AAT3174 Datasheet, PDF (11/14 Pages) Advanced Analogic Technologies – High Current, High Efficiency Charge Pump

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.
R1
(kΩ)
920
732
649
562
R2
(kΩ)
193
205
210
223
I_LED
Torch (mA)
122
153
173
200
I_LED
Flash (mA)
703
701
707
703
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.
As a reference, the equations applicable to the
PMOS case are:
600mA · 187kΩ
R₁ = _{ILED (flash)}
V_IO
0.7
-
1
R₂
=
1
R₁
-
I_LED (torch)
600mA · 187kΩ
The value to use for V_IO must come from the I/O
supply voltage used in the system. 0.7V is the typ-
ical value of the V_RSET parameter found in the
Electrical Characteristics.
For a push-pull I/O port output configuration, first
calculate the overall R_SET value needed for the
desired flash level LED current:
R_SET
=
600mA · 187kΩ
I_LED (flash)
Next, choose a reasonable value for R₁. A value
that is slightly larger than R_SET, calculated from
above, is appropriate. Calculate R₂ and then calcu-
late the torch mode current level that results:
R₂
=
R_SET · R₁
R₁ - R_SET
I_LED (torch) = 600mA · 187kΩ ·
⎛ R₂ - R₁
⎝ R₁ · R₂
-
V_IO ⎞
0.7V · R₂⎠
Once again, the current and resistance values used
in the equations come from the conditions placed on
the I_DX parameter of the Electrical Characteristics
table. 0.7V is the typical value for the V_RSET param-
eter. The value to use for V_IO must come from the
I/O supply voltage used in the system.
Example standard 1% values are provided in Table 4.
R1
(kΩ)
169
165
162
160
R2
(kΩ)
1000
1000
1000
1000
I_LED
Torch (mA)
95
111
124
132
I_LED
Flash (mA)
776
792
805
813
Table 4: Push-Pull I/O Example
Resistor Values.
In all of the approaches mentioned, the open-drain
NMOS or PMOS type configurations offer the most
flexibility for current level selection.
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.
3174.2006.05.1.2
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