LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 15

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

APPLICATIO S I FOR ATIO
voltage is between 4.7V and 5.2V, INTV
nected to the output and the gate drive is reduced. The
resulting increase in R
limit. Even applications with V
region during start-up and must take into account the
reduced current limit.
Topside MOSFET Driver Supply (C
An external bootstrap capacitor (C
gram) connected to the BOOST pin supplies the gate drive
voltage for the topside MOSFET. This capacitor is charged
through diode D
Note that the voltage across C
INTV
voltage rises to V
mately V
plies the top driver for as long as ten cycles between re-
freshes. Thus, the boost capacitance needs to store about
100 times the gate charge required by the top MOSFET. In
many applications 0.1 F to 0.47 F is adequate.
When adjusting the gate drive level , the final arbiter is the
total input current for the regulator. If you make a change
and the input current decreases, then you improved the
efficiency. If there is no change in input current, then there
is no change in efficiency.
External Gate Drive Buffer
The LTC1775 drivers are adequate for driving up to about
30nC into MOSFET switches. When using large single, or
multiple, MOSFET switches, external buffers may be re-
quired to provide additional gate drive capability. Special
purpose gate driver circuits such as the LTC1693 are ideal
in such cases. Alternately, the external buffer circuit shown
in Figure 7 can be used. Note that the bipolar devices
TG
CC
. When the top MOSFET turns on, the switch node
IN
BOOST
SW
+ INTV
Figure 7. Optional External Gate Driver
Q1
FMMT619
Q2
FMMT720
B
IN
from INTV
CC
U
and the BOOST pin rises to approxi-
. During dropout operation, C
GATE
OF M1
DS(ON)
U
CC
B
OUT
will also lower the current
is about a diode drop below
BG
when the SW node is low.
> 5.2V will traverse this
B
W
B
in the functional dia-
, D
INTV
PGND
B
)
CC
Q3
FMMT619
Q4
FMMT720
CC
will be con-
U
B
GATE
OF M2
sup-
1775 F07
reduce the signal swing at the gate by a diode drop. Thus,
the LTC1775 requires an increased EXTV
about 6V (such as provided by the Figure 6 circuit) when
using this driver.
Output Voltage Programming
The LTC1775 has a pin selectable output voltage deter-
mined by the V
Remote sensing of the output voltage is provided by the
V
internal resistive divider is used and the V
connected directly to the output voltage as shown in
Figure 8a. When using an external resistive divider, the
V
feedback resistors as shown in Figure 8b. The output
voltage is set by the divider as:
OSENSE
PROG
V
OUT
pin is left open and the V
pin. For fixed 3.3V and 5V output applications an
1 19
INTV
.
V
Open
CONNECT FOR
+
CONNECT FOR
V
PROG
0V
Figure 8a. Fixed 3.3V or 5V V
OUT
V
PROG
V
OUT
V
CC
V
OUT
OUT
Figure 8b. Adjustable V
C
OUT
= 3.3V
1
= 5V
pin as follows:
R
+
R
R2
R1
2
1
C
OPEN
OUT
OSENSE
V
V
SGND
PROG
OSENSE
LTC1775
V
V
SGND
PROG
OSENSE
LTC1775
INTV
OUT
pin is connected to
1775 F08a
Adjustable
CC
OUT
1775 F08b
V
3.3V
LTC1775
5V
OUT
CC
OSENSE
voltage of
15
pin is