Datasheet

FB
COMP
V
O
R1
R3
C1
C2
R2
CO RO
gm
ea
350 mA/V
0.8 V
Power Stage
gm 12 A/V
ps
SW
R
ESR
C
OUT
R
L
b
a
c
TPS54340
SLVSBK0B –OCTOBER 2012–REVISED MARCH 2014 www.ti.com
Feature Description (continued)
The OVP feature minimizes output overshoot when using a low value output capacitor by comparing the FB
terminal voltage to the rising OVP threshold which is nominally 109% of the internal voltage reference. If the FB
terminal voltage is greater than the rising OVP threshold, the high side MOSFET is immediately disabled to
minimize output overshoot. When the FB voltage drops below the falling OVP threshold which is nominally 106%
of the internal voltage reference, the high side MOSFET resumes normal operation.
8.3.13 Thermal Shutdown
The TPS54340 provides an internal thermal shutdown to protect the device when the junction temperature
exceeds 176°C. The high side MOSFET stops switching when the junction temperature exceeds the thermal trip
threshold. Once the die temperature falls below 164°C, the device reinitiates the power up sequence controlled
by the internal soft-start circuitry.
8.3.14 Small Signal Model for Loop Response
Figure 30 shows an equivalent model for the TPS54340 control loop which can be simulated to check the
frequency response and dynamic load response. The error amplifier is a transconductance amplifier with a gm
EA
of 3350 μA/V. The error amplifier can be modeled using an ideal voltage controlled current source. The resistor
R
o
and capacitor C
o
model the open loop gain and frequency response of the amplifier. The 1mV ac voltage
source between the nodes a and b effectively breaks the control loop for the frequency response measurements.
Plotting c/a provides the small signal response of the frequency compensation. Plotting a/b provides the small
signal response of the overall loop. The dynamic loop response can be evaluated by replacing R
L
with a current
source with the appropriate load step amplitude and step rate in a time domain analysis. This equivalent model is
only valid for continuous conduction mode (CCM) operation.
Figure 30. Small Signal Model for Loop Response
8.3.15 Simple Small Signal Model for Peak Current Mode Control
Figure 31 describes a simple small signal model that can be used to design the frequency compensation. The
TPS54340 power stage can be approximated by a voltage-controlled current source (duty cycle modulator)
supplying current to the output capacitor and load resistor. The control to output transfer function is shown in
Equation 9 and consists of a dc gain, one dominant pole, and one ESR zero. The quotient of the change in
switch current and the change in COMP terminal voltage (node c in Figure 30) is the power stage
transconductance, gm
PS
. The gm
PS
for the TPS54340 is 12 A/V. The low-frequency gain of the power stage is
the product of the transconductance and the load resistance as shown in Equation 10.
18 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated
Product Folder Links: TPS54340