Theory of Operation
5–18
1710J-Series Waveform Monitors
U5 is a current-mode pulse width modulator (PWM). A current-mode PWM uses
two feedback loops. See Figure 5–5. The inner current-feedback loop directly
controls the switcher MOSFET peak current. The outer voltage-feedback loop
programs the inner loop peak current trip point.
Oscillator
Latch
From peak
detector
+2.5 V
E
B
C
Q9
Switcher
Mosfet
R83
10.0K
R79
49.9K
R101
49.9
R87
22.1
R96
0.51
R
2R
6
3
1
2
R84
24.9K
Figure 5–5: Simplified representation of the pulse-width modulator circuit
U5 pin 2 is the inverting input of an internal operational amplifier. The non-in-
verting input is set to 2.5 V by an internal voltage reference. Current from the
peak detector flows through R83 and R79. R84 provides a 100
m
A offset. The
voltage at U5 pin 1 will vary in order to maintain U5 pin 2 at 2.5 V.
The voltage at U5 pin 1 is modified by an internal circuit and sets the trip point
of the internal comparator. U5 pin 3 is the external input to the comparator. R88
and C52, connected to U5 pin 4, set the internal oscillator to 80 kHz.
The circuit works as follows: The oscillator resets the latch and U5 pin 6 goes
high, turning the switcher MOSFET on. The current through the switcher
MOSFET increases, causing the voltage across R96 to increase. This voltage is
divided by R87 and R101, and is applied to the comparator (pin 3). When the
voltage at U5 pin 3 reaches the comparator trip point, the latch toggles and the
switcher MOSFET is turned off. This process is repeated at an 80 kHz rate.
C58 increases the PWM noise immunity by rolling off the internal operational
amplifier frequency response. R82 holds the switcher MOSFET off as the circuit
is powering up. R81 slows the turn-on of the switcher MOSFET while CR27
speeds up the turn off.
Pulse Width Modulator
Содержание 1710J Series
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