Linear Technology LTC3350, Руководство пользователя

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LTC3350
15
3350fc
For more information www.linear.com/LTC3350
operaTion
+
+
+
+
+–
+
–
+ –
30mV
INPUT
CURRENT
CONTROLLER
CHARGE
CURRENT
CONTROLLER
BIDIRECTIONAL
SWITCHING
CONTROLLER
STEP-DOWN MODE
V
REF
I
IN
V
IN
V
IN
LTC3350
INFET VOUTSP VOUTSN
V
OUT
(TO SYSTEM)
TGATE
I
CHG
BGATE
ICAP
VCAP
R
SNSC
R
SNSI
3350 F01
+
–
I
REF
V
REF
CAPACITOR
VOLTAGE
CONTROLLER
+
–
+
–
CAPFB
VC
37.5
D/A
vcapfb_dac[3:0]
+
–
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