Datasheet

LTC3822-1
11
38221f
Undervoltage Lockout
To prevent operation of the MOSFETs below safe input
voltage levels, an undervoltage lockout is incorporated
in the LTC3822-1. When the input supply voltage (V
IN
)
drops below 2.25V, the external MOSFETs and all internal
circuits are turned off except for the undervoltage block,
which draws only a few microamperes.
Peak Current Sense Voltage Selection and Slope
Compensation (IPRG Pin)
When the LTC3822-1 controller is operating below 20%
duty cycle, the peak current sense voltage (between the
V
IN
and SENSE
–
/SW pins) allowed across the external top
MOSFET is determined by:
∆≈VA
VV
SENSE MAX
ITH
()
•
–.07
10
where A is a constant determined by the state of the IPRG
pin. Floating the IPRG pin selects A = 1; tying IPRG to
V
IN
selects A = 5/3; tying IPRG to GND selects A = 2/3.
The maximum value of V
ITH
is typically about 1.98V, so
the maximum sense voltage allowed across the external
main MOSFET is 125mV, 200mV or 82mV for the three
respective states of the IPRG pin.
However, once the controller’s duty cycle exceeds 20%,
slope compensation begins and effectively reduces the
peak sense voltage by a scale factor (SF) given by the
curve in Figure 1.
The peak inductor current is determined by the peak sense
voltage and the on-resistance of the main MOSFET:
I
V
R
PK
SENSE MAX
DS ON
=
∆
()
()
If a sense resistor is used, ΔV
SENSE(MAX)
is the peak cur-
rent sense voltage (between the V
IN
and SENSE
–
/SW pins)
across the sense resistor. The peak inductor is determined
by the peak sense voltage and the resistance of the sense
resistor:
I
V
R
PK
SENSE MAX
SENSE
=
∆
()
Power Good (PGOOD) Pin (GN Only)
A window comparator monitors the feedback voltage and
pulls the open-drain PGOOD output pin low when the
feedback voltage is not within ±10% of the 0.6V reference
voltage. PGOOD is low when the LTC3822-1 is shut down
or in undervoltage lockout.
OPERATION
(Refer to Functional Diagram)
DUTY CYCLE (%)
10
SF = I/I
MAX
(%)
60
80
110
100
90
38221 F01
40
20
50
70
90
30
10
0
30
50
70
200
40
60
80
100
Figure 1. Maximum Peak Current vs Duty Cycle