ltc3873 Linear Technology Corporation, ltc3873 Datasheet - Page 10

• Manufacturer Part Number: ltc3873
• Description: No Rsense Constant Frequency Current Mode Boost/flyback/sepic Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3873
plus the secondary-to-primary refl ected voltage of the
fl yback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating.
Leakage Inductance
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
output switch (Q1) turn-off. This is increasingly prominent
at higher load currents where more stored energy must
be dissipated. In some cases a “snubber” circuit will be
required to avoid overvoltage breakdown at the MOSFET’s
drain node. Application Note 19 is a good reference on
snubber design. A bifi lar or similar winding technique is a
good way to minimize troublesome leakage inductances.
However, remember that this will limit the primary-to-
secondary breakdown voltage, so bifi lar winding is not
always practical.
Power MOSFET Selection
The power MOSFET serves two purposes in the LTC3873:
it represents the main switching element in the power path
and its R
for the control loop. Important parameters for the power
MOSFET include the drain-to-source breakdown voltage
(BV
(R
and gate-to-drain charges (Q
the maximum drain current (I
thermal resistances (R
For boost applications with R
the LTC3872 data sheet for the selection of MOSFET
R
MOSFETs have conduction losses (I
losses. For V
improves with large MOSFETs with low R
for V
point that the use of a higher R
reverse transfer capacitance, C
higher effi ciency.
10
DS(ON)
DS(ON)
DSS
DS
), the threshold voltage (V
> 20V the transition losses rapidly increase to the
.
) versus gate-to-source voltage, the gate-to-source
DS(ON)
DS
represents the current sensing element
< 20V, high current effi ciency generally
TH(JC)
and R
GS
D(MAX)
DS(ON)
GS(TH)
DS(ON)
and Q
RSS
TH(JA)
) and the MOSFET’s
, actually provides
2
), the on-resistance
R) and switching
device with lower
GD
sensing, refer to
).
, respectively),
DS(ON)
, while
Output Capacitors
The output capacitor is normally chosen by its effective
series resistance (ESR), which determines output ripple
voltage and affects effi ciency. Low ESR ceramic capaci-
tors are often used to minimize the output ripple. Boost
regulators have large RMS ripple current in the output
capacitor that must be rated to handle the current. The
output ripple current (RMS) is:
Output ripple is then simply:
The output capacitor for fl yback converter should have a
ripple current rating greater than:
Input Capacitors
The input capacitor of a boost converter is less critical due
to the fact that the input current waveform is triangular, and
does not contain large square wave currents as found in
the output capacitor. The input voltage source impedance
determines the size of the capacitor that is typically 10μF to
100μF. A low ESR is recommended although not as critical
as the output capacitor can be on the order of 0.3Ω.
The RMS input ripple current for a boost converter is:
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.
V
I
I
RMS COUT
I
RMS CIN
OUT
RMS
(
(
= R
=
I
)
OUT
ESR
= 0 3
)
≈
(ΔI
•
I
. •
OUT MAX
L(RMS)
– 1
V
(
D
IN MIN
L f
MAX
D
(
•
MAX
)
)
•
)
•
D
V
OUT
MAX
V
IN MIN
–
(
V
IN MIN
(
)
)
3873f