
VSENSE
COMP
VOUT
R1
R3
C1
C2
R2
Coea
Roea
gm
1300 mA/V
0.6 V
Power Stage
PH
RESR
CO
RL
b
a
c
16 A/V
SLVSA70
– MARCH 2011
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 of 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
175
°C typically. Once the junction temperature drops below 165°C typically, the internal thermal hiccup timer will
start to count. The device reinitiates the power up sequence after the built-in thermal shutdown hiccup time
(16384 cycles) is over.
Small Signal Model for Loop Response
Figure 25 shows an equivalent model for the device 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 1300
μA/V. The error amplifier can be modeled using an ideal voltage controlled current
source. The resistor Roea (2.38 M
) 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 RL with a current source with the
appropriate load step amplitude and step rate in a time domain analysis.
Figure 25. Small Signal Model for Loop Response
Simple Small Signal Model for Peak Current Mode Control
Figure 26 is a simple small signal model that can be used to understand how to design the frequency
compensation. The device power stage can be approximated to 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 10 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 pin voltage (node c in
Figure 25) is the power stage
transconductance (gmps) which is 16 A/V for the device. The DC gain of the power stage is the product of gmps
and the load resistance
L) as shown in Equation 11 with resistive loads. As the load current increases, the DC gain decreases. This variation with load may seem problematic at first glance, but fortunately the dominant pole
moves with load current (see
Equation 12). The combined effect is highlighted by the dashed line in
Figure 27.As the load current decreases, the gain increases and the pole frequency lowers, keeping the 0-dB crossover
frequency the same for the varying load conditions which makes it easier to design the frequency compensation.
Copyright
2011, Texas Instruments Incorporated
17