Datasheet

VSENSE
COMP
VOUT
R8
R4
C4
C6
R9
Coea
Roea
gm
1300 mA/V
0.8 V
Power Stage
PH
R
ESR
C
O
R
L
b
a
c
12 A/V
TPS54521
www.ti.com SLVS981C –JUNE 2010–REVISED AUGUST 2013
During normal operation, the device implements current mode control which uses the COMP pin voltage to
control the turn off of the high-side MOSFET and the turn on of the low-side MOSFET, on a cycle by cycle basis.
Each cycle, the switch current and the current reference generated by the COMP pin voltage are compared.
When the peak switch current intersects the current reference, the high-side switch is turned off.
Low-side MOSFET overcurrent protection
While the low-side MOSFET is turned on, its conduction current is monitored by the internal circuitry. During
normal operation, the low-side MOSFET sources current to the load. At the end of every clock cycle, the low-side
MOSFET sourcing current is compared to the internally set low-side sourcing current limit. If the low-side
sourcing current is exceeded, the high-side MOSFET is not turned on and the low-side MOSFET stays on for the
next cycle. The high-side MOSFET is turned on again when the low-side current is below the low-side sourcing
current limit at the start of a cycle.
The low-side MOSFET may also sink current from the load. If the low-side sinking current limit is exceeded, the
low-side MOSFET is turned off immediately for the rest of that clock cycle. In this scenario both MOSFETs are
off until the start of the next cycle.
Furthermore, if an output overload condition (as measured by the COMP pin voltage) has lasted for more than
the hiccup wait time which is programmed for 512 switching cycles, the device will shut down itself and restart
after the hiccup time which is set for 16384 cycles. The hiccup mode helps to reduce the device power
dissipation under severe overcurrent conditions.
Thermal Shutdown
The internal thermal shutdown circuitry forces the device to stop switching if the junction temperature exceeds
150°C typically. The device reinitiates the power up sequence when the junction temperature drops below 145 ° C
typically.
Small Signal Model for Loop Response
Figure 30 shows an equivalent model for the device's control loop which can be modeled in a circuit simulation
program to check frequency response and transient responses. The error amplifier is a transconductance
amplifier with a gm of 1300μA/V. The error amplifier can be modeled using an ideal voltage controlled current
source. The resistor Roea (2.38 MΩ) and capacitor Coea (20.7 pF) model the open loop gain and frequency
response of the error amplifier. The 1-mV ac voltage source between the nodes a and b effectively breaks the
control loop for the frequency response measurements. Plotting a/c and c/b show the small signal responses of
the power stage and frequency compensation respectively. Plotting a/b shows the small signal response of the
overall loop. The dynamic loop response can be checked by replacing the R
L
with a current source with the
appropriate load step amplitude and step rate in a time domain analysis.
Figure 30. Small Signal Model for Loop Response
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