LTC4425 Linear Technology, LTC4425 Datasheet - Page 12

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

LTC4425
APPLICATIONS INFORMATION
Programming the Output Voltage
In LDO mode, the LTC4425 output voltage can be pro-
grammed for any voltage between 2.7V and V
a resistor divider from V
such that:
where V
Typical values for R
small a resistor will result in a large quiescent current
whereas too large a resistor coupled with FB pin capaci-
tance will create an additional pole and may cause loop
instability.
Programming the Input Voltage Threshold for Power
Fail Status Indicator
The input voltage below which the power fail status pin
PFO indicates a power-fail condition is programmed by
using a resistor divider from the V
pin via the PFI pin such that:
V
where V
Typical values for R
shutdown mode, this divider network is disconnected from
ground via the PFI_RET pin to save the quiescent current
drawn by the network.
Programming the Charge Current
The LTC4425 charge current is programmed using a single
resistor from the PROG pin to ground. The charge current
out of the V
12
IN
V
,
OUT
PFO
Figure 3. Programming Output Voltage and Input
Threshold for Power Fail Comparator.
FB
PFI
= V
= V
is 1.2V. See Figure 3.
is 1.2V. See Figure 3.
FB
PFI
OUT
V
• (1 + R
IN
• (1 + R
pin is 1000 times the current out of the
R
R
FB
PF1
PF2
PF
FB1
are in the range of 40k to 1M. Too
PF1
are in the range of 40k to 1M. In
V
PFI_RET
PFI
IN
/R
LTC4425
OUT
/R
FB2
PF2
V
4425 F03
OUT
pin to GND via the FB pin
FB
)
)
IN
R
R
pin to the PFI_RET
V
FB1
FB2
OUT
IN
by using
PROG pin. The program resistor and the charge current
are calculated using the following equations:
where I
charge current out of the V
any time by monitoring the PROG pin voltage and using
the following equation:
Stability Considerations
In LDO mode, the LTC4425 supercapacitor charger
has two principal control loops: constant-voltage and
constant-current. The constant-voltage loop is stable
when con-nected to a supercap of at least 0.2F. However,
when disconnected from the supercap, the voltage loop
requires at least 10μF capacitance in series with 500Ω
resistance for stability.
In constant-current mode, the PROG pin voltage is in
the feedback loop, not the V
the additional pole created by the PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the charger is
stable with a program resistor as high as 100k. However,
any additional capacitance on this node reduces the
maximum allowed program resistor. The pole frequency
at the PROG pin should be kept above 100kHz. Therefore,
if the PROG pin is loaded with a capacitance, C
the following equation should be used to calculate the
maximum resistance value for R
Board Layout Considerations
To be able to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on the
backside of the LTC4425’s package has a good thermal
contact to the PC board ground. Correctly soldered to a
2500mm
a thermal resistance of approximately 43°C/W. Failure to
make thermal contact between the exposed pad on the
backside of the package and the copper board will result
in a thermal resistance far greater than 43°C/W.
R
I
R
CHRG
PROG
PROG
CHRG
= 1000 • (V
2
= 1000 • (1V/I
≤ 1/(2π • 100kHz • C
double-sided 1 oz. copper board, the part has
is the charge current out of the V
PROG
CHRG
/R
OUT
PROG
), I
OUT
PROG
CHRG
pin can be determined at
)
PROG
www.DataSheet4U.com
pin voltage. Because of
)
= 1000 • (1V/R
:
OUT
pin. The
PROG
PROG
4425p
)
,