Datasheet

( )
( )
´
´ W
= = = W » W
-
-
FB
OUT FB
V R11
0.600 V 20.0k
R10 2.73k 2.74k
5.0 V 0.600 V
V V
( )
( )
( )
( )
+ ´
+ ´
W
> ´ = ´ =
W
+ ´
+ ´
1
1
OCP(min) RIPPLE DS on Q1
2
2
OC
1
1
DS on Q2
OCP(min) RIPPLE
2
2
R
I I
8 A 1.86 A
11 m
A 1.45
R 7.6 m
8 A 1.86 A
I I
( )
= = = W » W
m
OC
ILIM
OCSET min
V
107.6mV
R 12.0k 12.1k
I 9.0 A
( )
( )
(
)
( )
= ´ + ´ ´ ´ = ´ + ´ ´ ´ W =
1 1
OC OCP(min) RIPPLE
DS on G2
2 2
V 1.3 I I 1.25 R (1.3 8 A 1.86 A) 1.25 7.6m 107.6mV
= = = »
SS
SS
t
4ms
C 44nF 47nF
0.09 0.09
TPS40170
SLUS970A –MARCH 2011–REVISED NOVEMBER 2013 www.ti.com
SS Timing Capacitor (C15)
The soft-start capacitor provides smooth ramp of the error amplifier reference voltage for controlled start-up. The
soft-start capacitor is selected by using Equation 38.
(38)
ILIM Resistor (R9, C17)
The TPS40170 use the negative drop across the low-side FET at the end of the "OFF" time to measure the
inductor current. Allowing for 30% over the minimum current limit for transient recovery and 20% rise in R
DS(on)Q2
for self-heating of the MOSFET, the voltage drop across the low-side FET at current limit is given by
Equation 39.
(39)
The internal current limit temperature coefficient helps compensate for the MOSFET R
DS(on)
temperature
coefficient, so the current limit programming resistor is selected by Equation 40.
(40)
A 1000-pF capacitor is placed in parallel to improve noise immunity of the current limit set-point.
SCP Multiplier Selection (R5)
The TPS40170 controller uses a multiplier (A
OC
) to translate the low-side over-current protection into a high-side
R
DS(on)
pulse-by-pulse short circuit protection. Ensure that Equation 41 is true.
(41)
A
OC
= 3 is selected as the next greater A
OC
. The value of R5 is set to 10 kΩ.
Feedback Divider (R10, R11)
The TPS40170 controller uses a full operational amplifier with an internally fixed 0.6-V reference. The value of
R11 is selected between 10 kΩ and 50 kΩ for a balance of feedback current and noise immunity. With the value
of R11 set to 20 kΩ, the output voltage is programmed with a resistor divider given by Equation 42.
(42)
Compensation: (R4, R13, C13, C14, C21)
Using the TPS40k Loop Stability Tool for a 60 kHz bandwidth and a 50° phase margin with an R11 value of
20.0 kΩ, the following values are obtained. The tool is available from the TI website, Literature Number
SLUC263.
• C21 = C1 = 1500 pF
• C13 = C2 = 8200 pF
• C14 = C3 = 220 pF
• R13 = R2 = 511 Ω
• R4 = R3 = 3.83 kΩ
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