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SLVS839B – JULY 2008 – REVISED OCTOBER 2008
CURRENT MODE COMPENSATION DESIGN
To simplify design efforts using the TPS54331, the typical designs for common applications are listed in
For designs using ceramic output capacitors, proper derating of ceramic output capacitance is recommended
when doing the stability analysis. This is because the actual ceramic capacitance drops considerably from the
nominal value when the applied voltage increases. Advanced users may refer to the
Step by Step Design
Procedure
in the Application Information section for the detailed guidelines or use SwitcherPro™ Software tool
Table 1. Typical Designs (Referring to Simplified Schematic on page 1)
VIN
(V)
12
12
12
12
12
12
12
12
VOUT
(V)
5
3.3
1.8
0.9
5
3.3
1.8
0.9
F
sw
(kHz)
570
570
570
570
570
570
570
570
L
o
(µH)
6.8
6.8
4.7
3.3
6.8
6.8
4.7
3.3
C
o
Ceramic 33
µFx2
Ceramic 47µFx2
Ceramic 100
µF
Ceramic 100
µFx2
Aluminum 330
µF/160
mΩ
Aluminum 470
µF/160
mΩ
SP 100
µF/15
mΩ
SP 330
µF/12
mΩ
R
O1
(kΩ)
10
10
10
10
10
10
10
10
R
O2
(kΩ)
1.91
3.24
8.06
80.6
1.91
3.24
8.06
80.6
C
2
(pF)
39
47
68
56
68
82
68
100
C
1
(pF)
4700
1000
5600
5600
120
220
5600
1200
R
3
(kΩ)
49.9
29.4
29.4
29.4
29.4
10
29.4
49.9
OVERCURRENT PROTECTION AND FREQUENCY SHIFT
The TPS54331 implements current mode control that uses the COMP pin voltage to turn off the high-side
MOSFET on a cycle by cycle basis. Every cycle the switch current and the COMP pin voltage are compared;
when the peak inductor current intersects the COMP pin voltage, the high-side switch is turned off. During
overcurrent conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high,
causing the switch current to increase. The COMP pin has a maximum clamp internally, which limit the output
current.
The TPS54331 provides robust protection during short circuits. There is potential for overcurrent runaway in the
output inductor during a short circuit at the output. The TPS54331 solves this issue by increasing the off time
during short circuit conditions by lowering the switching frequency. The switching frequency is divided by 8, 4, 2,
and 1 as the voltage ramps from 0V to 0.8V on VSENSE pin. The relationship between the switching frequency
and the VSENSE pin voltage is shown in
Table 2. Switching Frequency Conditions
SWITCHING FREQUENCY
570 kHz
570 kHz / 2
570 kHz / 4
570 kHz / 8
VSENSE PIN VOLTAGE
VSENSE
≥
0.6 V
0.6 V > VSENSE
≥
0.4 V
0.4 V > VSENSE
≥
0.2 V
0.2 V > VSENSE
OVERVOLTAGE TRANSIENT PROTECTION
The TPS54331 incorporates an overvoltage transient protection (OVTP) circuit to minimize output voltage
overshoot when recovering from output fault conditions or strong unload transients. The OVTP circuit includes an
overvoltage comparator to compare the VSENSE pin voltage and internal thresholds. When the VSENSE pin
voltage goes above 109% × Vref, the high-side MOSFET will be forced off. When the VSENSE pin voltage falls
below 107% × Vref, the high-side MOSFET will be enabled again.
THERMAL SHUTDOWN
The device implements an internal thermal shutdown to protect itself if the junction temperature exceeds 165°C.
The thermal shutdown forces the device to stop switching when the junction temperature exceeds the thermal
trip threshold. Once the die temperature decreases below 165°C, the device reinitiates the power up sequence.
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