Datasheet

( )
OUT IN DS(on) OUT
V V DCR r I= - + g
TPS61251
www.ti.com SLVSAF7 –SEPTEMBER 2010
charging the capacitor with a constant current set by resistor R
ILIM
. During constant current charging power
dissipation in the TPS61251 is increased resulting in a thermal rise or heating of the device. If the output
capacitor is very large charging time can be long and thermal rise high. To prevent overheating of the device
during the charge phase the current will be limited to a lower value when device temperature is high. Please refer
to THERMAL REGULATION.
POWER-SAVE MODE
The TPS61251 integrates a power save mode to improve efficiency at light load. In power save mode the
converter only operates when the output voltage trips below a set threshold voltage. It ramps up the output
voltage with several pulses and goes into power save mode once the output voltage exceeds the set threshold
voltage. During the power save operation when the output voltage is above the set threshold the converter turns
off some of the inner circuits to save energy.
The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM
mode.
SNOOZE MODE
During this enhanced power save mode, the converter will still maintain the output voltage with a tolerance of
±2%. The operating current in snooze mode, is however, drastically reduced to a typical value of 2 mA. This will
be achieved by turning off as much as possible of the inner regulation circuits. Load current in snooze mode is
limited to 2 mA. If the load current increases above 2 mA, the controller recognizes a further drop of the output
voltage and the device turns on again in order to charge the output capacitor to the programmed output voltage
again.
100% DUTY-CYCLE MODE
If V
IN
> V
OUT
the TPS61251 offers the lowest possible input-to-output voltage difference while still maintaining
current limit operation with the use of the 100% duty-cycle mode. In this mode, the PMOS switch is constantly
turned on. During this operation the output voltage follows the input voltage and will not fall below the
programmed value if the input voltage decreases below V
OUT
. The output voltage drop during 100% mode
depends on the load current and input voltage, and the resulting output voltage is calculated as:
(3)
with:
DCR is the DC resistance of the inductor
r
DS(on)
is the typical on-resistance of the PMOS switch
ENABLE
The device is enabled by setting EN pin to a voltage above 1 V. At first, the internal reference is activated and
the internal analog circuits are settled. Afterwards, the softstart is activated and the output voltage ramps up. The
output voltage reaches its nominal value as fast as the current limit settings and the load condition allows it.
The EN input can be used to control power sequencing in a system with several DC/DC converters. The EN pin
can be connected to the output of another converter, to drive the EN pin high and getting a sequencing of supply
rails. With EN = GND, the device enters shutdown mode.
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