EUP8202-4.2/8.4
range. Short the NTC pin to ground to disable the
temperature qualification feature. However the user may
modify these thresholds by adding two external resistor.
See figure 8.
for filtering and has the necessary RMS current rating.
Switching ripple current splits between the battery and
the output capacitor depending on the ESR of the output
capacitor and the battery impedance. EMI considerations
usually make it desirable to minimize ripple current in
the battery leads. Ferrite beads or an inductor may be
added to increase battery impedance at the 500kHz
switching frequency. If the ESR of the output capacitor is
0.2Ω and the battery impedance is raised to 4Ω with a
bead or inductor, only 5% of the current ripple will flow
in the battery.
Design Example
As a design example, take a charger with the following
specifications:
Figure 7. Temperature Sensing Configuration
For single cell charge, VIN = 5V to 20V, VBAT = 4V
nominal, IBAT =1.5A, fOSC = 500kHz, IEOC=0.375A, see
Figure 2.
First, calculate the SENSE resistor :
100mV
R
=
= 68mꢀ
SENSE
1.5A
Choose the inductor for about 65% ripple current at the
maximum VIN:
4V
500kHz)(0.65)(1.5A
4V
20V
L =
1 −
= 6.56µH
Figure 8. Temperature Sensing Thresholds
Input and Output Capacitors
(
)
Selecting a standard value of 6.8µH results in a
maximum ripple current of :
Since the input capacitor is assumed to absorb all input
switching ripple current in the converter, it must have an
adequate ripple current rating. Worst-case RMS ripple
current is approximately one-half of output charge
current. Actual capacitance value is not critical. Solid
tantalum capacitors have a high ripple current rating in a
relatively small surface mount package, but caution must
be used when tantalum capacitors are used for input
bypass. High input surge currents can be created when
the adapter is hot-plugged to the charger and solid
tantalum capacitors have a known failure mechanism
when subjected to very high turn-on surge currents.
Selecting the highest possible voltage rating on the
capacitor will minimize problems. Consult with the
manufacturer before use.
4V
4V
20V
∆I
=
1 −
= 941.2mA
L
(
500kHz)(6.8µH
)
∆I
941.2mA
2
L
ILPK = I
+
= 1.5A +
≈1.975A
CHG
2
Next, choose the P-channel MOSFET. For example, a
TSSOP-8 package with RDS(ON) = 42mΩ (nom), 55mΩ
(max) offers a small solution. The maximum power
dissipation with VIN = 5V and VBAT = 4V at 50℃
ambient temperature is:
2
(
1.5A) (55mΩ)(4V
)
The selection of output capacitor COUT is primarily
determined by the ESR required to minimize ripple
voltage and load step transients. The output ripple ∆VOUT
is approximately bounded by:
P
=
= 0.099W
D
5V
TJ = 50℃ + (0.099W)(65℃/W) = 56.5℃
C
IN is chosen for an RMS current rating of about 0.8A at
85℃. The output capacitor is chosen for an ESR similar
to the battery impedance of about 100mΩ The ripple
voltage on the BAT pin is:
1
ꢁV ≤ ꢁI ESR +
OUT
L
8f
C
OSC
OUT
Since ∆IL increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
DS8202 Ver 1.1 Nov.2007
18
EUTECH [ EUTECH MICROELECTRONICS INC ]
