AAT3174
High Current, High Efficiency Charge Pump
For examples of standard 1% values where the
LED flash current level is targeted for 700mA, see
Table 3.
Next, choose a reasonable value for R1. A value
that is slightly larger than RSET, calculated from
above, is appropriate. Calculate R2 and then calcu-
late the torch mode current level that results:
R1
R2
ILED
ILED
Ω
Ω
(k )
(k )
Torch (mA) Flash (mA)
RSET · R1
920
732
649
562
193
205
210
223
122
153
173
200
703
701
707
703
R2 =
R1 - RSET
R - R
2
VIO
⎛
⎞
1
ILED (torch) = 600mA · 187kΩ ·
-
⎝ R1 · R2 0.7V · R2⎠
Table 3: Open-Drain I/O Example
Resistor Values.
If the I/O port must be configured as an open-drain
PMOS type output, the appropriate equations can
be generated from these same concepts. As done
in the previous example, the necessary values can
then be calculated.
Once again, the current and resistance values used
in the equations come from the conditions placed on
the IDX parameter of the Electrical Characteristics
table. 0.7V is the typical value for the VRSET param-
eter. The value to use for VIO must come from the
I/O supply voltage used in the system.
As a reference, the equations applicable to the
PMOS case are:
Example standard 1% values are provided in Table 4.
R1
R2
ILED
ILED
600mA · 187kΩ
ILED (flash)
(kΩ)
(kΩ)
Torch (mA) Flash (mA)
R1 =
169
165
162
160
1000
1000
1000
1000
95
776
792
805
813
111
124
132
VIO
- 1
0.7
R2 =
1
ILED (torch)
-
Table 4: Push-Pull I/O Example
Resistor Values.
R1 600mA · 187kΩ
In all of the approaches mentioned, the open-drain
NMOS or PMOS type configurations offer the most
flexibility for current level selection.
The value to use for VIO must come from the I/O
supply voltage used in the system. 0.7V is the typ-
ical value of the VRSET parameter found in the
Electrical Characteristics.
When configured as an output, if the I/O port is only
push-pull type, then the equivalent open-drain
NMOS can also be realized. To realize this, acti-
vate the port as output only when driving the line
low. Otherwise, to release the line, set the port to
be tri-stated.
For a push-pull I/O port output configuration, first
calculate the overall RSET value needed for the
desired flash level LED current:
600mA · 187kΩ
ILED (flash)
RSET
=
3174.2006.05.1.2
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