Datasheet

LTC3809-1
11
38091fc
Dropout Operation
When the input supply voltage (V
IN
) approaches the output
voltage, the rate of change of the inductor current while the
external P-channel MOSFET is on (ON cycle) decreases.
This reduction means that the P-channel MOSFET will
remain on for more than one oscillator cycle if the inductor
current has not ramped up to the threshold set by the
EAMP on the I
TH
pin. Further reduction in the input supply
voltage will eventually cause the P-channel MOSFET to be
turned on 100%; i.e., DC. The output voltage will then be
determined by the input voltage minus the voltage drop
across the P-channel MOSFET and the inductor.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below
safe input voltage levels, an undervoltage lockout is
incorporated in the LTC3809-1. When the input supply
voltage (V
IN
) drops below 2.25V, the external P- and
N-channel 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 LTC3809-1 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
()
()
OPERATION
(Refer to Functional Diagram)
Figure 1. Maximum Peak Current vs Duty Cycle
DUTY CYCLE (%)
10
SF = I/I
MAX
(%)
60
80
110
100
90
38091 F01
40
20
50
70
90
30
10
0
30
50
70
200
40
60
80
100