LTC1625
U
W U U
APPLICATIONS INFORMATION
High input voltage applications in which large MOSFETs
arebeingdrivenathighfrequenciesmaycausetheLTC1625
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:
3. EXTVCC connectedtoanoutput-derivedboostnetwork.
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 isavailable in the 5V to 7Vrange (EXTVCC <VIN),
it may be used to power EXTVCC providing it is compat-
ible with the MOSFET gate drive requirements.
TJ = 70°C + (14mA)(30V)(130°C/W) = 125°C
Topreventthemaximumjunctiontemperaturefrombeing
exceeded, the input supply current must be checked when
operating in continuous mode at high VIN.
V
C
IN
+
EXTVCC Connection
V
SEC
IN
V
IN
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,shortcircuit)powerissuppliedfromtheinternal
regulator. Do not apply greater than 7V to the EXTVCC pin
and ensure that EXTVCC ≤ VIN.
TK
1N4148
TG
•
OPTIONAL
+
LTC1625
EXTV
C
SEC
EXTV
CC
SW
1µF
CC
CONNECTION
V
OUT
R4
R3
5V < V
< 7V
•
SEC
T1
1:N
+
C
FCB
OUT
BG
SGND
PGND
1625 F05a
Figure 5a: Secondary Output Loop and EXTVCC Connection
V
≈ 2(V
– V )
OUT D
PUMP
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
DutyCycle/Efficiency.For5Vregulatorsthissimplymeans
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.
+
1µF
V
C
IN
+
BAT85
IN
V
IN
BAT85
L1
0.22µF
TK
TG
BAT85
LTC1625
VN2222LL
SW
EXTV
CC
V
OUT
+
The following list summarizes the four possible connec-
tions for EXTVCC:
C
OUT
BG
PGND
1. EXTVCC left open (or grounded). This will cause INTVCC
tobepoweredfromtheinternal5.2Vregulatorresulting
in an efficiency penalty of up to 10% at high input
voltages.
1625 F05b
Figure 5b: Capacitive Charge Pump for EXTVCC
2. EXTVCC connected directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
13
Linear [ Linear ]
