Datasheet

15
LTC1775
reduce the signal swing at the gate by a diode drop. Thus,
the LTC1775 requires an increased EXTV
CC
voltage of
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
PROG
pin as follows:
V
PROG
V
OUT
0V 3.3V
INTV
CC
5V
Open Adjustable
Remote sensing of the output voltage is provided by the
V
OSENSE
pin. For fixed 3.3V and 5V output applications an
internal resistive divider is used and the V
OSENSE
pin is
connected directly to the output voltage as shown in
Figure 8a. When using an external resistive divider, the
V
PROG
pin is left open and the V
OSENSE
pin is connected to
feedback resistors as shown in Figure 8b. The output
voltage is set by the divider as:
VV
R
R
OUT
=+






119 1
2
1
.
Figure 8a. Fixed 3.3V or 5V V
OUT
Figure 8b. Adjustable V
OUT
V
PROG
INTV
CC
CONNECT FOR
V
OUT
= 5V
CONNECT FOR
V
OUT
= 3.3V
LTC1775
V
OSENSE
1775 F08a
SGND
C
OUT
V
OUT
+
V
PROG
OPEN
LTC1775
V
OSENSE
1775 F08b
SGND
C
OUT
V
OUT
R1
R2
+
voltage is between 4.7V and 5.2V, INTV
CC
will be con-
nected to the output and the gate drive is reduced. The
resulting increase in R
DS(ON)
will also lower the current
limit. Even applications with V
OUT
> 5.2V will traverse this
region during start-up and must take into account the
reduced current limit.
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor (C
B
in the functional dia-
gram) connected to the BOOST pin supplies the gate drive
voltage for the topside MOSFET. This capacitor is charged
through diode D
B
from INTV
CC
when the SW node is low.
Note that the voltage across C
B
is about a diode drop below
INTV
CC
. When the top MOSFET turns on, the switch node
voltage rises to V
IN
and the BOOST pin rises to approxi-
mately V
IN
+ INTV
CC
. During dropout operation, C
B
sup-
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 0.1µF to 0.47µF 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
Q1
FMMT619
GATE
OF M1
TG
BOOST
SW
Q2
FMMT720
Q3
FMMT619
GATE
OF M2
BG
1775 F07
INTV
CC
PGND
Q4
FMMT720
Figure 7. Optional External Gate Driver
APPLICATIO S I FOR ATIO
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