LTC3350_15 Datasheet, PDF (30/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Applications Information
The external LDO should be powered from V_OUT. It must
be enabled after the INTV_CC LDO has powered up and its
output must be less than 5.5V. INTV_CC should no longer
be tied to DRV_CC.
Minimum On-Time Considerations
Minimum on-time, t_ON(MIN), is the smallest time dura-
tion that the LTC3350 is capable of turning on the top
MOSFET in step-down mode. It is determined by internal
timing delays and the gate charge required to turn on the
top MOSFET. The minimum on-time for the LTC3350 is
approximately 85ns. Low duty cycle applications may
approach this minimum on-time limit and care should be
taken to ensure that:
t_ON(MIN)
<
V_CAP
V_OUT • f_SW
If the duty cycle falls below what can be accommodated
by the minimum on-time, the controller will begin to skip
cycles. The charge current and V_CAP voltage will continue to
be regulated, but the ripple voltage and current will increase.
Ideal Diode MOSFET Selection
An external N-channel MOSFET is required for the input and
output ideal diodes. Important parameters for the selection
of these MOSFETs are the maximum drain-source voltage,
V_DSS, gate threshold voltage and on-resistance (R_DS(ON)).
Achieving forward regulation will minimize power loss and
heat dissipation, but it is not a necessity. If a forward volt-
age drop of more than 30mV is acceptable, then a smaller
MOSFET can be used but must be sized compatible with
the higher power dissipation. Care should be taken to
ensure that the power dissipated is never allowed to rise
above the manufacturer’s recommended maximum level.
During backup mode, the output ideal diode shuts off
when the voltage on OUTFB falls below 1.3V. For high
V_OUT backup voltages (>8.4V), the output ideal diode will
shut off when V_CAP is more than a diode drop (~700mV)
above the V_OUT regulation point (i.e., OUTFB > 1.2V). The
body diode of the output ideal diode N-channel MOSFET
will carry the load current until V_CAP drops to within a
diode drop of the V_OUT regulation voltage at which point
the synchronous controller takes over. During this period
the power dissipation in the output ideal diode MOSFET
increases significantly. Diode conduction time is small
compared to the overall backup time but can be significant
when discharging very large supercapacitors (>600F). Care
should be taken to properly heat sink the MOSFET to limit
the temperature rise.
PCB Layout Considerations
When laying out the printed circuit board, the following
guidelines should be used to ensure proper operation of
the IC. Check the following in your layout:
When the input is grounded, either the supercapacitor stack
voltage or the step-up controller’s backup voltage is applied
across the input ideal diode MOSFET. Therefore, the V_DSS of
the input ideal diode MOSFET must withstand the maximum
voltage on V_OUT in backup mode. When the supercapaci-
tors are at 0V, the input voltage is applied across the output
ideal diode MOSFET. Therefore, the V_DSS of the output ideal
diode MOSFET must withstand the highest voltage on V_IN.
1. Keep MN1, MN2, D1, D2 and C_OUT close together.
The high di/dt loop formed by the MOSFETs, Schottky
diodes and the V_OUT capacitance, shown in Figure 9,
should have short, wide traces to minimize high
frequency noise and voltage stress from inductive
ringing. Surface mount components are preferred to
reduce parasitic inductances from component leads.
D1
The gate drive for both ideal diodes is 5V. This allows the
MN1
use of logic-level threshold N-channel MOSFETs.
V_OUT
L1 RSNSC
V_CAP
+
As a general rule, select MOSFETs with a low enough
R_DS(ON) to obtain the desired V_DS while operating at full
load current. The LTC3350 will regulate the forward voltage
drop across the input and output ideal diode MOSFETs to
30mV if R_DS(ON) is low enough. The required R_DS(ON) can be
calculated by dividing 0.030V by the load current in amps.
C_OUT
HIGH
FREQUENCY
CIRCULATING
PATH
MN2
D2
+
C_{CAP +}
+
Figure 9. High Speed Switching Path
3350 F09
3350fb
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