LTC3703IGN Datasheet, PDF (23/34 Pages) Linear Integrated Systems – 100V Synchronous Switching Regulator

LTC3703
Applications Information
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.
Boost Converter: Feedback Loop/Compensation
Compensating a voltage mode boost converter is unfor-
tunately 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 ad-
ditional 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 converter
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
frequency so that the overall loop gain is 0dB here. The
GAIN
(dB)
A_V
GAIN
0
–12dB/OCT
PHASE
PHASE
(DEG)
0
–90
–180
3703 F16
Figure 16. Transfer Function of Boost Modulator
compensation component to achieve this, using a Type 1
amplifier (see Figure 12), is:
G = 10^{–GAIN/20}
C1 = 1/(2π • 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.
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 0.9V puts the LTC3703 into a low
quiescent current shutdown (I_Q ≅ 50µA). This pin can be
driven directly from logic as shown in Figure 17. Releasing
the RUN/SS pin allows an internal 4µA current source to
RUN/SS
2V/DIV
V_OUT
5V/DIV
I_L
2A/DIV
V_IN = 50V
I_LOAD = 2A
C_SS = 0.01µF
2ms/DIV
3703 F17
Figure 17. LTC3703 Start-Up Operation
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