Datasheet

( )O U TIN IN
M IN
L O U T
V V V
L
I f V
´ -
»
D ´ ´
_
OUT
L MAX OUT
IN
V
I I
Vh
» ´
´
TPD12S015A
SLLSE74C –JUNE 2011–REVISED MARCH 2013 www.ti.com
Power-Save Mode
The TPD12S015A 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. The PFM mode is left and PWM mode entered in case the output current can not longer be supported in
PFM mode.
Under-Voltage Lockout
The under voltage lockout circuit prevents the DC/DC converter from malfunctioning at low input voltages and
from excessive discharge of the battery. It disables the output stage of the converter once the falling V
IN
trips the
under-voltage lockout threshold V
BATUV
. The under-voltage lockout threshold V
BATUV
for falling V
IN
is typically
2.0V. The device starts operation once the rising VIN trips under-voltage lockout threshold V
BATUV
again at typical
2.1 V.
Enable
The DC/DC converter is enabled when the CT_CP_HPD is set to high. At first, the internal reference is activated
and the internal analog circuits are settled. Afterwards, the soft start is activated and the output voltage is
ramped up. The output voltage reaches its nominal value in typically 250 μs after the device has been enabled.
The CT_CP_HPD input can be used to control power sequencing in a system with various DC/DC converters.
The CT_CP_HPD pin can be connected to the output of another converter, to drive the EN pin high and getting a
sequencing of supply rails. With CT_CP_HPD = GND, the dc/dc enters shutdown mode.
Soft Start
The DC/DC converter has an internal soft start circuit that controls the ramp up of the output voltage. The output
voltage reaches its nominal value within tStart of typically 250 μs after CT_CP_HPD pin has been pulled to high
level. The output voltage ramps up from 5% to its nominal value within t
Ramp
of 300 μs. This limits the inrush
current in the converter during start up and prevents possible input voltage drops when a battery or high
impedance power source is used. During soft start, the switch current limit is reduced to 300 mA until the output
voltage reaches V
IN
. Once the output voltage trips this threshold, the device operates with its nominal current
limit ILIMF.
Inductor Selection
To make sure that the TPD12S015A devices can operate, an inductor must be connected between pin V
BAT
and
pin L. A boost converter normally requires two main passive components for storing energy during the
conversion. A boost inductor and a storage capacitor at the output are required. To select the boost inductor, it is
recommended to keep the possible peak inductor current below the current limit threshold of the power switch in
the chosen configuration. The highest peak current through the inductor and the switch depends on the output
load, the input (V
BAT
), and the output voltage (5VOUT). Estimation of the maximum average inductor current can
be done using Equation 1.
(1)
For example, for an output current of 55 mA at 5VOUT, approx 150 mA of average current flows through the
inductor at a minimum input voltage of 2.3 V.
The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally, it is
advisable to work with a ripple of less than 20% of the average inductor current. A smaller ripple reduces the
magnetic hysteresis losses in the inductor, as well as output voltage ripple and EMI. But in the same way,
regulation time at load changes rises. In addition, a larger inductor increases the total system size and cost. With
these parameters, it is possible to calculate the value of the minimum inductance by using Equation 2.
(2)
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