LTC3350_15 Datasheet, PDF (15/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

Operation
LTC3350
V_IN
R_SNSI
V_OUT
(TO SYSTEM)
LTC3350
V_IN
INFET VOUTSP VOUTSN
INPUT
CURRENT
CONTROLLER
+–
30mV
V_REF
CHARGE
CURRENT
CONTROLLER
CAPACITOR
VOLTAGE
CONTROLLER
I_IN
BIDIRECTIONAL
SWITCHING
CONTROLLER
STEP-DOWN MODE
TGATE
BGATE
I_REF
ICAP
37.5
I_CHG
VCAP
V_REF
D/A
vcapfb_dac[3:0]
CAPFB
VC
R_SNSC
+
+
+
+
3350 F01
Figure 1. Power Path Block Diagram—Power Available from V_IN
The LTC3350 provides constant power charging (for a fixed
V_IN) by limiting the input current drawn by the switching
controller in step-down mode. The input current limit will
reduce charge current to limit the voltage across the input
sense resistor, R_SNSI, to 32mV. If the combined system
load plus supercapacitor charge current is large enough to
cause the switching controller to reach the programmed
input current limit, the input current limit loop will reduce
the charge current by precisely the amount necessary
to enable the external load to be satisfied. Even if the
charge current is programmed to exceed the allowable
input current, the input current will not be violated; the
supercapacitor charger will reduce its current as needed.
Note that the part’s quiescent and gate drive currents are
not included in the input current measurement.The input
current can be read from the meas_iin register.
Bidirectional Switching Controller—Step-Up Mode
The bidirectional switching controller acts as a step-up
converter to provide power from the supercapacitors to
V_OUT when input power is unavailable (Figure 2). The PFI
comparator enables step-up mode. V_OUT regulation is set
by a resistor divider between V_OUT and OUTFB. To disable
step-up mode tie OUTFB to INTV_CC.
Step-up mode can be used in conjunction with the output
ideal diode. The V_OUT regulation voltage can be set below
the capacitor stack voltage. Upon removal of input power,
power to V_OUT will be provided from the supercapacitor
stack via the output ideal diode. V_CAP and V_OUT will fall as
the load current discharges the supercapacitor stack. The
output ideal diode will shut off when the voltage on OUTFB
falls below 1.3V and V_OUT will fall a PN diode (~700mV)
below V_CAP. If OUTFB falls below 1.2V when the output
3350fb
For more information www.linear.com/LTC3350
15