(per charge) for a fully charged battery
ignores the effects of cell self-discharge.
Power control
The main function of the step-up regulator is
to provide a regulated LED supply voltage
despite the falling battery voltage. Taking a
closer look at the regulator circuit we find
that the energy (E) stored in the inductor (L)
by the current (I) can be expressed in the for-
mula:
The current I will increase linearly from 0 and
is a function of the time that transistor T1 is
switched on (T
ON
).
Substituting into equation (1) we get the
average power (P) for the period:
E
V
T
L
BATT
ON
= ⋅
⋅
1
2
2
2
I
V
T
L
BATT
ON
=
⋅
...(
)
2
E
L I
= ⋅ ⋅
1
2
2
1
...(
)
It can be seen that the power to the
LEDs is a factor of the battery volt-
age so that if no regulation were
used the power would vary by a fac-
tor of 2 as the battery voltage drops
from 4.1 to 2.8 V. The average current
is P/V
BATT
. The on to off ratio of the
transistor (T
ON
und T
OFF
) is altered
by the microcontroller to keep P con-
stant.
In order to regulate the power it is
necessary for the microcontroller to
know the battery cell potential
V
BATT
. To reduce the component
count the supply voltage is not
directly measured, instead the bat-
tery voltage is used as the reference
input (= full scale) and the input at
P
V
T
L T
T
BATT
ON
ON
OFF
= ⋅
⋅
⋅
+
(
)
1
2
2
2
3
(
)
...(
)
AN0 is measured. The TL431AC
Bandgap-reference produces a con-
stant 2.495 V. For the 8-bit ADC the
measurement result stored in the
internal register AD
RES
is given by:
Rearranging:
V
BATT
is measured every 100 ms to
regulate the output power.
The microcontroller uses this battery
voltage measurement to point to a
stored look-up table for one of 16
possible values to control the T
ON
and T
OFF
times at output pin GP4.
The value of ‘back e.m.f.’ (electro-
motive force) generated across
inductor L1 is approximately
6
×
V
BATT
when the transistor T1 is
switched off (T
OFF
). The transistor
off time will be T
OFF
> 1/6
×
T
ON
this
ensures that the current I has time to
fall to zero
.
Internal resistances of the Schot-
tky diode, inductor and battery pro-
duce losses in the circuit and reduce
the circuit efficiency. The efficiency
at maximum power setting reaches
about 94%, but even LEDs from the
V
BATT
V
AD
RES
=
−
2 495
1
255
5
.
/
...(
)
AD
RES
V
V
V
BATT
BATT
=
⋅
−
2 495
255
4
.
...)
GENERAL
INTEREST
60
Elektor Electronics
11/2002
012019 - 12
0
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
100
90
80
70
60
50
40
30
20
10
4.1
500
1000
1500 2000 2500 3000 3500 4000 4500 5000
Energy [mWh]
U
batt
[V]
8 6
5
4
3
2
0
1
Figure 2. Typical Li-Ion cell discharge characteristics.
Table 2. Light level settings
Setting
Power
Current drain
Duration
Timeout
V
batt
= 3.6 V
(1.3 Ah)
period
0
0.5 W
140 mA
10 hr
20 min
1
0.2 W
55 mA
24 hr
40 min
2
65 mW
18 mA
3 days
2 h
3
20 mW
6 mA
9 days
5 h
4
8 mW
2.4 mA
22 days
10 h
5
4 mW
1.0 mA
55 days
10 h
Standby
–
30
µ
A
5 years
24 h
Off
–
2
µ
A
70 years
–
Table 1. Function
Key pressed
Function
S1 briefly
Brightness level +
S2 briefly
Brightness level –
S1 and S2 briefly
Off
S1 longer
Flash mode +
S2 longer
Flash mode –
S1 and S2 longer
Battery check, then
Standby, Reset Continuous mode
Содержание EPROM
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