LTC3703IGN Datasheet, PDF (22/34 Pages) Linear Integrated Systems – 100V Synchronous Switching Regulator

LTC3703
Applications Information
However, note that the power dissipation equations for
the MOSFETs at maximum output current in a boost
converter are:
P_MAIN
=
D_MAX


I_MAX
1– D_MAX


2
(1+
δ
)R_DS(ON)
+
( )( ) ¹
2
V_O2U^{}_T1–I^M_D^AMXAX


R_DR
C_MILLER •



V_CC
–
1
V_TH(IL)
+
1
V_TH(IL)
_(f)

( ) P_SYNC
=
–


1–
1
D_MAX


I_MAX
2 _{(1+ δ)RDS(ON)}
Boost Converter: Output Capacitor Selection
In boost mode, the output capacitor requirements are
more demanding due to the fact that the current waveform
is pulsed instead of continuous as in a buck converter.
The choice of component(s) is driven by the acceptable
ripple voltage which is affected by the ESR, ESL and bulk
capacitance as shown in Figure 15. The total output ripple
voltage is:
∆V_OUT
=
I_O(MAX
)


1
f •C_OUT
+
ESR
1– D_MAX


where the first term is due to the bulk capacitance and
second term due to the ESR.
V_OUT
(AC)
∆V_COUT
∆V_ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
Figure 15. Output Voltage Ripple Waveform for a Boost Converter
The choice of output capacitor is driven also by the RMS
ripple current requirement. The RMS ripple current is:
I_RMS(COUT) ≈ I_O(MAX) •
V_O – V_IN(MIN)
V_IN(MIN)
At lower output voltages (less than 30V), it may be pos-
sible to satisfy both the output ripple voltage and RMS
ripple current requirements with one or more capacitors
of a single capacitor type. However, at output voltages
above 30V where capacitors with both low ESR and high
bulk capacitance are hard to find, the best approach is to
use a combination of aluminum and ceramic capacitors
(see discussion in Input Capacitor section for the buck
converter). With this combination, the ripple voltage can
be improved significantly. The low ESR ceramic capaci-
tor will minimize the ESR step, while the electrolytic will
supply the required bulk capacitance.
Boost Converter: Input Capacitor Selection
The input capacitor of a boost converter is less critical
than the output capacitor, due to the fact that the inductor
is in series with the input and the input current waveform
is continuous. The input voltage source impedance deter-
mines the size of the input capacitor, which is typically in
the range of 10µF to 100µF. A low ESR capacitor is recom-
mended though not as critical as for the output capacitor.
The RMS input capacitor ripple current for a boost con-
verter is:
I_RMS(CIN)
=
0.3
•
V_IN(MIN)
L•f
•
D_MAX
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to
the input of the converter and solid tantalum capacitors
can fail catastrophically under these conditions. Be sure
to specify surge-tested capacitors!
Boost Converter: Current Limit Programming
The LTC3703 provides current limiting in boost mode by
monitoring the V_DS of the main switch during its on-time
and comparing it to the voltage at I_MAX. To set the cur-
rent limit, calculate the expected voltage drop across the
MOSFET at the maximum desired inductor current and
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
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