Datasheet

'V
IN
=
I
OUT
x D x (1 ± D)
C
IN
x f
SW
I
OUT
+
+
'I
L
2
x R
ESR_CIN
I
CIN(RMS)
= 20A
0.5 x 0.5 = 10A
I
CIN(RMS)
= I
OUT
D(1 - D)
D =
V
OUT
V
IN
x

1
LM27402 Evaluation Board www.ti.com
7 LM27402 Evaluation Board
The LM27402 evaluation board is designed to support multiple applications and modifications and is
optimized for a 4.5V to 12V input voltage range. The maximum steady state output current is set at 20A
and will typically current limit at 24A. The PCB features bolt-on banana connections if heavy duty
connectors are needed.
8 Setup Procedure
1. Set the input power supply voltage to 12V. Adjust the input supply current limit level to 10A to protect
from any unanticipated shorts.
2. Turn the input power supply off. Connect the input supply positive terminal to the VIN terminal and the
input supply ground terminal to the GND IN terminal.
3. Turn the output electronic load off. Connect the electronic load positive terminal to the VOUT terminal
and the ground terminal to the GND OUT terminal of the LM27402 evaluation board
4. Turn the input supply on. The part will self enable and the output voltage should be 1.5V. Slowly
increase the load current to 20A. The input voltage can now be adjusted as well.
CAUTION: If the input voltage is below 5V, the internal LDO will be in a drop out state. The output of the
LDO provides the driving voltage across the gates of the MOSFETs. If the voltage at VDD decreases
enough, the efficiency may suffer if the MOSFETs are not fully enhanced in the on-state. The output
should maintain regulation.
9 Evaluation Board Component Selection
This section describes the design process for the LM27402 evaluation board. Unless otherwise indicated,
all formulae assume units of Amps (A) for current, Farads (F) for capacitance, Henries (H) for inductance,
and Volts (V) for voltage.
The first equation to calculate for any buck converter is duty ratio:
(1)
Due to the resistive powertrain losses, the duty ratio will increase based on the overall efficiency, η.
Setting η = 1 yields an approximate result for D.
9.1 Input Filter, R
in
, C
in
An RC filter is added to prevent any switching noise from interfering with the internal analog circuitry
connected to VIN. The RC filter can be seen in the evaluation board schematic as components R
in
and C
in
.
There is a practical limit to the value of resistor R
in
as the VIN pin of the LM27402 will draw large bias
currents to switch the gate of each MOSFET. If R
in
is too large, the resulting voltage drop can disrupt
normal operation. For the evaluation board, a 2.2Ω resistor in conjunction with a 1.0 µF 25V X5R ceramic
capacitor is used for the input RC filter.
9.2 Input Capacitors, C
in1
- C
in5
Input capacitors should be selected based on the required input voltage ripple and maximum RMS current
rating. The required RMS current rating of the input capacitor for a buck regulator can be estimated by the
following equation:
(2)
From this equation, it follows that the maximum I
CIN(RMS)
requirement will occur at a full 20A load current
with the system operating at 50% duty cycle. Under this condition, the maximum I
CIN(RMS)
is given by:
(3)
The voltage ripple can be calculated by:
(4)
6
AN-1992 LM27402 Evaluation Board SNVA406B–May 2010–Revised May 2013
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