Datasheet

0
100 200
150
25050
I
FB
- Feedback Current - μA
300
133
30
33
I
FB,CNR1
(165mA)
I
FB,CNR2
(210mA)
I
FB,CNR3
(280mA)
I
GM,HYST
(25mA)
I
FB,CNR2
- I
FB,CNR1
(45mA)
I
FB,CNR3
- I
FB,CNR2
(70mA)
FM
AM GM
100
I
DRV,PK
- Percent of Maximum Peak DRV Current - %
f
S
- Switching Frequency -
kHz
Peak
Current
Control
Current Modulator
I
FB
T
SW
1/ t
S
Freq. Modulator
I
FB
I
FB
V
GATE
3
R
CL
UCC28610
SLUS888F –JANUARY 2009–REVISED SEPTEMBER 2012 www.ti.com
Modulation Modes
Under normal operating conditions, the FB current commands the operating mode of the UCC28610, as shown
in Figure 25 and Figure 26. The FB current commands the UCC28610 to operate the converter in one of three
modes: Frequency Modulation (FM) mode, Amplitude Modulation (AM) mode, and Green Mode (GM).
The converter operates in FM mode with a large power load (22% to 100% the peak regulated power). The peak
HVMOSFET current reaches its maximum programmed value and FB current regulates the output voltage by
varying the switching frequency, which is inversely proportional to t
S
. The switching frequency is modulated from
30 kHz (22% peak power) to 133 kHz (100% peak power), the on time is constant, and the I
DRV
peak current is
constant. The maximum programmable HVMOSFET current, I
DRV,PK(max)
, is set by a resistor on the CL pin, as
described in Equation 10.
The converter operates in AM mode at moderate power levels (2.5% to 22% of the peak regulated power). The
FB current regulates the output voltage by modulating the amplitude of the peak HVMOSFET current from 33%
to 100% of the maximum programmed value while the switching frequency is fixed at approximately 30 kHz. The
UCC28610 modulates the voltage on the CL pin from 3 V to 1 V to vary the commanded peak current, as shown
in Figure 25 and Figure 26.
The converter operates in GM at light load (0% to 2.5% of the peak regulated power). The FB current regulates
the output voltage in the Green Mode with hysteretic bursts of pulses using FB current thresholds. The peak
HVMOSFET current is 33% of the maximum programmed value. The switching frequency within a burst of pulses
is approximately 30 kHz. The duration between bursts is regulated by the power supply control dynamics and the
FB hysteresis. The UCC28610 reduces internal bias power between bursts in order to conserve energy during
light-load and no-load conditions.
Figure 25. Modulation Control Blocks Figure 26. Control Diagram with Operating Modes
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