ltc1709-85 Linear Technology Corporation, ltc1709-85 Datasheet - Page 16

• Manufacturer Part Number: ltc1709-85
• Description: 2-phase, 5-bit Vid, Current Mode, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1709-85
a minimum of 4.7 F tantalum or electrolytic capacitor. An
additional 1 F ceramic capacitor placed very close to the
IC is recommended due to the extremely high instanta-
neous currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1709-85 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 LTC1709-85
V
Use of the EXTV
to:
The input supply current should be measured while the
controller is operating in continuous mode at maximum
V
prevent the maximum junction temperature from being
exceeded.
EXTV
The LTC1709-85 contains an internal P-channel MOSFET
switch connected between the EXTV
When the voltage applied to EXTV
internal regulator is turned off and an internal switch
closes, connecting the EXTV
thereby supplying internal and MOSFET gate driving power
to the IC. The switch remains closed as long as the voltage
applied to EXTV
MOSFET driver and control power to be derived from a
16
IN
IN
T
T
current is limited to less than 24mA from a 24V supply:
J
J
and the power dissipation calculated in order to
CC
CC
= 70 C + (24mA)(24V)(85 C/W) = 119 C
= 70 C + (24mA)(5V)(85 C/W) = 80.2 C
pin. When the voltage applied to the EXTV
Connection
CC
CC
U
pin reduces the junction temperature
remains above 4.5V. This allows the
U
CC
CC
CC
pin to the INTV
load current is supplied
IN
W
rises above 4.7V, the
)(V
CC
IN
and INTV
– INTV
U
CC
CC
CC
CC
) and
pins.
pin
pin
separate 5V supply during normal operation (4.7V <
V
external 5V supply is not available. Do not apply greater
than 7V to the EXTV
0.3V when using the application circuits shown. If an
external voltage source is applied to the EXTV
the V
series with the LTC1709-85’s V
between the EXTV
from backfeeding V
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
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to V
The value of the boost capacitor C
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of D
than V
The final arbiter when defining the best gate drive ampli-
tude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
Output Voltage
The LTC1709-85 has a true remote voltage sense capablity.
The sensing connections should be returned from the load
back to the differential amplifier’s inputs through a com-
mon, tightly coupled pair of PC traces. The differential
amplifier corrects for DC drops in both the power and
ground paths. The differential amplifier output signal is
divided down and compared with the internal precision
0.8V voltage reference by the error amplifier.
EXTVCC
CC
IN
IN(MAX).
when the SW pin is low. When the topside MOSFET
supply is not present, a diode can be placed in
< 7V) and from the internal regulator when the
IN
and the BOOST pin rises to V
CC
CC
IN
and the V
.
pin and ensure that EXTV
IN
IN
B
B1
pin and a Schottky diode
voltage across the gate-
B
pin, to prevent current
and C
needs to be 30 to 100
B
,D
B
B
) (Refer to
B2
must be greater
connected to
IN
CC
+ V
CC
pin when
B
B
INTVCC
< V
in the
170985f
from
IN
+
.