EUP8084
current from flowing backward through the inductor,
from the output capacitor to GND, or through the main
and synchronous switch to GND.
APPLICATIONS INFORMATION
BATTERY CHARGER
Programming Charge Current
Switching Regulator Undervoltage Lockout
The battery charge current is programmed using a single
resistor from the ISET pin to ground. The charge current
is 400 times the current out of the ISET pin. The program
resistor and the charge current are calculated using the
following equations:
Whenever VINB is less than 2.6V, an undervoltage lockout
circuit keeps the regulator off, preventing unreliable
operation. However, if the regulator is already running
and the battery voltage is dropping, the undervoltage
comparator does not shut down the regulator until VINB
drops below 2.4V.
1V
1V
R
= 400 ×
,
= 400 ×
I
CHG
ISET
I
R
ISET
CHG
Thermal Consideration
The charge current out of the BAT pin can be determined
at any time by monitoring the ISET pin voltage and using
the following equation:
To avoid the switching regulator from exceeding the
maximum junction temperature, the user will need to do
a thermal analysis. The goal of the thermal analysis is to
determine whether the operating conditions exceed the
maximum junction temperature of the part. The
temperature rise is given by:
V
ISET
ISET
I
=
× 400
CHG
Stability Considerations
R
TR=(PD)(θJA)
The EUP8084 battery charger contains two control loops:
constant-voltage and constant-current. The constant-
voltage loop is stable without any compensation when a
battery is connected with low impedance leads.
Excessive lead length, however, may add enough series
inductance to require a bypass capacitor of at least 1µF
from BAT to GND.
2
Where PD=ILOAD × RDS(ON) is the power dissipated by
the regulator ; θJA is the thermal resistance from the
junction of the die to the ambient temperature.
The junction temperature, TJ, is given by:
TJ=TA+TR
Where TA is the ambient temperature.
TJ should be below the maximum junction temperature
of 150°C.
In constant-current mode, the ISET pin voltage is in the
feedback loop, not the battery voltage. Because of the
additional pole created by ISET pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the ISET pin, the battery
charger is stable with ISET resistor values as high as 25k.
However, additional capacitance on this node reduces the
maximum allowed program resistor. The pole frequency
at the ISET pin should be kept above 100kHz. Therefore,
if the ISET pin is loaded with a capacitance, CISET, the
following equation should be used to calculate the
Linear Regulator Operation:
The EUP8084 includes a low-noise, low-dropout, linear
regulator operates from a 2.5V to 5.5V input and is
guaranteed to deliver 300mA.
The linear regulator is stable with small 2.2µF ceramic
capacitor. Its performance suits battery powered
applications because of its shutdown mode, low
quiescent current, and very low dropout voltage. The
low dropout voltage allows for more utilization of a
battery’s available energy by operating closer to its
end-of-life voltage.
maximum resistance value for RISET
:
1
R
≤
ISET
5
2π×10 × C
ISET
Average, rather than instantaneous, battery current may
be of interest to the user. For example, when the
switching regulator operating in low-current mode is
connected in parallel with the battery, the average current
being pulled out of the BAT pin is typically of more
interest than the instantaneous current pulses. In such a
case, a simple RC filter can be used on the ISET pin to
measure the average battery current as shown in Figure 3.
A 10k resistor has been added between the ISET pin and
the filter capacitor to ensure stability.
DS8084 Ver1.0 Apr. 2008
18
EUTECH [ EUTECH MICROELECTRONICS INC ]
