LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 14

• Manufacturer Part Number: LTC1775
• Description: High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

LTC1775
APPLICATIO S I FOR ATIO
EXTV
The LTC1775 contains an internal P-channel MOSFET
switch connected between the EXTV
Whenever the EXTV
regulator shuts off, the switch closes and INTV
supplied via EXTV
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 EXTV
and ensure that EXTV
Significant efficiency gains can be realized by powering
INTV
the driver and control currents will be scaled by a factor of
Duty Cycle/Efficiency. For 5V regulators this simply means
connecting the EXTV
3.3V and other lower voltage regulators, additional cir-
cuitry is required to derive INTV
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
2. EXTV
14
5V < V
CONNECTION
to be powered from the internal 5.2V regulator resulting
in a low current efficiency penalty of up to 10% at high
input voltages.
connection for a 5V regulator and provides the highest
efficiency.
Figure 5a: Secondary Output Loop and EXTV
OPTIONAL
SEC
CC
CC
EXTV
< 7V
from the output, since the V
CC
CC
Connection
CC
left open (or grounded). This will cause INTV
connected directly to V
R4
R3
CC
EXTV
FCB
SGND
:
CC
LTC1775
CC
U
CC
until EXTV
CC
CC
PGND
pin is above 4.7V the internal 5.2V
pin directly to V
SW
V
BG
TG
TK
IN
U
V
IN
.
CC
CC
+
drops below 4.5V. This
power from the output.
OUT
W
•
IN
CC
C
. This is the normal
V
IN
IN
1:N
T1
current supplying
OUT
and INTV
1N4148
CC
•
. However, for
Connection
+
CC
+
U
C
power is
CC
V
OUT
1775 F05a
SEC
C
1 F
CC
SEC
pins.
V
pin
OUT
CC
3. EXTV
4. EXTV
Figure 6 shows how one can easily generate a suitable
EXTV
drive current from V
tion from the LTC1775 internal regulator and increases the
gate drive voltage.
Note that R
gate drives other than the 5.2V INTV
be accounted for when selecting the MOSFET R
Particular care should be taken with applications where
EXTV
V
PUMP
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
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.
supply is available in the 5V to 7V range (EXTV
it may be used to power EXTV
EXTV
CC
CC
LTC1775
2(V
CC
CC
CC
Figure 5b: Capacitive Charge Pump for EXTV
voltage from V
is connected to the output. When the output
OUT
Figure 6. EXTV
connected to an output-derived boost network.
connected to an external supply. If an external
PGND
DS(ON)
– V
SW
V
BG
TG
TK
IN
D
) < 7V
also varies with the gate drive level. If
IN
V
IN
+
, but it removes the power dissipa-
R1
CC
IN
D1
6.8V
. This circuit still derives the gate
Power Supplied from V
C
V
IN
IN
Q1
BAT85
EXTV
L1
CC
1775 F06
.
CC
CC
+
are used, this must
VN2222LL
1 F
0.22 F
+
IN
CC
CC
C
CC
OUT
DS(ON)
< V
BAT85
BAT85
1775 F05b
to an
V
OUT
IN
),
.