TPS54231
www.ti.com .............................................................................................................................................................................................. SLUS851–OCTOBER 2008
CURRENT MODE COMPENSATION DESIGN
To simplify design efforts using the TPS54231, the typical designs for common applications are listed in Table 1.
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
(http://focus.ti.com/docs/toolsw/folders/print/switcherpro.html).
Table 1. Typical Designs (Referring to Simplified Schematic on page 1)
VIN
(V)
VOUT
(V)
Fsw
(kHz)
Lo
(µH)
Co
RO1
(kΩ)
RO2
(kΩ)
C2
(pF)
C1
(pF)
R3
(kΩ)
12
12
12
12
12
12
12
12
5
570
570
570
570
570
570
570
570
15
10
Ceramic 33 µF
Ceramic 47µF
10
10
10
10
10
10
10
10
1.91
3.24
8.06
80.6
1.91
3.24
8.06
80.6
47
47
47
47
47
47
47
47
1800
4700
4700
4700
220
21
21
3.3
1.8
0.9
5
6.8
4.7
15
Ceramic 100 µF
21
Ceramic 100 µFx2
Aluminum 330 µF/160 mΩ
Aluminum 470 µF/160 mΩ
SP 100 µF/15 mΩ
SP 220 µF/12 mΩ
21
40.2
21
3.3
1.8
0.9
10
220
6.8
4.7
4700
4700
40.2
40.2
OVERCURRENT PROTECTION AND FREQUENCY SHIFT
The TPS54231 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 TPS54231 provides robust protection during short circuits. There is potential for overcurrent runaway in the
output inductor during a short circuit at the output. The TPS54231 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.
Table 2. Switching Frequency Conditions
SWITCHING FREQUENCY
570 kHz
VSENSE PIN VOLTAGE
VSENSE ≥ 0.6 V
570 kHz / 2
0.6 V > VSENSE ≥ 0.4 V
0.4 V > VSENSE ≥ 0.2 V
0.2 V > VSENSE
570 kHz / 4
570 kHz / 8
OVERVOLTAGE TRANSIENT PROTECTION
The TPS54231 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 175°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 175°C, the device reinitiates the power up sequence.
Copyright © 2008, Texas Instruments Incorporated
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