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參數(shù)資料
| 型號： | CS4121ENF16G |
| 廠商： | ON Semiconductor |
| 文件頁數(shù)： | 10/12頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC TACH/SPEEDO DRIVER 16-DIP |
| 產(chǎn)品變化通告： | Product Obsolescence 13/Apr/2009 |
| 標(biāo)準(zhǔn)包裝： | 25 |
| 類型： | 傳感器接口 |
| 輸入類型： | 電壓 |
| 輸出類型： | 差分 |
| 電流 - 電源： | 125mA |
| 安裝類型： | 通孔 |
| 封裝/外殼： | 16-DIP（0.300"，7.62mm） |
| 供應(yīng)商設(shè)備封裝： | 16-DIP |
| 包裝： | 管件 |
CS4121
http://onsemi.com
7
CIRCUIT DESCRIPTION and APPLICATION NOTES
The CS4121 is specifically designed for use with aircore
meter movements. It includes an input comparator for
sensing an input signal from an ignition pulse or speed
sensor, a charge pump for frequency to voltage conversion,
a bandgap voltage regulator for stable operation, and a
function generator with sine and cosine amplifiers to
differentially drive the meter coils.
From the partial schematic of Figure 7, the input signal is
applied to the FREQIN lead, this is the input to a high
impedance comparator with a typical positive input
threshold of 2.0 V and typical hysteresis of 0.5 V. The output
of the comparator, SQOUT, is applied to the charge pump
input CP+ through an external capacitor CCP. When the
input signal changes state, CCP is charged or discharged
through R3 and R4. The charge accumulated on CCP is
mirrored to C4 by the Norton Amplifier circuit comprising
of Q1, Q2 and Q3. The charge pump output voltage, F/VOUT,
ranges from 2.0 V to 6.3 V depending on the input signal
frequency and the gain of the charge pump according to the
formula:
F VOUT + 2.0 V ) 2.0
FREQ
CCP
RT
(VREG * 0.7 V)
RT is a potentiometer used to adjust the gain of the F/V
output stage and give the correct meter deflection. The F/V
output voltage is applied to the function generator which
generates the sine and cosine output voltages. The output
voltage of the sine and cosine amplifiers are derived from the
onchip amplifier and function generator circuitry. The
various trip points for the circuit (i.e., 0
°, 90°, 180°, 270°)
are determined by an internal resistor divider and the
bandgap voltage reference. The coils are differentially
driven, allowing bidirectional current flow in the outputs,
thus providing up to 305
° range of meter deflection. Driving
the coils differentially offers faster response time, higher
current capability, higher output voltage swings, and
reduced external component count. The key advantage is a
higher torque output for the pointer.
The output angle,
q, is equal to the F/V gain multiplied by
the function generator gain:
q + AF V
AFG,
where:
AFG + 77° V(typ)
The relationship between input frequency and output
angle is:
q + AFG
2.0
FREQ
CCP
RT
(VREG * 0.7 V)
or,
q + 970
FREQ
CCP
RT
The ripple voltage at the F/V converter’s output is
determined by the ratio of CCP and C4 in the formula:
DV +
CCP(VREG * 0.7 V)
C4
Ripple voltage on the F/V output causes pointer or needle
flutter especially at low input frequencies.
The response time of the F/V is determined by the time
constant formed by RT and C4. Increasing the value of C4
will reduce the ripple on the F/V output but will also increase
the response time. An increase in response time causes a
very slow meter movement and may be unacceptable for
many applications.
Design Example
Maximum meter Deflection = 270
°
Maximum Input Frequency = 350 Hz
1. Select RT and CCP
q + 970
FREQ
CCP
RT + 270°
Let CT = 0.0033
mF, find RT
RT +
270
°
970
350 Hz
0.0033
mF
RT + 243 kW
RT should be a 250 k
W potentiometer to trim out any
inaccuracies due to IC tolerances or meter movement
pointer placement.
2. Select R3 and R4
Resistor R3 sets the output current from the voltage
regulator. The maximum output current from the voltage
regulator is 10 mA. R3 must ensure that the current does not
exceed this limit.
Choose R3 = 3.3 k
W
The charge current for CCP is
VREG * 0.7 V
3.3 k
W
+ 1.90 mA
CCP must charge and discharge fully during each cycle of
the input signal. Time for one cycle at maximum frequency
is 2.85 ms. To ensure that CCP is charged, assume that the
(R3 + R4) CCP time constant is less than 10% of the
minimum input period.
T
+ 10%
1
350 Hz
+ 285 ms
Choose R4 = 1.0 k
W.
Discharge time: tDCHG = R3 × CCP = 3.3 k
W × 0.0033 mF
= 10.9
ms
Charge time: tCHG = (R3 + R4)CCP = 4.3 k
W. × 0.0033 mF
= 14.2
ms
3. Determine C4
C4 is selected to satisfy both the maximum allowable
ripple voltage and response time of the meter movement.
C4
+
CCP(VREG * 0.7 V)
DVMAX
With C4 = 0.47
mF, the F/V ripple voltage is 44 mV.
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