VSENSE

COMP

VOUT

R8

R4

C4

C6

R9

Coea

Roea

gm

1300 mA/V

0.8 V

Power Stage

PH

CO

b

a

c

12 A/V

TPS54521

www.ti.com

SLVS981 – JUNE 2010

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 1300mA/V. The error amplifier can be modeled using an ideal voltage controlled current

source. The resistor Roea (2.38 M

Ω) 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

appropriate load step amplitude and step rate in a time domain analysis.

Figure 30. Small Signal Model for Loop Response

Simple Small Signal Model for Peak Current Mode Control

Figure 31 is a simple small signal model that can be used to understand how to design the frequency

compensation. The device's power stage can be approximated to a voltage controlled current source (duty cycle

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