LTC1435A Linear Technology, LTC1435A Datasheet - Page 11

• Manufacturer Part Number: LTC1435A
• Description: High Efficiency Low Noise Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
control power to be derived from the output during normal
operation (4.8V < V
lator when the output is out of regulation (start-up, short
circuit). Do not apply greater than 10V 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 supply means
connecting the EXTV
3.3V and other lower voltage regulators, additional circuitry
is required to derive INTV
The following list summarizes the four possible connections
for EXTV
1. EXTV
2. EXTV
3. EXTV
4. EXTV
Topside MOSFET Driver Supply (C
An external bootstrap capacitor C
pin supplies the gate drive voltage for the topside MOSFET.
Capacitor C
diode D
to be powered from the internal 5V regulator resulting
in an efficiency penalty of up to 10% at high input volt-
ages.
connection for a 5V regulator and provides the highest
efficiency.
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 4.8V. This can be done with either the in-
ductive boost winding as shown in Figure 4a or the
capacitive charge pump shown in Figure 4b. The charge
pump has the advantage of simple magnetics.
supply is available in the 5V to 10V range (EXTV
it may be used to power EXTV
ible with the MOSFET gate drive requirements. When
driving standard threshold MOSFETs, the external sup-
ply must always be present during operation to prevent
MOSFET failure due to insufficient gate drive.
CC
from the output, since the V
B
CC
CC
CC
CC
CC:
from INTV
left open (or grounded). This will cause INTV
connected to an output-derived boost network.
connected directly to V
connected to an external supply. If an external
B
in the Functional Diagram is charged through
U
OUT
CC
CC
CC
when the SW pin is low. When the
< 9V) and from the internal regu-
pin directly to V
< V
INFORMATION
CC
U
IN
power from the output.
.
CC
B
IN
OUT
connected to the Boost
providing it is compat-
W
B
current resulting from
, D
. This is the normal
OUT
B
)
. However, for
U
CC
CC
CC
to an
V
IN
pin
CC
),
topside MOSFET is to be turned on, the driver places the
C
hances the MOSFET and turns on the topside switch. The
switch node voltage SW rises to V
to V
to be 100 times greater than the total input capacitance of
the topside MOSFET. In most applications 0.1 F is ad-
equate. The reverse breakdown on D
V
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
B
IN(MAX).
5V V
voltage across the gate source of the MOSFET. This en-
Figure 4a. Secondary Output Loop and EXTV
V
CONNECTION
IN
OUT
OPTIONAL
+ INTV
SEC
EXT V
Figure 4b. Capacitive Charge Pump for EXTV
EXTV
9V
CC
1 19
LTC1435A
.
CC
CC
R6
R5
. The value of the boost capacitor C
PGND
V
EXTV
SFB
SGND
V
SW
TG
BG
LTC1435A
+
IN
1
CC
C
PGND
IN
R
R
V
SW
BG
2
TG
1
IN
+
N-CH
N-CH
V
,
IN
V
C
IN
OUT
BAT85
N-CH
N-CH
L1
V
IN
IN
VN2222LL
and the Boost pin rises
1 19
B
0.22 F
.
must be greater than
R
1:N
L1
SENSE
LTC1435A
1N4148
V
+
+
CC
R
Connection
BAT85
BAT85
1435A F04b
SENSE
C
1 F
OUT
V
1435A F04a
V
SEC
OUT
CC
+
+
B
11
needs
1 F
C
V
OUT
OUT