LTC1629 LINER [Linear Technology], LTC1629 Datasheet - Page 16

• Manufacturer Part Number: LTC1629
• Description: PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1629/LTC1629-PG
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1629 to be
exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTV
is less than 4.7V, all of the INTV
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (I
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC1629 V
current is limited to less than 24mA from a 24V supply:
Use of the EXTV
to:
The input supply current should be measured while the
controller is operating in continuous mode at maximum
V
vent the maximum junction temperature from being ex-
ceeded.
EXTV
The LTC1629 contains an internal P-channel MOSFET
switch connected between the EXTV
When the voltage applied to EXTV
internal regulator is turned off and the switch closes,
connecting the EXTV
supplying internal and MOSFET gate driving power. The
switch remains closed as long as the voltage applied to
EXTV
driver and control power to be derived from the output
during normal operation (4.7V < V
the internal regulator when the output is out of regulation
(start-up, short-circuit). Do not apply greater than 7V to
the EXTV
using the application circuits shown. If an external voltage
source is applied to the EXTV
16
IN
T
T
and the power dissipation calculated in order to pre-
J
J
CC
CC
CC
= 70 C + (24mA)(24V)(95 C/W) = 125 C
= 70 C + (24mA)(5V)(95 C/W) = 81.4 C
pin. When the voltage applied to the EXTV
Connection
remains above 4.5V. This allows the MOSFET
CC
pin and ensure that EXTV
CC
U
pin reduces the junction temperature
CC
pin to the INTV
U
CC
CC
pin when the V
CC
load current is supplied
IN
EXTVCC
W
rises above 4.7V, the
)(V
CC
CC
< V
IN
and INTV
< 7V) and from
CC
– INTV
IN
+ 0.3V when
pin thereby
IN
U
supply is
CC
CC
CC
) and
pins.
pin
IN
not present, a diode can be placed in series with the
LTC1629’s V
EXTV
V
Significant efficiency gains can be realized by powering
INTV
from the driver and control currents will be scaled by the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTV
ever, for 3.3V and other lower voltage regulators, addi-
tional circuitry is required to derive INTV
output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
to be powered from the internal 5V regulator resulting in
a significant efficiency penalty at high input voltages.
2. EXTV
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
supply is available in the 5V to 7V range, it may be used to
power EXTV
gate drive requirements. V
to the voltage applied to the EXTV
4. EXTV
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTV
derived voltage which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
inductive boost winding as shown in Figure 5a or the
capacitive charge pump shown in Figure 5b. The charge
pump has the advantage of simple magnetics.
Topside MOSFET Driver Supply (C
Functional Diagram)
External bootstrap capacitors C
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C
Functional Diagram is charged though diode D
INTV
turns on, the driver places the C
IN
.
CC
CC
CC
when the SW pin is low. When the topside MOSFET
and the V
CC
CC
CC
CC
from the output, since the V
left open (or grounded). This will cause INTV
connected to an output-derived boost network.
connected to an external supply. If an external
connected directly to V
CC
IN
CC:
providing it is compatible with the MOSFET
pin and a Schottky diode between the
IN
pin, to prevent current from backfeeding
IN
CC
must be greater than or equal
pin directly to V
B
B1
voltage across the gate-
OUT
CC
and C
B
,D
pin.
IN
. This is the normal
B
CC
current resulting
) (Refer to
CC
B2
power from the
to an output-
connected to
OUT
B
B
. How-
in the
from
CC