LTC3772B Datasheet, PDF (9/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

U
OPERATIO (Refer to the Functional Diagram)
However, once the controller’s duty cycle exceeds 20%,
slope compensation begins and effectively reduces the
peak sense voltage by an amount given by the curve in
Figure 1.
The peak inductor current is determined by the peak sense
voltage and the on-resistance of the external P-channel
MOSFET:
I_PEAK
=
∆V_SENSE(MAX)
R_DS(ON)
Soft-Start
The start-up of V_OUT is controlled by the LTC3772B inter-
nal soft-start. During soft-start, the error amplifier com-
pares the feedback signal V_FB to the internal soft-start
ramp (instead of the 0.8V reference), which rises linearly
from 0V to 0.8V in about 0.6ms. This allows the output
LTC3772B
voltage to rise smoothly from 0V to its final value, while
maintaining control of the inductor current. After the
soft-start is timed out, it is disabled until the part is put in
shutdown again or the input supply is cycled.
Light Load Current Operation
Under very light load current conditions, the I_TH/RUN pin
voltage will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will assure that the current
comparator remains tripped (even at zero load current)
and the regulator will start to skip cycles, as it must, in
order to maintain regulation. This behavior allows the
regulator to maintain constant frequency down to very
light loads, resulting in low output ripple as well as low
audio noise and reduced RF interference, while providing
high light load efficiency.
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3772B F01
Figure 1. Reduction in Sense Voltage Due to
Slope Compensation vs Duty Cycle
3772bfa
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