EUP8054/8054X
Application Information
Stability Considerations
Power Dissipation
The conditions that cause the EUP8054 to reduce charge
current through thermal feedback can be approximated
by considering the power dissipated in the IC. Nearly all
of this power dissipation is generated by the internal
MOSFET—this is calculated to be approximately:
The constant-voltage mode feedback loop is stable
without an output capacitor provided a battery is
connected to the charger output. With no battery present,
an output capacitor is recommended to reduce ripple
voltage. When using high value, low ESR ceramic
capacitors, it is recommended to add a 1Ω resistor in
series with the capacitor. No series resistor is needed if
tantalum capacitors are used.
PD=(VCC－VBAT) • IBAT
where PD is the power dissipated, VCC is the input supply
voltage, VBAT is the battery voltage and IBAT is the charge
current. The approximate ambient temperature at which
the thermal feedback begins to protect the IC is:
In constant-current mode, the PROG pin is in the
feedback loop, not the battery. The constant-current mode
stability is affected by the impedance at the PROG pin.
With no additional capacitance on the PROG pin, the
charger is stable with program resistor values as high as
20k. However, 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,
TA=120℃-PDθ
JA
TA=120℃-(VCC-VBAT)• IBAT • θ
JA
Example: An EUP8054 operating from a 5V USB supply
is programmed to supply 400mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.75V.
C
PROG, the following equation can be used to calculate
the maximum resistance value for RPROG
:
Assuming
θ
is 150℃/W (see Board Layout
ConsiderationsJA), the ambient temperature at which the
EUP8054 will begin to reduce the charge current is
approximately:
1
R
≤
PROG
5
2π•10 • C
PROG
TA=120℃-(5V-3.75V)• (400mA)• 150℃/W
TA=120℃-0.5W• 150℃/W=120℃-75℃
TA=45℃
Average, rather than instantaneous, charge current may
be of interest to the user. For example, if a switching
power supply 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 PROG pin to measure
the average battery current as shown in Figure 8. A 10k
resistor has been added between the PROG pin and the
filter capacitor to ensure stability.
The EUP8054 can be used above 45℃ ambient, but the
charge current will be reduced from 400mA. The
approximate current at a given ambient temperature can
be approximated by:
120°C −T
A
I
=
BAT
(
V
− V
BAT
)
• θ
CC
JA
Using the previous example with an ambient temperature
of 60℃, the charge current will be reduced to
approximately:
120°C − 60°C
5V − 3.75V •150°C / W 187.5°C / A
= 320mA
60°C
I
I
=
=
BAT
BAT
(
)
Moreover, when thermal feedback reduces the charge
current, the voltage at the PROG pin is also reduced
proportionally as discussed in the Operation section.
It is important to remember that EUP8054 applications
do not need to be designed for worst-case thermal
conditions since the IC will automatically reduce power
dissipation when the junction temperature reaches
approximately 120℃.
Figure 8. Isolating Capacitive Load on PROG Pin
and Filtering
DS8054 Ver1.1 Jan. 2007
13
EUTECH [ EUTECH MICROELECTRONICS INC ]
