Datasheet

LTC4425
1
4425f
TYPICAL APPLICATION
FEATURES DESCRIPTION
Linear SuperCap Charger
with Current-Limited Ideal
Diode and V/I Monitor
The LTC
®
4425 is a constant-current/constant-voltage linear
charger designed to charge a 2-cell supercap stack from
either a Li-Ion/Polymer battery, a USB port, or a 2.7V to
5.5V current-limited supply. The part operates as an ideal
diode with an extremely low 50m on-resistance making it
suitable for high peak-power/low average power applications.
The LTC4425 charges the output capacitors to an externally
programmed output voltage in LDO mode at a constant
charge current, or to V
IN
in normal mode with a smart charge
current profi le to limit the inrush current until the V
IN
to V
OUT
differential is less than 250mV. In addition the LTC4425 can
be set to clamp the output voltage to 4.9V or 5.4V.
Charge current (V
OUT
current limit) is programmed by
connecting a resistor between PROG and GND. The
voltage on the PROG pin represents the current fl owing
from V
IN
to V
OUT
for current monitoring. An internal active
balancing circuit maintains equal voltages across each
supercapacitor and clamps the peak voltage across each
cell to a pin-selectable maximum value. The LTC4425
operates at a very low 20µA quiescent current (shutdown
current <3µA) and is available in a low profi le 12-pin
3mm × 3mm DFN or a 12-lead MSOP package.
Charging 2-Cell Series Supercapacitor from Li-Ion Source
APPLICATIONS
n
50mΩ Ideal Diode from V
IN
to V
OUT
n
Smart Charge Current Profi le Limits Inrush Current
n
Internal Cell Balancer (No External Resistors)
n
Programmable Output Voltage (LDO Mode)
n
Programmable V
IN
to V
OUT
Current Limit
n
Continuous Monitoring of V
IN
to V
OUT
Current via
PROG Pin
n
Low Quiescent Current: 20μA
n
V
IN
Power Fail, PGOOD Indicator
n
2.45V/2.7V Cell Protection Shunts
(4.9V/5.4V SuperCap Max Top-Off Voltage)
n
3A Peak Current Limit, Thermal Limiting
n
Tiny Application Circuit, 3mm × 3mm × 0.75mm DFN
and 12-Lead MSOP Packages
n
High Peak Power Battery/USB Powered Equipment
n
Industrial PDAs
n
Portable Instruments/Monitoring Equipment
n
Power Meters, SuperCap Backup Circuits
n
PC Card/USB Modems
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
LTC4425
GND
+
–
Li-Ion
V
IN
V
OUT 
V
IN
V
IN
I
MONITOR
1.5M
1.2M
2.2µF
PFI
PFI_RET
SEL
FROM
µC
EN
2.45V/2.7V
2k
PROG
PFO
470k
FB
1F
1F
TO HIGH PEAK
POWER LOAD
V
MID
4425 TA01
+
V
IN
– V
OUT
(V)
CHARGE CURRENT (A)
0.6
0.5
4425 TA02
0
0.1
0.2
0.3
0.4
0 0.2 0.4 0.6 0.8 1
1/10 CHARGE
CURRENT
CURRENT LIMITED
BY PMOS RDSON
IDEAL DIODE
FORWARD VOLTAGE = 15mV
FULL CHARGE CURRENT
V
FB
= V
IN
R
PROG
= 2k
Charge Current vs V
IN
to V
OUT
Differential