AAT2820 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – Triple-Output Charge Pump Regulator

AAT2820
Triple-Output Charge Pump Regulator
V_NEG
The negative output voltage is adjusted by a resis-
tive divider from the output (V_NEG) to the FBN and
REF pin. The maximum reference voltage current
is 50µA; therefore, the minimum allowable value for
R2 of Figure 1 is 24kΩ. It is best to select the small-
est value possible for R2 as this will keep R1 to a
minimum. This limits errors due to the FBN input
bias current. The FBN input has a maximum input
bias current of 100nA. Using the full 50µA refer-
ence current for programming V_NEG:
I_PGM
=
V_REF
R2
=
1.2V
24kΩ
=
50μA
will limit the error due to the input bias current at
FBN to less than 0.2%.
I_FBN
I_PGM
=
0.1μA
50μA
= 0.2%
With R2 selected, R1 can be determined by:
R1
=
V_NEG · R2
-V_REF
See the example in Table 3.
V_NEG (V)
-7
-8
-9
-10
-11
-12
-13
-14
-15
-16
-17
-18
-19
-20
-21
-22
-23
-24
-25
R1, Closest Value (kΩ)
143
162
182
205
226
243
267
287
309
324
348
360
390
412
430
453
470
487
510
Table 3: Closest Value for R1 if Using 24.3kΩ
as R2.
V_POS
The positive output voltage is set by way of a resis-
tive divider from the output (V_POS) to the FBP and
ground pin. Limiting the size of R3 reduces the
effect of the FBP bias current. For less than 0.1%
error, limit R3 to less than 12kΩ.
I_PGM
=
V_REF
R3
=
1.2V
12kΩ
=
100μA
I_FBN
I_PGM
=
0.1μA
100μA
=
0.1%
Once R3 has been determined, then solve for R4
(see example in Table 4).
R4
=
R3
·
⎛ ^VPOS
⎝ V_REF
-1^⎞⎠
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2820.2006.04.1.4