LTC1625 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1625' PDF : 24 Pages View PDF
LTC1625
APPLICATIONS INFORMATION
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the LTC1625
to exceed its maximum junction temperature rating. Most
of the supply current drives the MOSFET gates unless an
external EXTVCC source is used. The junction temperature
can be estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC1625CGN
is limited to less than 14mA from a 30V supply:
TJ = 70°C + (14mA)(30V)(130°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in continuous mode at high VIN.
EXTVCC Connection
The LTC1625 contains an internal P-channel MOSFET
switch connected between the EXTVCC and INTVCC pins.
Whenever the EXTVCC pin is above 4.7V the internal 5.2V
regulator shuts off, the switch closes and INTVCC power is
supplied via EXTVCC until EXTVCC drops below 4.5V. This
allows the MOSFET gate drive and control power to be
derived from the output or other external source during
normal operation. When the output is out of regulation
(start-up, short circuit) power is supplied from the internal
regulator. Do not apply greater than 7V to the EXTVCC pin
and ensure that EXTVCC ≤ VIN.
Significant efficiency gains can be realized by powering
INTVCC from the output, since the VIN current supplying
the driver and control currents will be scaled by a factor of
Duty Cycle/Efficiency. For 5V regulators this simply means
connecting the EXTVCC pin directly to VOUT. However, for
3.3V and other lower voltage regulators, additional cir-
cuitry is required to derive INTVCC power from the output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC left open (or grounded). This will cause INTVCC
to be powered from the internal 5.2V regulator resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTVCC connected directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage which has been boosted to
greater than 4.7V. This can be done with either an
inductive boost winding as shown in Figure 5a or a
capacitive charge pump as shown in Figure 5b.
4. EXTVCC connected to an external supply. If an external
supply is available in the 5V to 7V range (EXTVCC < VIN),
it may be used to power EXTVCC providing it is compat-
ible with the MOSFET gate drive requirements.
OPTIONAL
EXTVCC
CONNECTION
5V < VSEC < 7V
VIN
TK
TG
LTC1625
EXTVCC SW
R4
FCB
R3
SGND
BG
PGND
+ VIN
CIN
VSEC
1N4148
•
+ CSEC
1µF
T1 • +
VOUT
1:N
COUT
1625 F05a
Figure 5a: Secondary Output Loop and EXTVCC Connection
VPUMP ≈ 2(VOUT – VD)
VIN
TK
TG
LTC1625
SW
EXTVCC
BG
PGND
+
+ VIN
1µF
CIN
BAT85
0.22µF
BAT85
VN2222LL
BAT85
L1
+
VOUT
COUT
1625 F05b
Figure 5b: Capacitive Charge Pump for EXTVCC
13
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