LTC3602_15 Datasheet, PDF (12/20 Pages) Linear Technology – 2.5A, 10V, Monolithic Synchronous Step-Down Regulator

LTC3602
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).
INTV_CC
LTC3602
BOOST
SW
3602 F03
D_BST
C_BST
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 amplifier.
Ramping up the effective reference into the error amp in
turn causes a smooth and controlled ramp on the output
3.3V OR 5V
LTC3602
RUN
PV_IN
4.7MΩ
LTC3602
RUN
3602 F04
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 cap,
resulting in a soft-start ramp time given by:
t_SS
=
C_SS
•
0.6
1.2µA
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_{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
=
(1.2µA)
•
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.
Figure 4. RUN Pin Interfacing
3602fb
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