APPLICATION NOTES
(continued)
CURRENT LIMITING
The current limiting circuitry of the SG1524 is shown in Figure 3.
By matching the base-emitter voltages of Q1 and Q2, and
assuming a negligible voltage drop across R1:
C.L. Threshold = V
BE
(Q1) + I
1
• R
2
- V
BE
(Q2) = I
1
• R
2
~ 200 mV
Although this circuit provides a relatively small threshold with a
negligible temperature coefficient, there are some limitations to
its use because of its simplicity.
The most important of these is the limited common-mode voltage
range: ±0.3 volts around ground. This requires sensing in the
ground or return line of the power supply. Also precautions
should be taken to not turn on the parasitic substrate diode of the
integrated circuit, even under transient conditions. A Schottky
clamp diode at Pin 5 may be required in some configurations to
achieve this.
A second factor to consider is that the response time is relatively
slow. The current limit amplifier is internally compensated by R
1
, C
1 ,
and Q1, resulting in a roll-off pole at approximately 300 Hz.
A third factor to consider is the bias current of the C.L. Sense
pins. A constant current of approximately 150µA flows out of Pin
4, and a variable current with a range of 0-150µA flows out of Pin
5. As a result, the equivalent source impedance seen by the
current sense pins should be less than 50 ohms to keep the
threshold error less than 5%.
Since the gain of this circuit is relatively low (42 dB), there is a
transition region as the current limit amplifier takes over pulse
width control from the error amplifier. For testing purposes,
threshold is defined as the input voltage required to get 25% duty
cycle (+2 volts at the error amplifier output) with the error amplifier
signaling maximum duty cycle.
APPLICATION NOTE: If the current limit function is not used on
the SG1524, the common-mode voltage range restriction re-
quires both current sense pins to be grounded.
FIGURE 3 - CURRENT LIMITING CIRCUITRY OF THE SG1524
In this conventional single-ended regulator circuit, the two out-
puts of the SG1524 are connected in parallel for effective 0 - 90%
duty-cycle modulation. The use of an output inductor requires
and R-C phase compensation network for loop stability.
Push-pull outputs are used in this transformer-coupled DC-DC
regulating converter. Note that the oscillator must be set at twice
the desired output frequency as the SG1524's internal flip-flop
divides the frequency by 2 as it switches the P.W.M. signal from
one output to the other. Current limiting is done here in the
primary so that the pulse width will be reduced should transformer
saturation occur.
Rev 1.1a
Copyright
©
1994
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