LTC3350_15 Datasheet, PDF (22/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Applications Information
The peak inductor current limit, I_PEAK, is 80% higher than
the maximum charge current and is equal to:
I_PEAK
=
58mV
R_SNSC
Note that the input current limit does not include the part’s
quiescent and gate drive currents. The total current drawn
by the part will be I_IN(MAX) + I_Q + I_G, where I_Q is the non-
switching quiescent current and I_G is the gate drive current.
Low Current Charging and High Current Backup
The LTC3350 can accommodate applications requiring
low charge currents and high backup currents. In these
applications, program the desired charge current using
R_SNSI. The higher current needed during backup can be
set using R_SNSC. The input current limit will override the
charge current limit when the supercapacitors are charging
while the charge current limit provides sufficient current
capability for backup operation.
The charge current will be limited to I_CHG(MAX) at low
V_CAP (i.e., low duty cycles). As V_CAP rises, the switching
controller’s input current will increase until it reaches
I_IN(MAX). The input current will be maintained at I_IN(MAX)
and the charge current will decrease as V_CAP rises further.
Some applications may want to use only a portion of the
input current limit to charge the supercapacitors. Two input
current sense resistors placed in series can be used to
accomplish this as shown in Figure 3. VOUTSP is kelvin
connected to the positive terminal of R_SNSI1 and VOUTSN
is kelvin connected to the negative terminal of R_SNSI2.
The load current is pulled across R_SNSI1 while the input
current to the charger is pulled across R_SNSI1 and R_SNSI2.
The input current limit is:
32mV = R_SNSI1 • I_LOAD + (R_SNSI1 + R_SNSI2) • I_INCHG
For example, suppose that only 2A of input current is de-
sired to charge the supercapacitors but the system load
and charger combined can pull a total of up to 4A from the
supply. Setting R_SNSI1 = R_SNSI2 = 8mΩ will set a 4A cur-
rent limit for the load + charger while setting a 2A limit for
the charger. With no system load, the charger can pull up
to 2A of input current. As the load pulls 0A to 4A of current
the charger’s input current will reduce from 2A down to 0A.
The following equation can be used to determine charging
input current as a function of system load current:
I_INCHG
=
32mV
R_SNSI1 +R_SNSI2
–
R_SNSI1
R_SNSI1 +R_SNSI2
•I_LOAD
The contact resistance of the negative terminal of R_SNSI1 and
the positive terminal of R_SNSI2 as well as the resistance of
the trace connecting them will cause variability in the input
current limit. To minimize the error, place both input current
sense resistors close together with a large PCB pad area
between them as the system load current is pulled from the
trace connecting the two sense resistors.
Note that the backup current will flow through R_SNSI2. The
R_SNSI2 package should be sized accordingly to handle the
power dissipation.
V_IN
R_SNSI1 R_SNSI2
V_IN INFET VOUTSP
LTC3350
VOUTSN
TGATE
BGATE
Figure 3
V_OUT (TO SYSTEM)
I_LOAD
I_INCHG
3350 F03
Setting V_CAP Voltage
The LTC3350 V_CAP voltage is set by an external feedback
resistor divider, as shown in Figure 4. The regulated output
voltage is determined by:
V_CAP
=


1+
R_FBC1
R_FBC2


CAPFBREF
where CAPFBREF is the output of the V_CAP DAC, pro-
grammed in the vcapfb_dac register. Great care should
be taken to route the CAPFB line away from noise sources,
such as the SW line.
Power-Fail Comparator Input Voltage Threshold
The input voltage threshold below which the power-fail
status pin, PFO, indicates a power-fail condition and the
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