Datasheet

R
KFF
+
ǒ
V
IN (min)
* 3.5
Ǔ
ǒ
65.27 R
T
) 1502
Ǔ
(W)
100
0
200
300
400
500
600
400 600 800 1000
700
200
800
FEED-FORWARD IMPEDANCE
vs
SWITCHING FREQUENCY
R
KFF
- Feed-Forward Impedance - kΩ
f
SW
- Switching Frequency - kHz
V
IN
= 25 V
V
IN
= 15 V
V
IN
= 9 V
R
T
- Timing Resistance - kΩ
f
SW
- Switching Frequency - kHz
TIMING RESISTANCE
vs
SWITCHING FREQUENCY
0
100
0
200
400 600 800 1000
200
300
400
500
600
R
KFF
+
ǒ
V
IN (min)
* 3.5
Ǔ
ǒ
65.27 R
T
) 1502
Ǔ
(W)
TPS40060
TPS40061
SLUS543F –DECEMBER 2002–REVISED JUNE 2013 www.ti.com
The PWM ramp must be faster than the master clock frequency or the PWM is prevented from starting. The
PWM ramp time is programmed via a single resistor (R
KFF
) pulled up to V
IN
. R
KFF
is related to R
T
, and the
minimum input voltage, V
IN(min)
through the following:
where:
• V
IN
is the desired start-up (UVLO) input voltage
• R
T
is the timing resistor in kΩ (2)
See the section on UVLO operation for further description.
The curve showing the feedforward impedance required for a given switching frequency, f
SW
, at various input
voltages is shown in Figure 3.
For low input voltage and high duty cycle applications, the voltage feed-forward may limit the duty cycle
prematurely. This does not occur for most applications. The voltage control loop controls the duty cycle and
regulates the output voltages. For more information on large duty cycle operation, refer to Application Note
(SLUA310).
Figure 2. Figure 3.
UVLO OPERATION
The TPS40060 and TPS40061 use both fixed and variable (user programmable) UVLO protection. The fixed
UVLO monitors the BP10 and BP5 bypass voltages. The UVLO circuit holds the soft-start low until the BP5 and
BP10 voltage rails have exceeded their thresholds and the input voltage has exceed the user programmable
undervoltage threshold.
The TPS40060 and TPS40061 use the feed-forward pin, KFF, as a user programmable low-line UVLO detection.
This variable low-line UVLO threshold compares the PWM ramp duration to the oscillator clock period. An
undervoltage condition exists if the device receives a clock pulse before the ramp has reached 90% of its full
amplitude. The ramp duration is a function of the ramp slope, which is directly related to the current into the KFF
pin. The KFF current is a function of the input voltage and the resistance from KFF to the input voltage. The KFF
resistor can be referenced to the oscillator frequency as described in Equation 3:
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