Datasheet

IN
ON
OUT
V
t
V
æ ö
µ
ç ÷
è ø
VFB
VREF
PWM
t
ON
t
OFF
UDG-10208
VFB
Compensation
Ramp
PWM
t
ON
t
OFF
VREF
UDG-10209
TPS53315
SLUSAE6 –DECEMBER 2010 www.ti.com
Adaptive On-Time D-CAP™ Control
The TPS53315 does not have a dedicated oscillator to determine switching frequency. However, the device
operates with pseudo-constant frequency by feed-forwarding the input and output voltages into the on-time
one-shot timer. The adaptive on-time control adjusts the on-time to be inversely proportional to the input voltage
and proportional to the output voltage .
This makes the switching frequency fairly constant in steady state conditions over a wide input voltage range.
The switching frequency is selectable from eight preset values by a resistor connected between the RF pin and
GND or between the RF pin and the VREG pin as shown in Table 2. (Leaving the resistance open sets the
switching frequency to 500 kHz.)
Table 2. Resistor and Switching Frequency
SWITCHING FREQUENCY
RESISTOR (R
RF
) CONNECTIONS
(kHz)
0 Ω to GND 250
187 kΩ to GND 300
619 kΩ to GND 400
Open 500
866 kΩ to VREG 600
309 kΩ to VREG 750
124 kΩ to VREG 850
0 Ω to VREG 970
The off-time is modulated by a PWM comparator. The VFB node voltage (the mid-point of resistor divider) is
compared to the internal 0.6-V reference voltage added with a ramp signal. When the signal values match, the
PWM comparator asserts a set signal to terminate the off-time (turn off the low-side MOSFET and turn on
high-side MOSFET). The set signal is valid if the inductor current level is below the OCP threshold, otherwise the
off-time is extended until the current level falls below the threshold.
Figure 35 and Figure 36 show two on-time control schemes.
Figure 35. On-Time Control Without Ramp Figure 36. On-Time Control With Ramp
Compensation Compensation
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