Datasheet

LTC3809
11
3809fc
OPERATION
(Refer to Functional Diagram)
Peak Current Sense Voltage Selection and Slope
Compensation (IPRG Pin)
When the LTC3809 controller is operating below 20%
duty cycle, the peak current sense voltage (between the
V
IN
and SW pins) allowed across the external P-channel
MOSFET is determined by:
Δ=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 P-
channel MOSFET is 125mV, 85mV or 204mV 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 external P-channel
MOSFET:
I
V
R
PK
SENSE MAX
DS ON
=
Δ
()
()
DUTY CYCLE (%)
10
SF = I/I
MAX
(%)
60
80
110
100
90
3809 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