TPS54331 Datasheet, PDF (8/24 Pages) Texas Instruments – 3A, 28V INPUT, STEP DOWN SWIFT™ DC/DC CONVERTER WITH Eco-mode

TPS54331
SLVS839 – JULY 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 TPS54331 operating
when the EN pin floats.
The TPS54331 is disabled when the VIN pin voltage falls below internal VIN UVLO threshold. It is recommended
to use an external VIN UVLO to add hysterisis unless VIN is greater than (VOUT + 2V). To adjust the VIN UVLO
with hysterisis, use the external circuitry connected to the EN pin as shown in Figure 12. Once the EN pin
voltage exceeds 1.25V, an additional 3µA of hysteresis is added. Use Equation 1 and Equation 2 to calculate the
resistor values needed for the desired VIN UVLO threshold voltages. The V_START is the input start threshold
voltage, the V_STOP is the input stop threshold voltage and the V_EN is the enable threshold voltage of 1.25V. The
V_STOP should always be greater than 3.5V.
TPS54331
VIN
Ren1
1 mA
3 mA
EN
Ren2
+
1.25 V -
Figure 12. Adjustable Input Under-Voltage Lockout
Ren1 = ^{VSTART - VSTOP}
3 mA
(1)
Ren2 =
V_EN
V_START - V_{EN + 1 mA}
Ren1
(2)
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 TPS54331 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 TPS54331 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.
T_SS (ms)
=
C_SS (nF) ´ V_ref
I_SS (mA )
(V )
(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
1.25V, or a thermal shutdown event occurs, the TPS54331 stops switching.
ERROR AMPLIFIER
The TPS54331 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
TPS54331 adds a built-in slope compensation which is a compensating ramp to the switch current signal.
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