Datasheet

12
LTC1159
LTC1159-3.3/LTC1159-5
4. EXTV
CC
Connected to an External Supply. If an external
supply is available in the 5V to 12V range, it may be used
to power EXTV
CC
providing it is compatible with the
MOSFET gate drive requirements. There are no restrictions
on the EXTV
CC
voltage relative to V
IN
. EXTV
CC
may be
higher than V
IN
providing EXTV
CC
does not exceed the 15V
absolute maximum rating.
When driving standard threshold MOSFETs, the exter-
nal supply must always be present during operation to
prevent MOSFET failure due to insufficient gate drive . The
LTC1149 family should also be considered for applications
which require the use of standard threshold MOSFETs.
Important Information About LTC1159 Adjustable
Applications
When an output voltage other than 3.3V or 5V is required,
the LTC1159 adjustable version is used with an external
resistive divider from V
OUT
to the V
FB
pin (Figure 6). The
regulated voltage is determined by:
V
OUT
= 1.25V
) )
1 +
R2
R1
The V
FB
pin is extremely sensitive to pickup from the
inductor switching node . Care should be taken to isolate
the feedback network from the inductor, and the 100pF
capacitor should be connected between the V
FB
and SGND
pins next to the package.
In LTC1159N and LTC1159S applications with V
OUT
>
5.5V, the V
CC
pin may self-power through the SENSE pins
when SHDN2 is taken high, preventing shutdown. In these
applications, a pull-down must be added to the SENSE
–
pin
as shown in Figure 6. This pull-down effectively takes the
place of the SHDN1 pin, ensuring complete shutdown.
Note: For versions in which both the SHDN1 and SHDN2
pins are available (LTC1159G and all fixed output ver-
sions), the two pins are simply connected to each other and
driven together to guarantee complete shutdown.
The Figure 6 circuit cannot be used to regulate a V
OUT
which
is greater than the maximum voltage allowed on the
LTC1159 SENSE pins (13V). In applications with V
OUT
>
13V, R
SENSE
must be moved to the ground side of the
output capacitor and load. This operates the current sense
comparator at 0V common mode, increasing the off-time
approximately 40% and requiring the use of a smaller
timing capacitor C
T
.
Inverting Regular Applications
The LTC1159 can also be used to obtain negative output
voltages from positive inputs. In these inverting applica-
tions, the current sense resistor connects to ground while
the LTC1159 and N-channel MOSFET connections, which
would normally go to ground, instead ride on the negative
output. This allows the negative output voltage to be set by
Figure 6. High Efficiency Adjustable Regulator with 5.5V < V
OUT
< 13V
4
3
21
0.15µF
V
IN
CAP
P-DRIVE
PGND
EXTV
CC
V
FB
LTC1159
I
TH
C
T
P-GATE
V
CC
V
CC
SHDN2
SENSE
–
SENSE
+
Si4840DY
Si4401DY
VN2222LL
0.1µF
100pF
5M
1N4148
V
IN
1N5819
0.01µF
100Ω
100Ω
100µF
50V
150µF
16V
OS-CON
1µF
0V = NORMAL
>3V = SHUTDOWN
3300pF
C
T
390pF
1k
N-GATE
SGND
V
OUT
R1
24.9k
LTC1159 • F06
R2
215k
V
OUT
=
()
1 +
R2
R1
VALUES SHOWN FOR V
OUT
= 12V/2.5A
100µH
R
SENSE
0.039Ω
1.25
+
+
+
APPLICATIO S I FOR ATIO
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