NE5560 Philipss, NE5560 Datasheet - Page 12

• Manufacturer Part Number: NE5560
• Description: Switched-mode power supply control circuit
• Manufacturer: Philipss

Philips Semiconductors
Dynamic Current Limit and Current Protection
(Pin 11)
In many applications, it is not necessary to have a real
constant-current output of the SMPS.
Protection of the power transistor will be the prime goal. This can be
realized with the NE5560 in an economical way. A resistor (or a
current transformer) in the emitter of the power transistor gives a
replica of the collector current. This signal must be connected to Pin
11. As can be seen in Figure 14, this input has two comparators with
different reference levels. The output of the comparator with the
lower 0.48V reference is connected to the same gate as the output
of the PWM.
When activated, it will immediately reset the output flip-flop, so
reducing the duty cycle. The effectiveness of this cycle-by-cycle
1994 Aug 31
Switched-mode power supply control circuit
V
Figure 15. Output Characteristics
11
NORMAL OPERATING
POINT
(CURRENT LIMITING)
R
R
2
1
LEVEL 1 LEVEL 2
C
.48
SS
OC
12
2
6
1
.60 (V)
OC
0.6V
V
2
Z
PWM
Figure 16. Start-Stop Circuit
100
SL00374
0.48V
LATCH
RESET
Q
11
12
1
START/
current limit diminishes at low duty cycle values. When becomes
very small, the storage time of the power transistor becomes
dominant. The current will now increase again, until it surpasses the
reference of the second comparator. The output of this comparator
activates the start-stop circuit and causes an immediate inhibit of the
output pulses. After a certain deadtime, the circuit starts again with
very narrow output pulses. The effect of this two-level current
protection circuit is visualized in Figure 15.
The Start-Stop Circuit
The function of this protection circuit is to stop the output pulses as
soon as a fault occurs and to keep the output stopped for several
periods. After this dead-time, the output starts with a very small,
gradually increasing duty cycle. When the fault is persistent, this will
cause a cyclic switch-off/switch-on condition. This “hiccup” mode
effectively limits the energy during fault conditions. The realization
and the working of the circuit are indicated in Figures 12 and 13.
The dead time and the soft-start are determined by an external
capacitor that is connected to Pin 6 (
An RS flip-flop can be set by three different functions:
1. Remote on/off on Pin 10.
2. Overcurrent protection on Pin 11.
3. Low supply voltage protection (internal).
As soon as one of these functions cause a setting of the flip-flop, the
output pulses are blocked via the output gate. In the same time
transistor Q1 is forward-biased, resulting in a discharge of the
capacitor on Pin 6.
The discharging current is limited by an internal 150 resistor in the
emitter of Q1. The voltage at Pin 6 decreases to below the lower
level of the sawtooth. When V6 has dropped to 0.6V, this will
activate a comparator and the flip-flop is reset. The output stage is
no longer blocked and Q1 is cut off. Now V
capacitor via R1 to the normal
delivering very narrow pulses as soon as V6 exceeds the lower
sawtooth level. The duty cycle of the output pulse now gradually
increases to a value determined by the feedback on Pin 3, or by the
static
STOP
0.6V
SET
MAX
Q
setting on Pin 6.
2
10
LOW SUPPLY
PROTECTION
VOLTAGE
14
15
MAX
voltage. The output starts
MAX
setting).
Z
will charge the
NE/SE5560
Product specification
SL00375