ISL24201_14 Datasheet, PDF (7/12 Pages) Intersil Corporation – Programmable VCOM Calibrator with EEPROM

ISL24201
The maximum voltage on the SET pin is A_VDD/20 and is added to
the minimum voltage difference between the V_OUT and SET pins to
calculate the minimum V_OUT voltage, as shown in Equation 5.
V_O
U
T
(
M
I
N
)
≥
A_{----V----D---D--}
20
+
M
i
ni
m
u
mSa
t
u
r
a
t
i
o
n
V
ol
t
ag
e
(EQ. 5)
Output Voltage
The output voltage, V_OUT, of the OUT pin can be calculated from
Equation 6:
V_OUT
=
A_VD
D
⎛
⎜
⎝
--_R----₁-^R--+--²--_R-----₂--_{⎠⎟⎞}
⎛
⎜1
⎝
–
R_-----e_---g_----i_--s_---t_---e_---r_---V_---a_----l_--u_----e_----+-----1_--
256
⎛
⎜
⎝
2^----0^---R-R^---1-_S^---_E^----_T^-_{⎠⎟⎞}
⎞
⎟
⎠
(EQ. 6)
While Equation 6 can be used to calculate the output voltage, it
does not help select the values of R₁, R₂ and R_SET to obtain a
specific range of V_COM voltages.
Output Voltage Span Calculation
The span of the output voltage is typically centered around the
nominal V_COM voltage value, which is typically near half of the
A_VDD voltage. The high V_COM voltage occurs with the register
value of zero, while the low V_COM voltage occurs with the register
value of 255. Figure 7 shows the definition of several terms used
later in the text.
A _VDD
HIGH VCOM VOLTAGE
NOMINAL VCOM VOLTAGE
SPAN
LOW VCOM VOLTAGE
GND
FIGURE 7. VOLTAGE LEVELS FOR V_COM
There are three variables that control the V_COM calibrator’s
operating point; the span of the V_COM voltage, the maximum
current sink and the source impedance of the resistive divider.
Figure 8 shows a range of operating points for these three variables
and a quick way to estimate a specific operating point. The X-axis is
the span of the V_COM voltage (High V_COM Voltage - Low V_COM
Voltage), and the Y-axis is the maximum sink current set by R_SET.
The individual plots of each R_TH show the V_COM span plotted against
the maximum OUT sink current given that value of source
impedance of the voltage divider. R_TH is the Thevenin equivalent
resistance of the voltage divider R₁ and R₂, which is the resistance
of the parallel combination of R₁ and R₂, as shown in Equation 7.
R_TH
=
---^R-----¹---^R----²-----
R₁ + R₂
(EQ. 7)
The span of the V_COM voltage is shown by Equation 8.
V_COMSPAN = I_SET(R_TH)
(EQ. 8)
0.6
0.5
0.4
0.3
0.2
= ^25kΩ
R TH
0.1
RTH ^{= 50kΩ}
R_TH ^{= 100kΩ}
0
0
1
2
3
4
5
6
V_COM SPAN (V)
FIGURE 8. GRAPH of V_COM SPAN vs MAXIMUM OUTCURRENT
AND R_TH
To make a final selection of the resistor values for R₁ and R₂, The
supply voltage A_VDD and the value of R_SET are specified. The
calculations for R₁ and R₂ are shown in Equations 9 and 10:
R₁
=
4_----0_----R_{----S---E----T---}(_--S_----P_----A_----N_-----)
A_VDD + SPAN
(EQ. 9)
R₂
=
-⁴---⁰----^R----^S----^E---^T---⁽--^S----^P----^A----^N-----⁾
A_VDD – SPAN
(EQ. 10)
The R₁ and R₂ calculations are based on the span of the V_COM
voltage being centered at half the A_VDD voltage.
As an example, A_VDD = 15V, the maximum value for I_SET is
selected to be 100µA and the required span is 2V. Using Figure 8
as a guide, the V_COM maximum is equal to 8.5V and the V_COM
minimum is equal to 6.5V. Rearranging equation and calculation
the value of R_SET:
R_SET
=
------------^A----^V---^D----^D-------------
20 I _{O U T} ( M A X )
=
2^----0^----(^--0^----.^-0^-1---0^-5--0^----1^---0^----0^----)
=
7500 Ω
(EQ. 11)
Calculating the value of R₁ is shown in Equation 12.
R₁
=
4_----0_----(_--7_----5_---0_----0_----)_--(_---2_----)
15 + 2
=
39.29 k Ω
(EQ. 12)
Calculating the value of R₂ is shown in Equation 13.
R₂
=
4_----0_----(_--7_----5_---0_----0_----)_--(_---2_----)
15 – 2
=
46.15 k Ω
(EQ. 13)
7
FN7586.1
December 9, 2010