Datasheet

4.6
4.8
5
5.2
5.4
5.6
0 0.05 0.10 0.15 0.20
I – Output Current – A
OUT
V – Input Voltage – V
IN
V = 5 V
OUT
Start
Stop
V – Input Voltage – V
IN
I – Output Current – A
OUT
V = 3.3 V
OUT
OUT
V 0.8 V
R1 = R2
0.8 V
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TPS5401
SLVSAB0 –DECEMBER 2010 www.ti.com
DETAILED DESCRIPTION (continued)
voltage and output voltage is greater than 2.1 V, at which point the BOOT UVLO threshold is exceeded, and the
device starts switching again until the desired output voltage is reached. This operating condition persists until
the input voltage and/or the load current increases. It is recommended to adjust the VIN stop voltage greater
than the BOOT UVLO trigger condition at the minimum load of the application using the adjustable VIN UVLO
feature with resistors on the EN pin.
The start and stop voltages for typical 3.3-V and 5-V output applications are shown in Figure 26 and Figure 27.
The voltages are plotted versus load current. The start voltage is defined as the input voltage needed to regulate
the output within 3.5%. The stop voltage is defined as the input voltage at which the output drops by 5% or stops
switching.
During high-duty-cycle conditions, the inductor current ripple increases while the BOOT capacitor is being
recharged, resulting in an increase in ripple voltage on the output. This is due to the recharge time of the boot
capacitor being longer than the typical high-side off time, when switching occurs every cycle.
Figure 26. 3.3-V Start/Stop Voltage Figure 27. 5-V Start/Stop Voltage
Error Amplifier
The TPS5401 has a transconductance amplifier as the error amplifier. The error amplifier compares the VSENSE
voltage to the lower of the SS/TR pin voltage or the internal 0.8-V voltage reference. The transconductance (gm)
of the error amplifier is 97 mA/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 0.8 V and the device is
regulating using the SS/TR voltage, the gm is 26 mA/V.
The frequency compensation components (series resistor and capacitor) are connected between the COMP pin
and ground.
Voltage Reference
The voltage reference system produces a precise ±3.5% 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 10 kΩ for the R2 resistor and use Equation 1 to calculate
R1. To improve efficiency at light loads, consider using larger-value resistors. If the values are too high, the
regulator becomes more susceptible to noise, and voltage errors from the VSENSE input current become
noticeable.
(1)
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