TPS54240 Datasheet, PDF (13/44 Pages) Texas Instruments – 3.5V to 42V STEP DOWN SWIFT™ DC/DC CONVERTER WITH ECO-MODE™

TPS54240
www.ti.com
SLVSAA6 – APRIL 2010
DETAILED DESCRIPTION (continued)
Attention must be taken in maximum duty cycle applications which experience extended time periods with light
loads or no load. When the voltage across the BOOT capacitor falls below the 2.1V UVLO threshold, the high
side MOSFET is turned off, but there may not be enough inductor current to pull the PH pin down to recharge the
BOOT capacitor. The high side MOSFET of the regulator stops switching because the voltage across the BOOT
capacitor is less than 2.1V. The output capacitor then decays until the difference in the input voltage and output
voltage is greater than 2.1V, 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 3.3V and 5V output applications are shown in Figure 25 and Figure 26.
The voltages are plotted versus load current. The start voltage is defined as the input voltage needed to regulate
the output within 1%. 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.
4
5.6
V_O = 3.3 V
V_O = 5 V
3.8
5.4
3.6
Start
3.4
Stop
3.2
5.2
Start
5
Stop
4.8
3
0
0.05
0.10
0.15
0.20
I_O - Output Current - A
Figure 25. 3.3V Start/Stop Voltage
4.6
0
0.05
0.10
0.15
0.20
I_O - Output Current - A
Figure 26. 5.0V Start/Stop Voltage
Error Amplifier
The TPS54240 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 310mA/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 70mA/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 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.
Copyright © 2010, Texas Instruments Incorporated
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