Datasheet

( )
( )
f
L ripple
IN
SW
IN ripple
I
1.02 A
C 7.0 F
4 V 4 60mV 600kHz
> = = m
´ ´ ´ ´
( )
( )
IN ripple
L ripple
V
60mV
ESR 30m
2 I 2 1.02 A
< = = W
´ ´
( )
(
)
OCP(min)
ISNS
Drive
L peak
V
110mV
R 14.2m
1.1 6.57 A 0.50 A
1.1 I I
< = = W
´ +
´ +
f
MAX SW
ISNS
OUT fd IN
VDD L
14 V 10 H 600kHz
R 133m
60 (V V V ) 60 (24 V 0.48 V 14 V)
´ ´
´ m ´
< = = W
´ + - ´ + -
f
MIN
IFLT
SW IFLT
0.1 D
0.1 0.428
C 71pF
R 600kHz 1k
´
´
= = =
´ ´ W
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
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 13.0 nC
2 V I 2 24 V 2 A 600kHz
´ ´
´ ´
< = =
´ ´ ´ ´ ´ ´
TPS40210-Q1, TPS40211-Q1
www.ti.com SLVS861D –AUGUST 2008–REVISED APRIL 2010
(47)
(48)
For this design, to meet a maximum input ripple of 60 mV, a minimum 7.0-mF input capacitor with ESR less than
30 mΩ is needed. A 10-mF X7R ceramic capacitor is selected.
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)
The current limit requires a resistor less than 14.2 mΩ, and stability requires a sense resistor less than 133 mΩ.
A 10-mΩ resistor is selected. Approximately 2-mΩ of routing resistance is added in compensation calculations.
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 51.
(51)
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.
(52)
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.
(53)
This leaves 740 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 54 and Equation 55.
(54)
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.
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