Datasheet

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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
high impedance corresponding to logic-high. So, when
the output is at logic-high, the C1 capacitor charges
through the resistor network formed by R1 to R5 as shown
in Figure 8. An accurate calculation of t
HIGH
would have
involved applying thevenin’s theorem to compute the
equivalent thevenin voltage (V
THEVENIN
) and thevenin
resistance (R
THEVENIN
) in series with the capacitor
C1. t
HIGH
can then be computed using the basic time
domain equations for the charging RC circuit as:

−
=


−

T_RISE
THEVENIN
HIGH
THEVENIN
T_FALL
THEVENIN
V
V
R C1 ln
t
V
V
[ ]
[ ]
THEVENIN
CC
CC
THEVENIN
R (R2 R4) R3 R1 R5
V (R2 R4) R3
V x R4
V
(R2 R4) R3 R1 R2 R4
R1
x
(R2 R4) R3 R1
= ++
+
= +
++ +
++




The t
HIGH
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 V
CC
when at logic-high (that is, V
THEVENIN
~ V
CC
and R
THEVENIN
~ R5). With this selection, t
HIGH
can be approximated as:

−
=


−

T_RISE
CC
HIGH
T_FALL
CC
V
V
R5C1 ln
t
V
V
The frequency of the relaxation oscillator is:
( )
( )
HIGH LOW
CC T_RISE
T_FALL
T_RISE
T_FALL
CC
11
f
VV
tt
V
R 5 C1 1n
V
V
V
= =
+ 
−


−

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-
mines the switching frequency and the analog control
voltage determines the pulse width.
Figure 7. Relaxation Oscillator Waveforms Figure 9. PWM Generator
Figure 8. Charging Network Corresponding to Logic-High Output
V
CC
V
CC
V
CC
INA-
C1
GND
R2
OUT
MAX44269
R3
ANALOG
CONTROL
VOLTAGE
R6
R1
R5
R4
V
T_RISE
C1 WAVEFORM
OUT
WAVEFORM
V
T_FALL
R2
R3 R5
C1
R4
V
CC
R1
V
CC
R
THEVENIN
V
THEVENIN
C1