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TPS51117PWRG4 Fiches technique(PDF) 8 Page - Texas Instruments

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No de pièce TPS51117PWRG4
Description  SINGLE SYNCHRONOUS STEP-DOWN CONTROLLER
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Fabricant  TI [Texas Instruments]
Site Internet  http://www.ti.com
Logo TI - Texas Instruments

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PWM FREQUENCY AND ADAPTIVE ON-TIME CONTROL
T
ON
19
10[12
R
TON
(2 3)V
OUT *
100 mV
V
IN
* 50 ns
(3)
0
100
200
300
400
500
600
100
R
- k
TON
W
400
500
600
700
200
300
V
= 15 V,
V
= 2.5 V,
PWM
IN
OUT
LOW-SIDE DRIVER
TPS51117
SLVS631 – DECEMBER 2005
DETAILED DESCRIPTION (continued)
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
the output voltage, so that the duty ratio is kept as VOUT/VIN technically with the same cycle time. Equation
Equation 3 shows a simplified calculation of the on time.
Here, RTON is the external resistor connected from TON pin to the LL node. In the equation, 19 pF represents the
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.
Figure 1 shows the relationship of RTON to the switching frequency.
Figure 1. Switching Frequency vs RTON
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.
The low-side driver is designed to drive high-current, low RDS(on) N-channel MOSFET(s). The drive capability is
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
average drive current is calculated by the FET gate charge at Vgs = 5 V times the switching frequency. The
instantaneous drive current is supplied by an input capacitor connected between V5DRV and GND.
8


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