Datasheet

ENA
START ENA
1
V
R2
V V
I
R1
=
-
+
START STOP
HYS
V V
R1
I
-
=
EN
VIN
+
-
TPS54140
R1
R2
Ihys
I1
0.9 Am
1.25V
2.9 Am
( )
æ ö
-
= ´
ç ÷
ç ÷
è ø
OUT
V 0.8 V
R1 R2
0.8 V
TPS54140
SLVS889B –OCTOBER 2008 –REVISED SEPTEMBER 2013 www.ti.com
DETAILED DESCRIPTION (continued)
Error Amplifier
The TPS54140 has a transconductance amplifier for the error amplifier. The error amplifier compares the
VSENSE voltage to the lower of the SS/TR pin voltage or the internal 0.8V voltage reference. The
transconductance (gm) of the error amplifier is 97μA/V during normal operation. During the slow start operation,
the transconductance is a fraction of the normal operating gm. When the voltage of the VSENSE pin is below
0.8V and the device is regulating using the SS/TR voltage, the gm is 26 μA/V.
The frequency compensation components (capacitor, series resistor and capacitor) are added to the COMP pin
to ground.
Voltage Reference
The voltage reference system produces a precise ±2% voltage reference over temperature by scaling the output
of a temperature stable bandgap circuit.
Adjusting the Output Voltage
The output voltage is set with a resistor divider from the output node to the VSENSE pin. It is recommended to
use 1% tolerance or better divider resistors. Start with a 10 kΩ for the R2 resistor and use the Equation 1 to
calculate R1. To improve efficiency at very light loads consider using larger value resistors. If the values are too
high the regulator will be more susceptible to noise and voltage errors from the VSENSE input current will be
noticeable
(1)
Enable and Adjusting Undervoltage Lockout
The TPS54140 is disabled when the VIN pin voltage falls below 2.5 V. If an application requires a higher
undervoltage lockout (UVLO), use the EN pin as shown in Figure 28 to adjust the input voltage UVLO by using
the two external resistors. Though it is not necessary to use the UVLO adjust registers, for operation it is highly
recommended to provide consistent power up behavior. The EN pin has an internal pull-up current source, I1, of
0.9μA that provides the default condition of the TPS54140 operating when the EN pin floats. Once the EN pin
voltage exceeds 1.25V, an additional 2.9μA of hysteresis, Ihys, is added. This additional current facilitates input
voltage hysteresis. Use Equation 2 to set the external hysteresis for the input voltage. Use Equation 3 to set the
input start voltage.
Figure 28. Adjustable Undervoltage Lockout (UVLO)
(2)
(3)
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