MAX44269 Ver la hoja de datos (PDF) - Maxim Integrated
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MAX44269 Datasheet PDF : 14 Pages
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MAX44269
1.3mm x 1.3mm, Low-Power
Dual Comparator
Since the comparator’s output is open drain, it goes to The frequency of the relaxation oscillator is:
high impedance corresponding to logic-high. So, when
the output is at logic-high, the C1 capacitor cha= rges f
through the resistor network formed by R1 to R5 as shown
in Figure 8. An accurate calculation of tHIGH would have
involved applying thevenin’s theorem to compute the
= 1
tHIGH + tLOW
1
( ) R5C11n
V T _FALL
VCC − VT_RISE
( ) VT_RISE VCC − VT_FALL
equivalent thevenin voltage (VTHEVENIN) and thevenin
resistance (RTHEVENIN) in series with the capacitor
C1. tHIGH can then be computed using the basic time
domain equations for the charging RC circuit as:
Simple PWM Generation Circuit
A pulse-width modulated (PWM) signal generator can be
made utilizing both comparators in the IC (Figure 9). The
capacitor/feedback resistor combination on INA- deter-
tHIGH
=
R
THEVENIN
C1
ln
VTHEVENIN
VTHEVENIN
−
−
V T _RISE
V T _FALL
mines the switching frequency and the analog control
voltage determines the pulse width.
R THEVENIN= [(R2 R4) + R3] R1+ R5
VCC
VCC
= VTHEVENIN
VCC [(R2 R4) + R3] + VCC x R4
(R2 R4) + R3 + R1 R2 + R4
x
R1
(R2 R4) + R3 + R1
The tHIGH calculation can be simplified by selecting the
component values in such a way that R3 >> R1 and R5
>> R1. This ensures that the output of the comparator
goes close to VCC when at logic-high (that is, VTHEVENIN
~ VCC and RTHEVENIN ~ R5). With this selection, tHIGH
can be approximated as:
tHIGH
=
R5C1
ln
VCC
VCC
−
−
V T _RISE
V T _FALL
R2
R3
R1
R5
C1
R4
RTHEVENIN
VTHEVENIN
C1
Figure 8. Charging Network Corresponding to Logic-High Output
R4
VCC
VCC
R2
R1
VT_FALL
C1 WAVEFORM
VT_RISE
OUT
WAVEFORM
R3
C1
ANALOG
CONTROL
VOLTAGE
INA-
R5
MAX44269
OUT
VCC
R6
GND
Figure 7. Relaxation Oscillator Waveforms
Figure 9. PWM Generator
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