LTC3862 Datasheet, PDF (21/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
OPERATION
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = SGND
INDUCTOR
CURRENT
1A/DIV
GATE
2V/DIV
SW NODE
20V/DIV
V_IN = 30V
200ns/DIV
V_OUT = 48V
MEASURED ON-TIME = 180ns
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = FLOAT
INDUCTOR
CURRENT
1A/DIV
GATE
2V/DIV
SW NODE
20V/DIV
V_IN = 30V
200ns/DIV
V_OUT = 48V
MEASURED ON-TIME = 260ns
MINIMUM ON-TIME AT LIGHT LOAD WITH BLANK = 3V8
INDUCTOR
CURRENT
1A/DIV
GATE
2V/DIV
SW NODE
20V/DIV
V_IN = 30V
200ns/DIV
V_OUT = 48V
MEASURED ON-TIME = 340ns
3862 F13
Figure 13. Leading Edge Blanking Effects
on the Minimum On-Time
In order to satisfy these different applications require-
ments, 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.
96% MAXIMUM DUTY CYCLE WITH D_MAX = SGND
SW NODE
10V/DIV
INDUCTOR
CURRENT
2A/DIV
1μs/DIV
84% MAXIMUM DUTY CYCLE WITH D_MAX = FLOAT
SW NODE
10V/DIV
INDUCTOR
CURRENT
2A/DIV
1μs/DIV
75% MAXIMUM DUTY CYCLE WITH D_MAX = 3V8
SW NODE
10V/DIV
INDUCTOR
CURRENT
2A/DIV
1μs/DIV
3862 F14
Figure 14. SW Node Waveforms with Different Duty Cycle Limits
The LTC3862 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.
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