Datasheet

LTC3862
19
3862fc
For more information www.linear.com/LTC3862
operaTion
The SS pin has an internal open-drain NMOS pull-down
transistor that turns on when the RUN pin is pulled low,
when the voltage on the INTV
CC
pin is below its under-
voltage lockout threshold, or during an overtemperature
condition. In multi-phase applications
that use more than
one LTC3862 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 capacitor will be
I
SS
= n • 5μA, where n is the number of SS pins connected
together. Figure 9 illustrates the start-up waveforms for a
2-phase LTC3862 application.
Pulse Skip 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 180ns (with
the blanking time set to its minimum value), the majority
of which is leading edge blanking. Figure 10 illustrates
the LTC3862 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 sub-harmonic oscillation. For the
LTC3862, 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
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 sub-harmonic oscillation.
The LTC3862 contains a pin that allows the user to pro
-
gram the slope compensation gain in order to optimize
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
RUN
5V/DIV
I
L1
5A/DIV
I
L2
5A/DIV
V
OUT
50V/DIV
1ms/DIVV
IN
= 12V
V
OUT
= 48V
100Ω LOAD
3862 F09a
Figure 9. Typical Start-Up Waveforms for a
Boost Converter Using the LTC3862
Figure 10. Light Load Switching Waveforms for
the LTC3862 at the Onset of Pulse Skipping
SW1
10V/DIV
SW2
10V/DIV
I
L1
1A/DIV
I
L2
1A/DIV
1µs/DIV
3862 F10
V
IN
= 17V
V
OUT
= 24V
LIGHT LOAD (10mA)