LTC3350
21
3350fc
For more information
www.linear.com/LTC3350
applicaTions inForMaTion
Digital Configuration
Although the LTC3350 has extensive digital features, only
a few are required for basic use. The shunt voltage should
be programmed via the vshunt register if a value other
than the default 2.7V is required. The capacitor voltage
feedback reference defaults to 1.2V; it may be changed
in the vcapfb_dac register.
All other digital features are optional and used for moni-
toring. The ADC automatically runs and stores conver-
sions to registers (e.g., meas_vcap). Capacitance and ESR
measurements only run if requested, however, they may
be scheduled to repeat if desired (ctl_strt_capesr and cap_
esr_per). Each measured parameter has programmable
limits (e.g., vcap_uv_lvl and vcap_ov_lvl) which may
trigger an alarm and SMBALERT when enabled. These
alarms are disabled by default.
Capacitor Configuration
The LTC3350 may be used with one to four supercapaci-
tors. If less than four capacitors are used, the capacitors
must be populated from CAPRTN to CAP4, and the unused
CAP pins must be tied to the highest used CAP pin. For
example, if three capacitors are used, CAP4 should be tied
to CAP3. If only two capacitors are used, both CAP4 and
CAP3 should be tied to CAP2. The number of capacitors
used must be programmed on the CAP_SLCT0 and
CAP_SLCT1 pins by tying the pins to VCC2P5 for a one
and ground for a zero as shown in Table 1. The value
programmed on these pins may be read back from the
num_caps register via I
2
C/SMBus.
Table 1
CAP_SLCT1
CAP_SLCT0
num_caps
REGISTER VALUE
NUMBER OF
CAPACITORS
0
0
0
1
0
1
1
2
1
0
2
3
1
1
3
4
Capacitor Shunt Regulator Programming
V
SHUNT
is programmed via the I
2
C/SMBus interface and
defaults to 2.7V at initial power-up. V
SHUNT
serves to limit
the voltage on any individual capacitor by turning on a
shunt around that capacitor as the voltage approaches
V
SHUNT
. CAPRTN, CAP1, CAP2, CAP3 and CAP4 must be
connected to the supercapacitors through resistors which
serve as ballasts for the internal shunts. The shunt cur-
rent is approximately V
SHUNT
divided by twice the shunt
resistance value. For a V
SHUNT
of 2.7V, 2.7Ω resistors
should be used for 500mA of shunt current. The shunts
have a duty cycle of up to 75%. The power dissipated in
a single shunt resistor is approximately:
P
SHUNT
≈
3V
SHUNT
2
16R
SHUNT
and the resistors should be sized accordingly. If the shunts
are disabled, make R
SHUNT
100Ω.
Since the shunt current is less than what the switcher can
supply, the on-chip logic will automatically reduce the
charging current to allow the shunt to protect the capacitor.
This greatly reduces the charge rate once any one shunt is
activated. For this reason, V
SHUNT
should be programmed
as high as possible to reduce the likelihood of it activating
during a charge cycle. Ideally, V
SHUNT
would be set high
enough so that any likely capacitor mismatches would not
cause the shunts to turn on. This keeps the charger operat-
ing at the highest possible charge current and reduces the
charge time. If the shunts never turn on, the charge cycle
completes quickly and the balancers eventually equalize
the voltage on the capacitors. The shunt setting may also
be used to discharge the capacitors for testing, storage
or other purposes.
Setting Input and Charge Currents
The maximum input current is determined by the resis-
tance across the VOUTSP and VOUTSN pins, R
SNSI
. The
maximum charge current is determined by the value of
the sense resistor, R
SNSC
, used in series with the induc-
tor. The input and charge current loops servo the voltage
across their respective sense resistor to 32mV. Therefore,
the maximum input and charge currents are:
I
IN(MAX)
=
32mV
R
SNSI
I
CHG(MAX)
=
32mV
R
SNSC
