LTC3603 Linear Dimensions Semiconductor, LTC3603 Datasheet - Page 12

• Manufacturer Part Number: LTC3603
• Description: Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Dimensions Semiconductor

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APPLICATIONS INFORMATION
LTC3603
Topside MOSFET Driver Supply (BOOST Pin)
The LTC3603 uses a bootstrapped supply to power the
gate of the internal topside MOSFET (Figure 3). When the
topside MOSFET is off and the SW pin is low, diode D
charges capacitor C
ply. In order to turn on the topside MOSFET, the voltage on
the BOOST pin is then applied to its gate. As the topside
MOSFET turns on, the SW pin rises to the PV
and the BOOST pin rises to PV
ing the MOSFET fully enhanced. For most applications, a
0.22μF ceramic capacitor is appropriate for C
diode D
is greater than PV
Run and Soft-Start/Tracking Functions
The LTC3603 has a low power shutdown mode which is
controlled by the RUN pin. Pulling the RUN pin below 0.7V
puts the LTC3603 into a low quiescent current shutdown
mode (I
the controller is enabled. The RUN pin can be driven directly
from logic as shown in Figure 4.
Soft-start and tracking are implemented by limiting the
effective reference voltage as seen by the error amplifi er.
Ramping up the effective reference into the error amp in
turn causes a smooth and controlled ramp on the output
12
3.3V OR 5V
BST
Q
< 1μA). When the RUN pin is greater than 0.7V,
should have a reverse breakdown voltage that
Figure 3. Topside MOSFET Supply
LTC3603
Figure 4. RUN Pin Interfacing
RUN
IN(MAX)
LTC3603
INTV
BOOST
BST
3603 F03
SW
CC
to the voltage on the INTV
.
IN
D
C
4.7MΩ
BST
BST
+ INTV
PV
IN
CC
C
INTVCC
RUN
, thereby keep-
LTC3603
BST
IN
. Schottky
3603 F04
CC
voltage
sup-
BST
voltage of the converter. To use the default, internal 1ms
soft-start ramp, leave the TRACK/SS pin fl oating. Do not
tie the TRACK/SS pin to INTV
soft-start time above 1ms, place a cap on the TRACK/SS
pin. A 1.2μA internal pull-up current will charge this cap,
resulting in a soft-start ramp time given by:
When the LTC3603 detects a fault condition (either
undervoltage lockout or overtemperature), the TRACK/SS
pin is quickly pulled to ground and the internal soft-start
timer is also reset. This ensures an orderly restart when
using an external soft-start capacitor.
To implement tracking, a resistor divider is placed between
an external supply (V
in Figure 5a. This technique can be used to cause V
ratiometrically track the V
to the following:
For coincident tracking, as shown in Figure 5c, (V
V
Note that the 1.2μA current that is sourced from the
TRACK/SS pin will cause a slight offset in the voltage seen
on the TRACK/SS pin and consequently on the V
age during tracking. This V
current is given by:
For most applications, this offset is small and has minimal
effect on tracking performance. For improved tracking ac-
curacy, reduce the parallel impedance of R
X
R
t
V
during start-up),
V
SS
TA
OS TRK
OUT
V
X
,
=
= R
C
=
SS
A
R
=
R
, R
TA
•
A
(
1
TB
1 2
0 6
µA
.
•
.
= R
R
µA
) •
V
R
TA
A
R
B
X
R R
+
+
TA
) and the TRACK/SS pin as shown
TA TA
R
R
+
B
TB
X
OUT
R
supply (Figure 5b), according
TB
CC
offset due to the TRACK/SS
or to PV
•
R
A
R
+
A
R
IN
B
. To increase the
TA
and R
OUT
OUT
OUT
TB
volt-
3603f
.
to
=