ltc3862gn-1 Linear Technology Corporation, ltc3862gn-1 Datasheet - Page 19

• Manufacturer Part Number: ltc3862gn-1
• Description: Multi-phase Current Mode Step-up Dc/dc Controller
• Manufacturer: Linear Technology Corporation

LTC3862-1 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
together. Figure 9 illustrates the start-up waveforms for a
2-phase LTC3862-1 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 210ns (with the
blanking time set to its minimum value), the majority of
which is leading edge blanking. Figure 10 illustrates the
LTC3862-1 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
LTC3862-1, this ramp compensation is internal and user
adjustable. Having an internally fi xed ramp compensation
waveform normally places some constraints on the value
of the inductor and the operating frequency. For example,
with a fi xed amount of internal slope compensation, using
OPERATION
SS
= n • 5μA, where n is the number of SS pins connected
100V/DIV
5V/DIV
2A/DIV
2A/DIV
V
RUN
OUT
I
I
L1
L2
Figure 9. Typical Start-Up Waveforms for a
Boost Converter Using the LTC3862-1
V
V
R
IN
OUT
L
= 100Ω
= 24V
= 72V
1ms/DIV
38621 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 sub-harmonic oscillation.
The LTC3862-1 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 fl oating, 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 sub-harmonic oscillation. When
500mA/DIV
500mA/DIV
50V/DIV
50V/DIV
SW1
SW2
Figure 10. Light Load Switching Waveforms for
the LTC3862-1 at the Onset of Pulse Skipping
I
I
L1
L2
V
V
LIGHT LOAD (10mA)
IN
OUT
= 51V
= 72V
2μs/DIV
LTC3862-1
3862 F10
19
38621f