ENABLE AND ADJUSTABLE INPUT UNDER-VOLTAGE LOCKOUT (VIN UVLO)

EN

1.25V

VIN

+

-

Ren1

Ren2

1

A

m

3

A

m

START

STOP

V

- V

Ren1 =

3

A

m

(1)

EN

START

EN

V

Ren2 =

V

- V

+ 1

A

Ren1

m

(2)

PROGRAMMABLE SLOW START USING SS PIN

(

)

( )

( )

(

)

SS

ref

SS

SS

C

nF

V

V

T

ms

=

I

A

´

m

(3)

ERROR AMPLIFIER

SLOPE COMPENSATION

TPS54231

The EN pin has an internal pull-up current source that provides the default condition of the TPS54231 operating

when the EN pin floats.

The TPS54231 is disabled when the VIN pin voltage falls below internal VIN UVLO threshold. It is recommended

to use an external VIN UVLO to add Hysteresis unless VIN is greater than (VOUT + 2V). To adjust the VIN

UVLO with Hysteresis, use the external circuitry connected to the EN pin as shown in Figure 12. Once the EN

pin voltage exceeds 1.25 V, an additional 3

µA of hysteresis is added. Use Equation 1 and Equation 2 to

Figure 12. Adjustable Input Under-Voltage Lockout

It is highly recommended to program the slow start time externally because no slow start time is implemented

internally. The TPS54231 effectively uses the lower voltage of the internal voltage reference or the SS pin

voltage as the power supply’s reference voltage fed into the error amplifier and will regulate the output

accordingly. A capacitor (Css) on the SS pin to ground implements a slow start time. The TPS54231 has an

internal pull-up current source of 2

µA that charges the external slow start capacitor. The equation for the slow

start time (10% to 90%) is shown in Equation 3 . The Vref is 0.8V and the Iss current is 2

µA.

The slow start time should be set between 1ms to 10ms to ensure good start-up behavior. The slow start

capacitor should be no more than 27nF.

If during normal operation, the input voltage drops below the VIN UVLO threshold, or the EN pin is pulled below

1.25 V, or a thermal shutdown event occurs, the TPS54231 stops switching.

The TPS54231 has a transconductance amplifier for the error amplifier. The error amplifier compares the

VSENSE voltage to the internal effective voltage reference presented at the input of the error amplifier. The

transconductance of the error amplifier is 92

µA/V during normal operation. Frequency compensation

components are connected between the COMP pin and ground.

In order to prevent the sub-harmonic oscillations when operating the device at duty cycles greater than 50%, the

TPS54231 adds a built-in slope compensation which is a compensating ramp to the switch current signal.

8

Submit Documentation Feedback

Copyright © 2008, Texas Instruments Incorporated

Product Folder Link(s): TPS54231