LTC3603 Datasheet, PDF (13/22 Pages) Linear Technology – 2.5A, 15V Monolithic Synchronous Step-Down Regulator

LTC3603
APPLICATIONS INFORMATION
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).
INTV_CC
LTC3603
BOOST
SW
3603 F03
D_BST
C_BST
C_INTVCC
Figure 3. Topside MOSFET Supply
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_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.
3.3V OR 5V
LTC3603
RUN
PV_IN
4.7MΩ
LTC3603
RUN
3603 F04
Figure 4. RUN Pin Interfacing
Soft-start and tracking are implemented by limiting the
effective reference voltage as seen by the error amplifier.
Ramping up the effective reference into the error amp in
turn causes a smooth and controlled ramp on the output
voltage of the converter. To use the default, internal 1ms
soft-start ramp, leave the TRACK/SS pin floating. 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 capacitor,
resulting in a soft-start ramp time given by:
t_SS
=
C_SS
•
0.6V
1.2µA
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_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_{OUT =} R_{TA •} R_A + R_B
V_X R_A R_TA + R_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
= (1µA) •
R_TAR_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.
V_OUT
V_X
R_B
V_FB
R_TB
LTC3603
R_A
TRACK/SS
R_TA
3603 F05a
Figure 5a. Using the TRACK/SS Pin to Track V_X
3603fa
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