Datasheet

P1
C P
1
f
2 R Cp
=
´ ´
CO OUT OUT
C
PS ref EA
2 f C V
R = ×
gm V gm
p ´ ´
´
C
Pmod
1
C =
2 fp ´
P
Zmod
1
C =
2 fp ´
V = 20 V / div
IN
V = 2 V / div
OUT
EN = 2 V / div
Time = 5 msec / div
V = 50 mV / div (ac coupled)
OUT
I = 200 mA / div
OUT
Time = 200 µsec / div
TPS5401
SLVSAB0 –DECEMBER 2010 www.ti.com
And, the mid-frequency pole is given by Equation 32.
(32)
The first step is to choose the closed-loop crossover frequency f
CO
. In general, the closed-loop crossover
frequency could be less than 1/10 of the minimum operating frequency. But for the TPS5401, it is recommended
that the maximum closed-loop crossover frequency be not greater than 40 kHz. For this example, we choose
15-kHz crossover frequency. Next, by definition, the magnitude of the loop gain at the crossover frequency is 0
dB. By placing the compensation zero at the power stage pole, and the mid-compensation pole at the power
stage zero, the R
C
, C
C
and C
P
can be approximately calculated by Equation 33 through Equation 35.
(33)
(34)
(35)
Where:
• gm
PS
is the power-stage transconductance. It is 1.9 A/V
• gm
EA
is the error-amplifier transconductance. It is 97 mA/V
For this design example, R4, C7, and C8 make the compensation network. R4 = 698 kΩ, C7 = 3300 pF, C8 = 82
pF.
Discontinuous Mode and Eco-mode Control-Scheme Boundary
With an input voltage of 34 V, the power supply enters discontinuous mode when the output current is less than
50 mA. The power supply enters Eco-mode control scheme when the output current is lower than 30 mA.
Application Curves
Figure 42. Load Transient (100 mA to 350 mA) Figure 43. Start-Up With Input Voltage
30 Submit Documentation Feedback Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): TPS5401