LTC3703-5
23
37035fa
maximum junction temperature. The maximum inductor
current is a function of both duty cycle and maximum load
current, so the limit must be set for the maximum expected
duty cycle (minimum V
IN
) in order to ensure that the
current limit does not kick in at loads < I
O(MAX)
:
V
I
D
R
V
V
I
R
PROG
O MAX
MAX
DS ON
OUT
IN MIN
O MAX
DS ON
=
+
=
⎛
⎝⎜
⎞
⎠⎟
+
(
)
(
)
(
)
(
)
(
)
–
(
)
•
(
)
1
1
1
δ
δ
Once V
PROG
is determined, R
IMAX
is chosen as follows:
R
IMAX
= V
PROG
/12
µ
A
Note that in a boost mode architecture, it is only possible
to provide protection for “soft” shorts where V
OUT
> V
IN
.
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 converter.
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:
GAIN (COMP-to-V
OUT
DC gain) = 20Log(V
OUT
2
/V
IN
)
Dominant Pole: f
P
=
V
V
LC
IN
OUT
•
1
2
π
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 11), is:
G = 10
–GAIN/20
C1 = 1/(2
π
• f • G • R1)
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 LTC3703-5.
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 LTC3703-5 into a low
quiescent current shutdown (I
Q
≅
25
µ
A). This pin can be
driven directly from logic as shown in Figure 17. Releasing
APPLICATIO S I FOR ATIO
W
U
U
U
Figure 16. Transfer Function of Boost Modulator
GAIN
(dB)
PHASE
(DEG)
37035 F16
A
V
0
0
–90
–180
PHASE
GAIN
–12dB/OCT
Figure 17. LTC3703-5 Startup Operation
2ms/DIV
V
OUT
5V/DIV
V
IN
= 50V
I
LOAD
= 2A
C
SS
= 0.01
µ
F
RUN/SS
2V/DIV
I
L
2A/DIV
37035 F17