TPS54231
SLUS851–OCTOBER 2008.............................................................................................................................................................................................. www.ti.com
ENABLE AND ADJUSTABLE INPUT UNDER-VOLTAGE LOCKOUT (VIN UVLO)
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
calculate the resistor values needed for the desired VIN UVLO threshold voltages. The VSTART is the input start
threshold voltage, the VSTOP is the input stop threshold voltage and the VEN is the enable threshold voltage of
1.25 V. The VSTOP should always be greater than 3.5 V.
VIN
Ren1
Ren2
1 mA
3 mA
+
-
EN
1.25 V
Figure 12. Adjustable Input Under-Voltage Lockout
VSTART - VSTOP
Ren1 =
3 mA
(1)
(2)
VEN
Ren2 =
VSTART - V
EN + 1 mA
Ren1
PROGRAMMABLE SLOW START USING SS PIN
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 2µA 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 2µA.
CSS nF ´ V
V
( ) ref ( )
TSS ms =
( )
ISS mA
( )
(3)
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.
ERROR AMPLIFIER
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.
SLOPE COMPENSATION
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.
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