LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 20

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3862-2
one LTC3862-2 chip, connect all of the SS pins together
and use one external capacitor to program the soft-start
time. In this case, the current into the soft-start capaci-
tor will be I
pins connected together. Figure 9 illustrates the start-up
waveforms for a 2-phase LTC3862-2 application.
Pulse-Skipping Operation at Light Load
As the load current is decreased, the controller enters
discontinuous mode (DCM). The peak inductor current can
be reduced until the minimum on-time of the controller
is reached. Any further decrease in the load current will
cause pulse-skipping to occur, in order to maintain output
regulation, which is normal. The minimum on-time of the
controller in this mode is approximately 210ns (with the
blanking time set to its minimum value), the majority of
which is leading edge blanking. Figure 10 illustrates the
LTC3862-2 switching waveforms at the onset of pulse-
skipping.
Programmable Slope Compensation
For a current mode boost regulator operating in CCM,
slope compensation must be added for duty cycles above
50%, in order to avoid subharmonic oscillation. For the
LTC3862-2, this ramp compensation is internal and user
adjustable. Having an internally fixed ramp compensation
waveform normally places some constraints on the value
of the inductor and the operating frequency. For example,
with a fixed amount of internal slope compensation, using
operaTion
20
100V/DIV
2A/DIV
2A/DIV
5V/DIV
V
RUN
OUT
I
I
L1
L2
Figure 9. Typical Start-Up Waveforms for a
Boost Converter Using the LTC3862-2
V
V
R
IN
OUT
L
SS
= 100Ω
= 24V
= 72V
= n • 5μA, where n is the number of SS
1ms/DIV
38622 F09
an excessively large inductor would result in too much
effective slope compensation, and the converter could
become unstable. Likewise, if too small an inductor were
used, the internal ramp compensation could be inadequate
to prevent subharmonic oscillation.
The LTC3862-2 contains a pin that allows the user to
program the slope compensation gain in order to opti-
mize performance for a wider range of inductance. With
the SLOPE pin left floating, the normalized slope gain is
1.00. Connecting the SLOPE pin to ground reduces the
normalized gain to 0.625 and connecting this pin to the
3V8 supply increases the normalized slope gain to 1.66.
With the normalized slope compensation gain set to 1.00,
the design equations assume an inductor ripple current of
20% to 40%, as with previous designs. Depending upon
the application circuit, however, a normalized gain of 1.00
may not be optimum for the inductor chosen. If the ripple
current in the inductor is greater than 40%, the normalized
slope gain can be increased to 1.66 (an increase of 66%)
by connecting the SLOPE pin to the 3V8 supply. If the
inductor ripple current is less than 20%, the normalized
slope gain can be reduced to 0.625 (a decrease of 37.5%)
by connecting the SLOPE pin to SGND.
To check the effectiveness of the slope compensation, apply
a load step to the output and monitor the cycle-by-cycle
behavior of the inductor current during the leading and
trailing edges of the load current. Vary the input voltage
over its full range and check for signs of cycle-by-cycle
SW node instability or subharmonic oscillation. When the
500mA/DIV
500mA/DIV
50V/DIV
50V/DIV
SW1
SW2
Figure 10. Light Load Switching Waveforms for
the LTC3862-2 at the Onset of Pulse-Skipping
I
I
L1
L2
V
V
LIGHT LOAD (10mA)
IN
OUT
= 51V
= 72V
2µs/DIV
38622 F10
38622f