
ON
is included in
)(6)
FBT
LR P-P
to im-
,
10
C
The LMZ14203 module contains an internal 0.47 µF input ce-
ramic capacitor. Additional input capacitance is required ex-
ternal to the module to handle the input ripple current of the
application. This input capacitance should be located in very
close proximity to the module. Input capacitor selection is
generally directed to satisfy the input ripple current require-
ments rather than by capacitance value. Worst case input
ripple current rating is dictated by the equation:
I(C
where D
(As a point of reference, the worst case ripple current will oc-
cur when the module is presented with full load current and
when V
Recommended minimum input capacitance is 10uF X7R ce-
ramic with a voltage rating at least 25% higher than the
maximum applied input voltage for the application. It is also
recommended that attention be paid to the voltage and tem-
perature deratings of the capacitor selected. It should be
noted that ripple current rating of ceramic capacitors may be
missing from the capacitor data sheet and you may have to
contact the capacitor manufacturer for this rating.
If the system design requires a certain minimum value of input
ripple voltage ΔV
may be used.
C
If ΔV
this equals 240 mV and f
C
≥
Additional bulk capacitance with higher ESR may be required
to damp any resonant effects of the input capacitance and
parasitic inductance of the incoming supply lines.
R
Many designs will begin with a desired switching frequency in
mind. For that purpose the following equation can be used.
f
This can be rearranged as
R
The selection of RON and f
tations in the on-time and off-time for the COT control section.
The on-time of the LMZ14203 timer is determined by the re-
sistor R
t
The inverse relationship of t
switching frequency as V
such that the on-time at maximum V
The on-timer has a limiter to ensure a minimum of 150 ns for
t
governed by the following equation:
f
This equation can be used to select R
frequency is desired so long as the minimum on-time of 150
ns is observed. The limit for R
R
If R
termined in (14) a lower frequency should be selected. Alter-
SW(CCM)
ON
ON
SW(MAX)
IN
IN
IN
ON
ON
ON
3.7μF
IN(RMS)
. This limits the maximum operating frequency, which is
≥
ON
= (1.3 * 10
SELECTION
≥
RESISTOR SELECTION
≊
≥
IN
3A * 3.3V/24V * (1– 3.3V/24V) / (400000 * 0.240 V)
I
O
calculated in (11) is less than the minimum value de-
V
V
is 1% of V
IN
ON
O
= V
IN(MAX)
* D * (1–D) / f
≊
≊
)
= 2 * V
/ (1.3 * 10
and the input voltage V
≊
V
V
O
O
O
1 /2 * I
/ (V
/ (1.3 * 10
-10
/ V
* 150 nsec / (1.3 * 10
O
IN
* R
IN
IN
IN(MAX)
).
be maintained then the following equation
O
for a 24V input to 3.3V output application
ON
-10
*
SW-CCM
√
) / V
* f
-10
* 150 nsec) (13)
SW
IN
(D / 1-D) (8)
SW(CCM)
IN
* R
ON
SW(CCM)
is varied. R
= 400 kHz.
ON
(12)
* ΔV
and V
ON
can be calculated as follows:
) (10)
IN
(11)
IN
. It is calculated as follows:
must be confined by limi-
IN
(9)
IN
-10
ON
gives a nearly constant
is greater than 150 ns.
ON
) (14)
if a certain operating
should be selected