Datasheet

( )
(
)
ISNS(OC)MIN
ISNS
Drive
L peak
V
120mV
R 15.4m
1.1 (6.57 A 0.50 A)
1.1 I I
< = = W
´ +
´ +
IN(MAX) SW
ISNS
OUT FD IN
V L
14 V 10 H 600kHz
R 134m
60 (V V V ) 60 (24 V 0.48 V 14 V)
´ ´
´ m ´
< = = W
´ + - ´ + -
f
2
R LRMS ISNS
P (I ) R D= × ×
ISNS
MIN
IFLT
SW IFLT
0.1 D
0.1 0.429
C 71pF
R 600kHz 1k
´
´
= = =
´ ´ Wf
DISS(total) OUT OUT OUT
1 1 1
P P 1 V I 1 24 V 2 A 1 2.526 W
0.95
æ ö æ ö
æ ö
» ´ - = ´ ´ - = ´ ´ - =
ç ÷ ç ÷
ç ÷
h h
è ø
è ø è ø
( )
FET L D Risns IN(max) VDD(max)
DISS total
P P P P P V I< - - - - ´
f
FET DRIVE
GS
OUT OUT SW
3 P I
3 0.50 W 0.50 A
Q 13.0nC
2 V I 2 24 V 2 A 600kHz
´ ´
´ ´
< = =
´ ´ ´ ´ ´ ´
( )
( )
FET
DS on
2 2
RMS
P
0.50 W
R 9.9m
2 6.13 0.673
2 I D
< = = W
´ ´
´ ´
TPS40210, TPS40211
www.ti.com SLUS772E –MARCH 2008– REVISED OCTOBER 2011
Current Sense and Current Limit
The maximum allowable current sense resistor value is limited by both the current limit and sub-harmonic
stability. These two limitations are given by Equation 49 and Equation 50.
(49)
(50)
With 10% margin on the current limit trip point (the 1.1 factor) and assuming a maximum gate drive current of
500 mA, the current limit requires a resistor less than 15.4 mΩ and stability requires a sense resistor less than
134 mΩ. A 10-mΩ resistor is selected. Approximately 2-mΩ of routing resistance is added in compensation
calculations.
The power dissipation in R
ISNS
is calculated by Equation 51.
(51)
At maximum duty cycle, this is 0.253 W.
Current Sense Filter
To remove switching noise from the current sense, an R-C filter is placed between the current sense resistor and
the ISNS pin. A resistor with a value between 1 kΩ and 5 kΩ is selected and a capacitor value is calculated by
Equation 52.
(52)
For a 1-kΩ filter resistor, 71 pF is calculated and a 100-pF capacitor is selected.
Switching MOSFET Selection
The TPS40210 drives a ground referenced N-channel FET. The R
DS(on)
and gate charge are estimated based on
the desired efficiency target.
(53)
For a target of 95% efficiency with a 24 V Input voltage at 2 A, maximum power dissipation is limited to 2.526 W.
The main power dissipating devices are the MOSFET, inductor, diode, current sense resistor and the integrated
circuit, the TPS40210.
(54)
This leaves 812 mW of power dissipation for the MOSFET. This can likely cause an SO-8 MOSFET to get too
hot, so power dissipation is limited to 500 mW. Allowing half for conduction and half for switching losses, we can
determine a target R
DS(on)
and Q
GS
for the MOSFET by Equation 55 and Equation 56.
(55)
A target MOSFET gate-to-source charge of less than 13.0 nC is calculated to limit the switching losses to less
than 250 mW.
(56)
A target MOSFET R
DS(on)
of 9.9 mΩ is calculated to limit the conduction losses to less than 250 mW. Reviewing
30-V and 40-V MOSFETs, an Si4386DY 9-mΩ MOSFET is selected. A gate resistor was added per Equation 30.
The maximum gate charge at V
gs
=8 V for the Si4386DY is 33.2 nC, this implies R
G
= 3.3 Ω.
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