Datasheet

LTC3862-1
21
38621f
OPERATION
Figure 13. Leading Edge Blanking Effects
on the Minimum On-Time
Figure 14. SW Node Waveforms with Different Duty Cycle Limits
In order to satisfy these different applications require-
ments, the LTC3862-1 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 LTC3862-1 contains an oscillator that runs at a multiple
of the 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.
SW NODE
20V/DIV
GATE
5V/DIV
500ns/DIV
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = SGND
V
IN
= 36V
V
OUT
= 72V
MEASURED ON-TIME = 210ns
V
IN
= 36V
V
OUT
= 72V
MEASURED ON-TIME = 290ns
V
IN
= 36V
V
OUT
= 72V
MEASURED ON-TIME = 375ns
INDUCTOR
CURRENT
200mA/DIV
SW NODE
20V/DIV
GATE
5V/DIV
INDUCTOR
CURRENT
200mA/DIV
SW NODE
20V/DIV
GATE
5V/DIV
INDUCTOR
CURRENT
200mA/DIV
500ns/DIV
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = FLOAT
500ns/DIV
38621 F13
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = 3V8
SW NODE
20V/DIV
SW NODE
20V/DIV
1µs/DIV
96% MAXIMUM DUTY CYCLE WITH D
MAX
= SGND
INDUCTOR
CURRENT
1A/DIV
1µs/DIV
84% MAXIMUM DUTY CYCLE WITH D
MAX
= FLOAT
INDUCTOR
CURRENT
1A/DIV
SW NODE
20V/DIV
INDUCTOR
CURRENT
1A/DIV
1µs/DIV
3862 F14
75% MAXIMUM DUTY CYCLE WITH D
MAX
= 3V8