LTC3703-5_15 Datasheet, PDF (23/32 Pages) Linear Technology – 60V Synchronous Switching Regulator Controller

LTC3703-5
APPLICATIO S I FOR ATIO
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_PROG
=
I_O(MAX)
1– D_MAX
R_DS(ON) (1 +
δ)
=
⎛
⎝⎜
V_OUT
V_IN(MIN)
⎞
⎠⎟
I_O(MAX)
•
R_DS(ON)
(1+
δ)
Once V_PROG is determined, R_IMAX is chosen as follows:
R_IMAX = V_PROG/12µA
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.
GAIN
(dB)
A_V
GAIN
0
–12dB/OCT
PHASE
PHASE
(DEG)
0
–90
–180
37035 F16
Figure 16. Transfer Function of Boost Modulator
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/(2π • f • G • R1)
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_OUT2/V_IN)
Dominant Pole: f_P =
V_IN
V_OUT
•1
2π LC
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-
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 ≅ 25µA). This pin can be
driven directly from logic as shown in Figure 17. Releasing
RUN/SS
2V/DIV
V_OUT
5V/DIV
I_L
2A/DIV
V_IN = 50V
I_LOAD = 2A
C_SS = 0.01µF
2ms/DIV
37035 F17
Figure 17. LTC3703-5 Startup Operation
37035fa
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