LM3524 National Semiconductor, LM3524 Datasheet - Page 7

• Manufacturer Part Number: LM3524
• Description: Regulating Pulse Width Modulator
• Manufacturer: National Semiconductor

Functional Description
applied to pin 9 which will override the error amplifier and
force a particular duty cycle to the outputs. An example of
this could be a non-regulating motor speed control where a
variable voltage was applied to pin 9 to control motor speed.
A graph of the output duty cycle vs the voltage on pin 9 is
shown in Figure 5 .
The duty cycle is calculated as the percentage ratio of each
output’s ON-time to the oscillator period. Paralleling the out-
puts doubles the observed duty cycle.
The amplifier’s inputs have a common-mode input range of
1.5V–5.5V. The on board regulator is useful for biasing the
inputs to within this range.
FIGURE 5.
(Continued)
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CURRENT LIMITING
The function of the current limit amplifier is to override the er-
ror amplifier’s output and take control of the pulse width. The
output duty cycle drops to about 25% when a current limit
sense voltage of 200 mV is applied between the +C
−C
mately 5% results in a 0% output duty cycle. Care should be
taken to ensure the −0.7V to +1.0V input common-mode
range is not exceeded.
In most applications, the current limit sense voltage is pro-
duced by a current through a sense resistor. The accuracy of
this measurement is limited by the accuracy of the sense re-
sistor, and by a small offset current, typically 100 µA, flowing
from +CL to −CL.
OUTPUT STAGES
The outputs of the LM3524D are NPN transistors, capable of
a maximum current of 200 mA. These transistors are driven
180˚ out of phase and have non-committed open collectors
and emitters as shown in Figure 6 .
L
sense terminals. Increasing the sense voltage approxi-
FIGURE 6.
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L
and