LTC3772_15 Datasheet, PDF (9/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

U
OPERATIO (Refer to the Functional Diagram)
Overvoltage Protection
If V_FB exceeds its regulation point of 0.8V by more than
10% for any reason, such as an output short-circuit to a
higher voltage, the overvoltage comparator will hold the
external P-channel MOSFET off. This comparator has a
typical hysteresis of 40mV.
Peak Current Sense Voltage Selection and Slope
Compensation (I_PRG Pins)
When a controller is operating below 20% duty cycle, the
peak current sense voltage (between the SENSE⁺ and SW
pins) allowed across the external P-channel MOSFET is
determined by:
∆V_SENSE(MAX)
=
A(V_ITH – 0.7V)
10
– 0.015
where A is a constant determined by the state of the I_PRG
pins. Floating the I_PRG pin selects A = 1.58; tying I_PRG to
V_IN selects A = 2.2; tying I_PRG to SGND selects A = 0.97.
The maximum value of V_ITH is typically about 1.98V, so the
maximum sense voltage allowed across the external
P-channel MOSFET is 175mV, 100mV or 250mV for the
three respective states of the I_PRG pin.
LTC3772
However, once the controller’s duty cycle exceeds 20%,
slope compensation begins and effectively reduces the
peak sense voltage by a scale factor 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 LTC3772 internal
soft-start. During soft-start, the error amplifier EAMP
compares 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
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.
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3772 F01
Figure 1. Reduction in Sense Voltage Due to
Slope Compensation vs Duty Cycle
3772f
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