Datasheet

R
en1
=
V
IN
- 1.17V
1.17V - I
EN
x R
en2
R
en2
I
LIMIT
R
DCR
=
I
cs-
R
SET
=
27.2A x 2.34 m:
10 PA
=
6.36 k:
L
out
=
C
s
R
DCR
R
s
=
1.32 k:
0.68 PH
220 nF x 2.34 m:
=
L
=
R
DCR
R
S
C
S
R
f
=
100
-
5
=
100
= 45 k:
-
5
300
-
100
1
f
SW
-
100
1
Evaluation Board Component Selection www.ti.com
9.7 Frequency Adjust Resistor, R
f
The LM27402 switching frequency can be adjusted from 200 kHz to 1.2 MHz using an external resistor
labeled on the evaluation board as R
f
.The frequency of the LM27402 evaluation board was selected to be
300 kHz. A 300 kHz switching frequency enables the LM27402 to deliver high currents by reducing the
MOSFET related losses while maintaining the ability to achieve satisfactory transient response. To find the
value of resistance needed for a given frequency use the following equation: (f
SW
(kHz), R
f
(kΩ))
(10)
A value of 45.7 kΩ was chosen for the R
f
resistor on the LM27402 evaluation board.
9.8 Current Limit Circuitry, R
s
, C
s
, R
s1
, R
s5
, R
tc
The current limit circuitry included on the LM27402 evaluation board sets the current limit at 24A.
Components R
s
and C
s
connect directly under the inductor pads and create an RC filter. The time constant
of R
s
C
s
should match the time constant of the inductance and DCR of the inductor:
(11)
A typical range of capacitance used in the R
s
C
s
network is 100 nF to 1 µF. A 220 nF capacitor was chosen
for the C
s
filter capacitor resulting in an R
s
resistor of:
(12)
A standard value resistor of 1.3 kΩ was selected for R
s
. The current limit level is set through a resistor
from CS- to the VOUT pad of the inductor. The LM27402 evaluation board is set to current limit at 24A
I
OUT
. The maximum inductor current is I
OUT
+ ΔI
L
/2 = 24 + 6.4/2 = 27.2A. The next equation describes the
current limit resistor calculation:
(13)
Copper resistance changes by about 3900 ppm/°C and can cause a significant error in the current limit
setpoint. The LM27402 evaluation board is equipped with a 5.6 kΩ positive temperature coefficient resistor
(R
tc
) to compensate the effects of copper resistance and a 750Ω resistor R
s3
in series with R
tc
to
approximately provide the 6.36 kΩ needed for R
SET
. R
tc
was chosen to be a Vishay TFPT1206L5601F 5.6
kΩ resistor which has a temperature coefficient of 4110 ppm/°C. An optional 6.34 kΩ resistor (R
s1
) was
placed between CS+ and the R
s
C
s
filter to mirror the impedance of the CS- pin in addition to a 100 pF
capacitor placed between CS+ and CS- near the IC to reduce the effects of noise.
The internal 10 µA current source is powered from VIN. If the voltage between VIN and CS- is below 1V,
the current source will supply less than 10 µA. If this happens, the common mode voltage of the current
sense comparator inputs (CS+ and CS-) can be decreased to ensure 10 µA of current. Extra resistor pads
(R
s1
, R
s2
, R
s4
) are included in the LM27402 evaluation board to lower the common mode voltage. Please
refer to AN-2060 LM27402 Current Limit Application Circuits (SNVA441) for design guidelines to adjust
the common mode voltage of the current sense comparator.
9.9 Enable Resistors, R
en1
, R
en2
The LM27402 evaluation board is equipped with an enable connection tied directly to EN. Resistor
footprints R
en1
and R
en2
provide an optional voltage divider network from VIN to GND to program the
LM27402 to enable at a certain input voltage. The following equation will guide the user in choosing
resistors values to create a resistor divider for EN:
(14)
8
AN-1992 LM27402 Evaluation Board SNVA406B–May 2010–Revised May 2013
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