Datasheet

LTC3602
12
3602fb
APPLICATIONS INFORMATION
Topside MOSFET Driver Supply (BOOST Pin)
The LTC3602 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
BST
charges capacitor C
BST
to the voltage on the INTV
CC
sup-
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
IN
voltage
and the BOOST pin rises to PV
IN
+ INTV
CC
, thereby keep-
ing the MOSFET fully enhanced. For most applications, a
0.22μF ceramic capacitor is appropriate for C
BST
. Schottky
diode D
BST
should have a reverse breakdown voltage that
is greater than PV
IN(MAX)
.
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
CC
or to PV
IN
. To increase the
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:
t
SS
=C
SS
•
0.6
1.2µ A
When the LTC3602 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
X
) and the TRACK/SS pin as shown
in Figure 5a. This technique can be used to cause V
OUT
to
ratiometrically track the V
X
supply (Figure 5b), according
to the following:
V
V
R
R
RR
RR
OUT
X
TA
A
AB
TA TB
=
+
+
•
For coincident tracking, as shown in Figure 5c, (V
OUT
=
V
X
during start-up),
R
TA
= R
A
, R
TB
= R
B
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
OUT
volt-
age during tracking. This V
OUT
offset due to the TRACK/SS
current is given by:
V
OS,TRK
=(1.2µA)•
R
TA
R
TA
R
TA
+R
TB
•
R
A
+R
B
R
A
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
TA
and R
TB
.
4.7MΩ
LTC3602
3602 F04
PV
IN
RUN
LTC3602
3.3V OR 5V
RUN
Figure 4. RUN Pin Interfacing
D
BST
LTC3602
C
BST
3602 F03
BOOST
SW
INTV
CC
C
INTVCC
Figure 3. Topside MOSFET Supply
Run and Soft-Start/Tracking Functions
The LTC3602 has a low power shutdown mode which is
controlled by the RUN pin. Pulling the RUN pin below 0.7V
puts the LTC3602 into a low quiescent current shutdown
mode (I
Q
< 1μA). When the RUN pin is greater than 0.7V,
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