Datasheet
LTC3862
21
3862fc
For more information www.linear.com/LTC3862
operaTion
Figure 13. Leading Edge Blanking Effects
on the Minimum On-Time
Figure 14. SW Node Waveforms with Different Duty Cycle Limits
SW NODE
20V/DIV
GATE
2V/DIV
200ns/DIV
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = SGND
V
IN
= 30V
V
OUT
= 48V
MEASURED ON-TIME = 180ns
V
IN
= 30V
V
OUT
= 48V
MEASURED ON-TIME = 260ns
V
IN
= 30V
V
OUT
= 48V
MEASURED ON-TIME = 340ns
INDUCTOR
CURRENT
1A/DIV
SW NODE
20V/DIV
GATE
2V/DIV
INDUCTOR
CURRENT
1A/DIV
SW NODE
20V/DIV
GATE
2V/DIV
INDUCTOR
CURRENT
1A/DIV
200ns/DIV
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = FLOAT
200ns/DIV
3862 F13
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = 3V8
SW NODE
10V/DIV
SW NODE
10V/DIV
1µs/DIV
96% MAXIMUM DUTY CYCLE WITH D
MAX
= SGND
INDUCTOR
CURRENT
2A/DIV
1µs/DIV
84% MAXIMUM DUTY CYCLE WITH D
MAX
= FLOAT
INDUCTOR
CURRENT
2A/DIV
SW NODE
10V/DIV
INDUCTOR
CURRENT
2A/DIV
1µs/DIV
3862 F14
75% MAXIMUM DUTY CYCLE WITH D
MAX
= 3V8
undesirable, due to the need for V • sec reset during the
primary switch off-time.
In order to satisfy these different applications requirements,
the LTC3862 has a simple way to program the maximum
duty cycle. Connecting the D
MAX
pin to SGND limits the
maximum duty cycle to 96%. Floating this pin limits the
duty cycle to 84% and connecting the D
MAX
pin to the 3V8
supply limits it to 75%. Figure 14 illustrates the effect of
limiting the maximum duty cycle on the SW node waveform
of a boost converter.
The LTC3862 contains an oscillator that runs at 12x the
programmed switching frequency, in order to provide for
2-, 3-, 4-, 6- and 12-phase operation. A digital counter is
used to divide down the fundamental oscillator frequency in
order to obtain the operating frequency of the gate drivers.
Since the maximum duty cycle limit is obtained from
this digital counter, the percentage maximum duty cycle
does not vary with process tolerances or temperature.
The SENSE
+
and SENSE
–
Pins
The SENSE
+
and SENSE
–
pins are high impedance inputs
to the CMOS current comparators for each channel.
Nominally, there is no DC current into or out of these