參數(shù)資料
型號(hào)： LTC4425IDD#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 電源管理
英文描述： 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO12
封裝： 3 X 3 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, DFN-12
文件頁(yè)數(shù)： 20/20頁(yè)
文件大?。?/td> 239K
代理商： LTC4425IDD#TRPBF
LTC4425
9
4425f
OPERATION
The LTC4425 is a linear charger designed to charge a
two-cell series supercap stack by employing a constant-
current, constant-voltage, and constant-temperature
architecture. It has two modes of operation: charge current
prole 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
xed charge current that is also externally programmable. In
charge current prole mode, the LTC4425 charges the top of
the stack to the input voltage VIN with a charge current that
varies based on the input-to-output differential voltage.
LDO Mode
In LDO mode, the output voltage VOUT is programmed
by an external resistor divider network consisting of
RFB1 and RFB2 via the FB pin and the charge current is
programmed by an external resistor RPROG via the PROG
pin. Please refer to the Block Diagram shown in Figure 1.
The charger control circuitry consists of a constant-
current amplier and a constant-voltage amplier. When
the part is enabled to charge a discharged supercap stack,
initially the constant-current amplier 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 VOUT gets close to the programmed value,
the constant-voltage amplier 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 amplier is in control.
Charge Current Prole or Normal Mode
The LTC4425 is in charge current prole mode when the
FB pin is shorted to the input voltage VIN. In this mode
of operation, the constant-voltage amplier is internally
disabled but the charge current is still programmed by the
external RPROG resistor. The charger provides 1/10 of the
programmed charge current if the input-to-output voltage
differential (VIN–VOUT) is more than 750mV to limit the
power dissipation within the chip. As this differential voltage
decreases from 750mV, the charge current increases
linearly to its full programmed value when VOUT is within
250mV or closer to VIN. As VOUT rises further, the voltage
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 RDS(ON) is
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 VOUT is within
about 100mV of VIN.
Figure 2. Different Regions of Charge Current Prole
IDEAL DIODE
CONTROL REGION
OHMIC
REGION
FULL CHARGE
CURRENT
REGION
LINEAR CHARGE
CURRENT REGION
1/10 CHARGE
CURRENT REGION
CHARGE
CURRENT
(A)
0.3A
2A
0.2A
15
100
250
750
VIN – VOUT (mV)
4425 F02
The Ideal Diode Controller
When the input-to-output differential approaches 15mV,
the ideal diode controller takes over the control from the
constant-current amplier 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, VOUT can only be charged to
15mV below VIN. In the event VIN suddenly drops below
VOUT, the controller will quickly turn the FET completely
off to prevent any loss of charge due to the reverse ow
of charge from the supercap back to the supply.
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