LTC4425 Linear Technology, LTC4425 Datasheet - Page 9

• Manufacturer Part Number: LTC4425
• Description: Linear SuperCap Charger
• Manufacturer: Linear Technology

OPERATION
The LTC4425 is a linear charger designed to charge a
two-cell supercap stack by employing a constant-current,
constant-voltage, and constant-temperature architecture. It
has two modes of operation: charge current profi le mode
(also referred to as normal mode) and LDO mode. In LDO
mode, the LTC4425 charges the top of the stack to an
externally programmed output voltage with a fi xed charge
current that is also externally programmable. In charge
current profi le mode, the LTC4425 charges the top of the
stack to the input voltage V
varies based on the input-to-output differential voltage.
LDO Mode
In LDO mode, the output voltage V
by an external resistor divider network consisting of
R
programmed by an external resistor R
pin. Please refer to the Block Diagram shown in Figure
1. The charger control circuitry consists of a constant-
current amplifi er and a constant-voltage amplifi er. When
the part is enabled to charge a discharged supercap stack,
initially the constant-current amplifi er is in control and
servos the PROG pin voltage to 1V. The current through
the PROG resistor gets multiplied by approximately 1000,
the ratio of the sense MOSFET (MPSNS) and the power
MOSFET (MPSW), to charge the supercap stack. As the
output voltage V
the constant-voltage amplifi er takes over and backs off
the charge current as necessary to maintain the FB pin
voltage equal to an internal reference voltage of 1.2V.
Since the PROG pin current is always about 1/1000 of the
charge current, the PROG pin voltage continues to give
an indication of the actual charge current even when the
constant-voltage amplifi er is in control.
Charge Current Profi le or Normal Mode
The LTC4425 is in charge current profi le mode when the
FB pin is shorted to the input voltage V
of operation, the constant-voltage amplifi er is internally
disabled but the charge current is still programmed by the
external R
programmed charge current if the input-to-output voltage
FB1
and R
PROG
FB2
via the FB pin and the charge current is
resistor. The charger provides 1/10 of the
OUT
gets close to the programmed value,
IN
with a charge current that
OUT
PROG
IN
is programmed
. In this mode
via the PROG
differential (V
power dissipation within the chip. As this differential voltage
decreases from 750mV, the charge current increases
linearly to its full programmed value when V
250mV or closer to V
across the charger FET gets too small to support the full
charge current. So the charge current gradually falls off
and the charger FET enters into its triode (ohmic) region of
operation (see Figure 2). Since the charger FET R
approximately 50mΩ, with a programmed charge current
of 2A, the FET will enter the ohmic (triode) region and the
charge current will start to fall off when V
about 100mV of V
The Ideal Diode Controller
When the input-to-output differential approaches 15mV,
the ideal diode controller takes over the control from the
constant-current amplifi er and backs off the charge cur-
rent by pulling up the gate of the charger FET as much
as necessary to maintain a 15mV delta across the FET
(see Figure 2). As a result, V
15mV below V
V
off to prevent any loss of charge due to the reverse fl ow
of charge from the supercap back to the supply.
0.3A
OUT
REGION
CONTROL REGION
OHMIC
15
, the controller will quickly turn the FET completely
Figure 2. Different Regions of Charge Current Profi le
IDEAL DIODE
100
FULL CHARGE
CURRENT
REGION
IN
2A
IN
–V
. In the event V
IN
OUT
250
.
IN
V
) is more than 750mV to limit the
CURRENT REGION
IN
. As V
LINEAR CHARGE
– V
OUT
OUT
(mV)
OUT
IN
rises further, the voltage
www.DataSheet4U.com
can only be charged to
suddenly drops below
LTC4425
750
OUT
OUT
CURRENT REGION
0.2A
1/10 CHARGE
is within
is within
DS(ON)
4425 F02
9
4425p
is