Datasheet

( )
12
OUT
ON
T on
IN
(2 / 3)V 100mV
T 19 10 R 50ns
V
-
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+
= ´ ´ +
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Not Recommended for New Designs
TPS51315
SLUS881B –MAY 2009–REVISED AUGUST 2012 www.ti.com
PWM Frequency and Adaptive On Time Control
TPS51315 employs an adaptive on-time control scheme and does not include a dedicated oscillator on the
device. However, the device runs with pseudo-constant frequency by feed-forwarding the input voltage and
output voltage to the on-time, one-shot timer. The on-time control is inversely proportional to the input voltage
and proportional to the output voltage, so that the duty ratio is maintained as V
OUT
/V
IN
with the same cycle time,
as shown in Equation 3.
where
• R
T(on)x
is the external resistor connected from T
ON
to LL (3)
For the TPS51315, the input voltage is monitored at the LL pin during the ON state. An advantage of switching-
node monitoring is that any loss in the high-side n-channel FET becomes part of the on-time calculation, thereby
making the frequency more stable with load.
Another consideration regarding frequency is jitter. Jitter may have many causes, but the constant on-time D-
CAP mode scheme has some amount of inherent jitter. Because the output voltage ripple height is in the range
of approximately 20 mV, on the order of a milli-volt of noise on the feedback signal can affect the frequency
between approximately 3% and 10%. This is normal operation and risks only a small amount of influence to the
power supply performance.
Soft-Start
The TPS51315 includes an independent, internal, 1.06-ms, voltage servo soft-start function with overcurrent limit.
When the EN_PSV pin goes high, an internal digita-to-analog converter (DAC) begins ramping up the reference
voltage to the error amplifier. Smooth control of the output voltage is maintained during start-up.
Powergood
The TPS51315 includes a powergood output function. The powergood function is activated after the soft-start
function has completed. If the output voltage comes within ±5% of the target value, internal comparators detect a
powergood state and the powergood signal becomes high after a 64-μs internal delay. If the output voltage rises
above or falls below 10% of the target value, the powergood signal goes low immediately.
Output Discharge Control
The TPS51315 discharges output when EN_PSV is low or when the channel is in either a UVP or OVP fault
condition. The TPS51315 discharges output using an internal 20-Ω MOSFET that is connected to VOUT_DS and
PGND. The current capability of the MOSFET is limited to discharge slowly. An external resistor can be
connected between VOUT_DS and VOUT to program discharge slow rate. In case the discharge function is not
wanted, leave VOUT_DS open.
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