T

ON +

19

R

TON

(2 3)V

OUT )

100 mV

V

IN

) 50 ns

0

100

200

300

400

500

600

100

R

-k

TON

W

400

500

600

700

200

300

V

=15V,

V

=2.5V,

PWM

IN

OUT

TPS51117

DETAILED DESCRIPTION (continued)

PWM FREQUENCY AND ADAPTIVE ON-TIME CONTROL

The TPS51117 employs an adaptive on-time control scheme and does not have a dedicated oscillator on board.

However, the device emulates a constant frequency by feed-forwarding the input and output voltages into the

on-time one-shot timer. The ON time is controlled inverse proportional to the input voltage, and proportional to

shows a simplified calculation of the on time.

(3)

internal timing capacitor with some typical parasitic capacitance at the TON pin. Also, 50 nsec is the turn-off

delay time contributed by the internal circuit and that of the high-side MOSFET. Although this equation provides a

good approximation to start with, the accuracy depends on each design and selection of the high-side MOSFET.

The TPS51117 does not have a pin connected to VIN, but the input voltage information comes from the switch

node (LL node) during the ON state. An advantage of LL monitoring is that the loss in the high-side NFET is now

a part of the on-time calculation, thereby making the frequency more stable with load.

Another consideration about frequency is jitter. Jitter may be caused by many reasons, but the constant on-time

D-CAP mode scheme has some amount of inherent jitter. Since the output voltage ripple height is in the range of

a couple of tens of milli-volts. A milli-volt order of noise on the feedback signal can affect the frequency by a few

to ten percent. This is normal operation and has little harm to the power supply performance.

LOW-SIDE DRIVER

represented by its internal resistance, which is 5

Ω for V5DRV to DRVL and 1.5 Ω for DRVL to PGND. A dead

time to prevent shoot through is internally generated between high-side MOSFET off to low-side MOSFET on,

and low-side MOSFET off to high-side MOSFET on. A 5-V bias voltage is delivered from V5DRV supply. The

instantaneous drive current is supplied by an input capacitor connected between V5DRV and GND.

8

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