LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 23

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Once V
Note that in a boost mode architecture, it is only possible
to provide protection for “soft” shorts where V
For hard shorts, the inductor current is limited only by the
input supply capability. Refer to Current Limit Program-
ming for buck mode for further considerations for current
limit programming.
Boost Converter: Feedback Loop/Compensation
Compensating a voltage mode boost converter is unfortu-
nately more difficult than for a buck converter. This is due
to an additional right-half plane (RHP) zero that is present
in the boost converter but not in a buck. The additional
phase lag resulting from the RHP zero is difficult if not
impossible to compensate even with a Type 3 loop, so the
best approach is usually to roll off the loop gain at a lower
frequency than what could be achievable in buck con-
verter.
A typical gain/phase plot of a voltage-mode boost con-
verter is shown in Figure 16. The modulator gain and
phase can be measured as described for a buck converter
or can be estimated as follows:
R
GAIN (COMP-to-V
Dominant Pole: f
IMAX
GAIN
PROG
(dB)
Figure 16. Transfer Function of Boost Modulator
A
= V
V
0
is determined, R
PROG
GAIN
PHASE
/12µA
U
P
OUT
=
V
–12dB/OCT
V
OUT
DC gain) = 20Log(V
U
IN
IMAX
•
2π
is chosen as follows:
1
W
LC
3703 F16
OUT
0
–90
–180
PHASE
(DEG)
OUT
U
2
/V
> V
IN
IN
)
.
Since significant phase shift begins at frequencies above
the dominant LC pole, choose a crossover frequency no
greater than about half this pole frequency. The gain of the
compensation network should equal –GAIN at this fre-
quency so that the overall loop gain is 0dB here. The
compensation component to achieve this, using a Type 1
amplifier (see Figure 12), is:
Run/Soft-Start Function
The RUN/SS pin is a multipurpose pin that provide a soft-
start function and a means to shut down the LTC3703.
Soft-start reduces the input supply’s surge current by
gradually increasing the duty cycle and can also be used
for power supply sequencing.
Pulling RUN/SS below 1V puts the LTC3703 into a low
quiescent current shutdown (I
driven directly from logic as shown in Figure 17. Releasing
the RUN/SS pin allows an internal 4µA current source to
charge up the soft-start capacitor C
on RUN/SS reaches 1V, the LTC3703 begins operating at
its minimum on-time. As the RUN/SS voltage increases
from 1V to 3V, the duty cycle is allowed to increase from
0% to 100%. The duty cycle control minimizes input
supply inrush current and elimates output voltage over-
shoot at start-up and ensures current limit protection even
with a hard short. The RUN/SS voltage is internally clamped
at 4V.
G = 10
C1 = 1/(2π • f • G • R1)
RUN/SS
2V/DIV
5V/DIV
2A/DIV
V
OUT
–GAIN/20
I
L
V
I
C
Figure 17. LTC3703 Startup Operation
LOAD
IN
SS
= 50V
= 0.01µF
= 2A
2ms/DIV
Q
≅ 50µA). This pin can be
SS
. When the voltage
LTC3703
3703 F17
23
3703fa