LTC3350
22
3350fc
For more information
www.linear.com/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.
Figure 3
V
IN
V
IN
INFET VOUTSP
R
SNSI1
R
SNSI2
LTC3350
VOUTSN
I
INCHG
I
LOAD
V
OUT
(TO SYSTEM)
TGATE
BGATE
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
