EUP8051
It is recommended to choose a package with a lower θJA
1. Determine the maximum power dissipation, PD , in
the transistor.
than the number calculated above.
The worst case power dissipation happens when the
cell voltage, V(constant), is at its lowest (typically 3.1V
at the beginning of current regulation phase) and VI
is at its maximum. Where VD is the forward voltage
drop across the reverse-blocking diode (if one is
used), and VCS is the voltage drop across the current
sense resistor.
3. Select a drain-source voltage, V(DS), rating greater
than the maximum input voltage. A 12V device will
be adequate in this example.
4. Select a device that has at least 50% higher drain
current (ID) rating than the desired charge current
I(REG).
5. Verify that the available drive is large enough to
supply the desired charge current.
PD=(VI(MAX)－VD－VCS－VBAT)×IREG --------- (7)
PD=(5.5－0.4－0.2－3.1)×1A
PD=1.8W
V
(GS)=(VD+V(CS)+VOL(CC))－VI(min) -------------- (9)
V(GS)=(0.4+0.2+1) － 4.5
V(GS)=-2.9
Where V(GS) is the gate-to-source voltage, VD is the
forward voltage drop across the reverse-blocking diode
(if one is used), and VCS is the voltage drop across the
current sense resistor, and VOL(CC) is the CC pin output
low voltage specification for the EUP8051.
2. Determine the package size needed in order to keep
the junction temperature below the manufacturer’s
recommended value, TJMAX. Calculate the total theta,
θ(°C/W), needed.
(T
- T
)
max(J) A(max)
Select a MOSFET with gate threshold voltage, V(GSth)
,
θ
=
--------------(8)
JA
rating less than the calculated V(GS)
.
P
D
Reverse Blocking Protection
(150 - 40)
1.8
θ
=
θJA = 61°C/W
The optional reverse-blocking protection diode,
depicted in Figure1&2 provides protection from a
faulted or shorted input, or from a reversed-polarity
input source. Without the protection diode, a faulted of
shorted input would discharge the battery pack through
the body diode of the external pass transistor.
JA
If a reverse-protection diode is incorporated in the
design, it should be chosen to handle the fast charge
current continuously at the maximum ambient
temperature. In addition, the reverse-leakage current of
the diode should be kept as small as possible.
DS8051 Ver 1.6 Sep. 2006
14
EUTECH [ EUTECH MICROELECTRONICS INC ]
