Datasheet

IN
PW M
RAMP
V
K
V
@
( )
g TOT
BOOST
BOOST
Q
C
ΔV
>
( )
( ) ( ) ( )
( )
ILIM max ILIM max ILIMOFST max
SCP max
DS(on) min
I R V
I
R
´ +
=
( )
( ) ( ) ( )
( )( )
ILIM min ILIM min ILIMOFST min
SCP min
DS on max
I R V
I
R
´ +
=
( )
( ) ( ) ( )
( )
( )
3
DS on (max) SCP min ILIMOFSET min
ILIM min
6
LIM(min)
R I V
(4.88 10 14) 20mV
R 12.6k
I
7.0 10
-
-
´ -
´ ´ +
= = = W
´
( )
( )
6
OUT OUT
SCP PK
3
START
300 10 1.8
C V
I I 11.05 11.32 A
t
2 10
-
-
´ ´
´
³ + = + =
´
TPS40195
SLUS720E –FEBRUARY 2007–REVISED JULY 2012 www.ti.com
Short Circuit Protection, R
ILIM
Short circuit protection is programmed using the R
ILIM
resistor. Selection of this resistor depends on the R
DS(on)
of
the switching MOSFET and the required short circuit current trip point, I
SCP
. The minimum I
SCP
must exceed the
sum of the output current, the peak ripple current, and the output capacitor charging current during start up.
Equation 30 gives this minimum.
(32)
The minimum short circuit current trip point for this design is set to 14 A. Equation 33 is then used to calculate
the minimum R
ILIM
value.
(33)
R
ILIM
is calculated to be 12.6 kΩ . The closest standard value of 12.7 kΩ is used. The minimum and maximum
short circuit current can be calculated using Equation 34 and Equation 35 .
(34)
(35)
The minimum I
SCP
is 14 A and the maximum is 46 A.
Voltage Decoupling Capacitors, C
BP
, and C
VDD
Two pins on the TPS40195 have DC voltages. It is recommended to add small decoupling capacitors to these
pins. Below are the recommended values.
• C
BP
= 4.7 μF
• C
VDD
= 0.1 μF
Boost Voltage, C
BOOST
and D
BOOST (
optional)
Selection of the boost capacitor is based on the total gate charge of the switching MOSFET and the allowable
ripple on the boost voltage, V
BOOST
. A ripple of 0.2 V is assumed for this design. Using these two parameter and
equation (26) the minimum value for C
BOOST
can be calculated.
(36)
The total gate charge of the switching MOSFET is 13.3 nC. A minimum C
BOOST
of 0.066-μF is required. A 0.1-μF
capacitor was chosen. This capacitor must be able to withstand the maximum input voltage plus the maximum
voltage on BP. This is 16 V plus 5.4 V which is 21.4 V. A 50-V capacitor is used.
To reduce losses in the TPS40195 and to increase the available gate voltage for the switching MOSFET an
external diode can be added between the BP pin and the BOOST pin of the device. A small signal schottky
should be used here, such as the BAT54.
Closing the Feedback Loop R
Z1
, R
P1
, R
PZ2
, R
SET1
, R
SET2
, C
Z2
, C
P2
AND C
PZ1
A graphical method is used to select the compensation components. This is a standard feedforward buck
converter. Its PWM gain is given by the following equation.
(37)
26 Copyright © 2007–2012, Texas Instruments Incorporated