Datasheet

SS
SS
I t
C (Farads)
V
´ D
=
D
SW
9
SW
X
f (X 24 k MHz)
RT
10
f 24
RT
= = W ´
= ´
TPS43330-Q1
TPS43332-Q1
SLVSA82E –MARCH 2011–REVISED APRIL 2013 www.ti.com
DETAILED DESCRIPTION
BUCK CONTROLLERS: NORMAL MODE PWM OPERATION
Frequency Selection and External Synchronization
The buck controllers operate using constant-frequency peak-current-mode control for optimal transient behavior
and ease of component choices. The switching frequency is programmable between 150 kHz and 600 kHz,
depending upon the resistor value at the RT pin. A short circuit to ground at this pin sets the default switching
frequency to 400 kHz. Using a resistor at RT, one can set another frequency according to the formula:
For example,
600 kHz requires 40 kΩ
150 kHz requires 160 kΩ
It is also possible to synchronize to an external clock at the SYNC pin in the same frequency range of 150 kHz to
600 kHz. The device detects clock pulses at this pin, and an internal PLL locks on to the external clock within the
specified range. The device can also detect a loss of clock at this pin, and on detection of this condition, the
device sets the switching frequency to the internal oscillator. The two buck controllers operate at identical
switching frequencies, 180 degrees out-of-phase.
Enable Inputs
Independent enable inputs from the ENA and ENB pins enable the buck controllers. These are high-voltage pins,
with a threshold of 1.7 V for the high level, and with which direct connection to the battery is permissible for self-
bias. The low threshold is 0.7 V. Both these pins have internal pullup currents of 0.5 µA (typical). As a result, an
open circuit on these pins enables the respective buck controllers. When both buck controllers are disabled, the
device shuts down and consumes a current of less than 4 µA.
Feedback Inputs
The right resistor feedback divider network connected to the FBx (feedback) pins sets the output voltage. Choose
this network such that the regulated voltage at the FBx pin equals 0.8 V. The FBx pins have a 100-nA pullup
current source as a protection feature in case the pins open up as a result of physical damage.
Soft-Start Inputs
In order to avoid large inrush currents, each buck controller has an independent programmable soft-start timer.
The voltage at the SSx pin acts as the soft-start reference voltage. The 1-µA pullup current available at the SSx
pins, in combination with a suitably chosen capacitor, generates a ramp of the desired soft-start speed. After
start-up, the pullup current ensures that SSx is higher than the internal reference of 0.8 V; 0.8 V then becomes
the reference for the buck controllers. The following equation calculates the soft-start ramp time:
where,
I
SS
= 1 µA (typical)
∆V = 0.8 V
C
SS
is the required capacitor for ∆t, the desired soft-start time.
An alternative use of the soft-start pins is as tracking inputs. In this case, connect them to the supply to be
tracked via a suitable resistor-divider network.
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