Datasheet

( )
ON T COMP
T K V 125mV= -
ON(max) T
T K 4.825 V= ´
TS T
K 2 K ; active during single-phase operation= ´
TSET
T
R
s
K 4.0
133k volt
m
= ´
W
TSET
(MIN)
R
T 2.2 s ; Minimum Switching Period
133k
= ´ m
W
UCC28063
SLUSAO7 – SEPTEMBER 2011 www.ti.com
On-Time Control, Maximum Frequency Limiting, and Restart Timer
Gate-drive on-time varies proportionately with the error-amplifier output voltage by a factor called K
T
(in units of
μs/V), as shown in Equation 3.
(3)
Where:
• V
COMP
is the output voltage of the error amplifier and 125 mV is a modulator offset voltage.
The maximum output of the error amplifier is limited to 4.95 V. This value, minus the 125-mV modulator offset,
limits maximum on-time as determined by Equation 4.
(4)
This on-time limit sets the maximum power that can be delivered by the converter at a given input voltage.
At lower power, one boost channel (phase) may be turned off to achieve efficiency benefits (see Phase
Management section, below). To provide a smooth transition between two-phase and single-phase operation, K
T
increases by a factor of two in single-phase mode:
(5)
The maximum switching frequency of each phase is limited by minimum-period timers. If inductor current decays
to zero before the minimum-period timer elapses, the next turn-on will be delayed, resulting in discontinuous
phase current.
A restart timer ensures starting under all circumstances by restarting both phases if the ZCD input of either
phase has not transitioned from high-to-low within approximately 200 µs. To prevent the circuit from operating in
continuous conduction mode (CCM), the restart timer does not trigger turn-on until both phase-currents return to
zero.
The on-time factors (K
T
, K
TS
) and the minimum switching period, T
(MIN)
, are proportional to the time-setting
resistor R
TSET
(the resistor from the TSET pin to ground), and these factors can be calculated by Equation 5,
Equation 6 and Equation 7:
(6)
(7)
The proper value of R
TSET
will result in the clamped maximum on-time, T
ON(max)
, required by the converter
operating at the minimum input line voltage and maximum load.
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