CURRENT MODE COMPENSATION DESIGN

OVERCURRENT PROTECTION AND FREQUENCY SHIFT

OVERVOLTAGE TRANSIENT PROTECTION

THERMAL SHUTDOWN

TPS54331

To simplify design efforts using the TPS54331, 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

VOUT

(V)

(V)

(kHz)

(

µH)

(k

Ω)

(k

Ω)

(pF)

(pF)

(k

Ω)

12

5

570

6.8

Ceramic 33

µFx2

10

1.91

39

4700

49.9

12

3.3

570

6.8

Ceramic 47

µFx2

10

3.24

47

1000

29.4

12

1.8

570

4.7

Ceramic 100

µF

10

8.06

68

5600

29.4

12

0.9

570

3.3

Ceramic 100

µFx2

10

80.6

56

5600

29.4

12

5

570

6.8

Aluminum 330

µF/160 mΩ

10

1.91

68

120

29.4

12

3.3

570

6.8

Aluminum 470

µF/160 mΩ

10

3.24

82

220

10

12

1.8

570

4.7

SP 100

µF/15 mΩ

10

8.06

68

5600

29.4

12

0.9

570

3.3

SP 330

µF/12 mΩ

10

80.6

100

1200

49.9

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.

Table 2. Switching Frequency Conditions

SWITCHING FREQUENCY

VSENSE PIN VOLTAGE

570 kHz

VSENSE

≥ 0.6 V

570 kHz / 2

0.6 V > VSENSE

≥ 0.4 V

570 kHz / 4

0.4 V > VSENSE

≥ 0.2 V

570 kHz / 8

0.2 V > VSENSE

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.

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.

Copyright © 2008, Texas Instruments Incorporated

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