Datasheet

LTC3862
28
3862fc
For more information www.linear.com/LTC3862
Figure 21. Maximum Sense Voltage Variation
with Duty Cycle and Slope Gain
Programming the Current Limit
The peak sense voltage threshold for the LTC3862 is 75mV
at low duty cycle and with a normalized slope gain of
1.00, and is measured from SENSE
+
to SENSE
–
. Figure 21
illustrates the change in the sense threshold with varying
duty cycle and slope gain.
The average power dissipated in the sense resistor can
easily be calculated as:
P
I
nD
R
RSENSE
OMAX
MAX
SEN()
()
.•
•–
•=
()






13
1
2
SSE MAX
D•
This equation assumes no temperature coefficient for
the sense resistor. If the resistor chosen has a significant
temperature coefficient, then substitute the worst-case
high resistance value into the equation.
The resistor temperature can be calculated using the
equation:
T
D
= T
A
+ P
R(SENSE)
• R
TH(JA)
Selecting the Output Diodes
To maximize efficiency, a fast switching diode with low
forward drop and low reverse leakage is required. The
output diode in a boost converter conducts current during
the switch off-time. The peak reverse voltage that the diode
must withstand is equal to the regulator output voltage.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to the
peak inductor current:
I
n
I
D
DPEAK
OMAX
MAX
()
()
••
–
=+






1
1
21
χ
Although the average diode current is equal to the output
current, in very high duty cycle applications (low V
IN
to
high V
OUT
) the peak diode current can be several times
higher than the average, as shown in Figure 22. In this
applicaTions inForMaTion
Figure 22. Diode Current Waveform for a
High Duty Cycle Application
For a boost converter where the current limit value is
chosen to be 30% higher than the maximum load current,
the peak current in the MOSFET and sense resistor is:
II
n
I
SW MAXRSENS E
OMAX
() ()
(
••
.•
==+






1
1
2
13
χ
))
–1D
MAX
The sense resistor value is then:
R
VnD
SENSE
SENSEMAX MAX
=
()
+






()
•• –
.•
1
13 1
2
χ
••
()
I
OMAX
Again, the factor n is the number of phases used, and χ
represents the percentage ripple current in the inductor.
The number 1.3 represents the factor by which the current
limit exceeds the maximum load current, I
O(MAX)
. For
example, if the current limit needs to exceed the maxi-
mum load current by 50%, then the 1.3 factor should be
replaced with 1.5.
SW NODE
10V/DIV
INDUCTOR
CURRENT
2A/DIV
DIODE
CURRENT
2A/DIV
1µs/DIV
3862 F22
V
IN
= 6V
V
OUT
= 24V
DUTY CYCLE (%)
50
MAXIMUM CURRENT SENSE THRESHOLD (mV)
60
70
80
55
65
75
20 40 60 80
3862 F21
100100 30 50 70 90
SLOPE = 0.625
SLOPE = 1.66
SLOPE = 1