Datasheet

SS
FB
V
V =
1.56
SS SS
1.25V
t = C x
7.5μA
TPS62090 , TPS62091 , TPS62092 , TPS62093
SLVSAW2B –MARCH 2012–REVISED APRIL 2014 www.ti.com
Device Functional Modes (continued)
In Power Save Mode the output voltage rises slightly above the nominal output voltage in PWM mode, as shown
in Figure 16 and Figure 17. This effect can be reduced by increasing the output capacitance or the inductor
value. This effect can also be reduced by programming the output voltage of the TPS62090 lower than the target
value. As an example, if the target output voltage is 3.3 V, then the TPS62090 can be programmed to 3.3V -
0.8%. As a result the output voltage accuracy is now -2.2% to +2.2% instead of -1.4% to 3%. The output voltage
accuracy in PFM operation is reflected in the electrical specification table and given for a 22 µF output
capacitance.
9.4.3 Low Dropout Operation (100% Duty Cycle)
The device offers low input to output voltage difference by entering 100% duty cycle mode. In this mode the high
side MOSFET switch is constantly turned on. This is particularly useful in battery powered applications to achieve
longest operation time by taking full advantage of the whole battery voltage range. The minimum input voltage
where the output voltage falls below its nominal regulation value is given by:
V
IN(min)
= V
OUT(max)
+ I
OUT
x ( R
DS(on)
+ R
L
) (2)
Where
R
DS(on)
= High side FET on-resistance
R
L
= DC resistance of the inductor
V
OUT(max)
= nominal output voltage plus maximum output voltage tolerance
9.4.4 Softstart (SS)
To minimize inrush current during start up, the device has an adjustable softstart depending on the capacitor
value connected to the SS pin. The device charges the softstart capacitor with a constant current of typically 7.5
µA. The feedback voltage follows this voltage with a fraction of 1.56 until the internal reference voltage of 0.8 V is
reached. The softstart operation is completed once the voltage at the softstart capacitor has reached typically
1.25 V. The soft-start time can be calculated using Equation 3. The larger the softstart capacitor the longer the
softstart time. The relation between softstart voltage and feedback voltage can be estimated using Equation 4.
(3)
(4)
This is also the case for the fixed output voltage option having the internal regulation voltage. During start-up the
switch current limit is reduced to 1/3 (~1.5 A) of its typical current limit of 4.6 A. Once the output voltage exceeds
typically 0.6 V the current limit is released to its nominal value. The device provides a reduced load current of
~1.5A when the output voltage is below typ 0.6V. Due to this, a small or no softstart time may trigger the short
circuit protection during start-up especially for larger output capacitors >22uF. This can be avoided by using
larger softstart capacitance extending the softstart time. See Short Circuit Protection (Hiccup-Mode) for details of
the reduced current limit during startup. Leaving the softstart pin floating sets the minimum start-up time.
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