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Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
DESCRIPTION
The NE/SE5560 is a control circuit for use in switched-mode power supplies. This single monolithic chip incorporates all the control and housekeeping (protection) functions required in switched-mode power supplies, including an internal temperature-compensated reference source, internal Zener references, sawtooth generator, pulse-width modulator, output stage and various protection circuits.
PIN CONFIGURATION
D, F, N Packages
VCC 1 VZ 2 16 FEEDFORWARD 15 OUTPUT (COLL) 14 OUTPUT (EMIT) 13 DEMAG: OVERVOLTAGE 12 GND 11 CURRENT LIMITING 10 REMOTE ON/OFF 9 EXTERNAL SYNC
FEEDBACK 3 GAIN 4
FEATURES
MODULATOR 5 DUTY CYCLE CONTROL 6 RT 7
* Stabilized power supply * Temperature-compensated reference source * Sawtooth generator * Pulse-width modulator * Remote on/off switching * Current limiting * Low supply voltage protection * Loop fault protection * Demagnetization/overvoltage protection * Maximum duty cycle clamp * Feed-forward control * External synchronization
ORDERING INFORMATION
DESCRIPTION 16-Pin Plastic Dual In-Line Package (DIP) 16-Pin Plastic Small Outline Large (SOL) Package 16-Pin Plastic Dual In-Line Package (DIP) 16-Pin Cerdip Dual In-Line Package (CERDIP)
CT 8
SL00360
Figure 1. Pin Configuration
TEMPERATURE RANGE 0 to 70C 0C to 70C -55C to 125C -55C to 125C
ORDER CODE NE5560N NE5560D SE5560N SE5560F
DWG # SOT38-4 SOT162-1 SOT38-4 0582B
ABSOLUTE MAXIMUM RATINGS
SYMBOL Supply1 VCC ICC IOUT Voltage-forced mode Current-fed mode Output transistor (at 20-30V max) Output current Collector voltage (Pin 15) Max. emitter voltage (Pin 14) TA Operating ambient temperature range SE5560 NE5560 TSTG Storage temperature range NOTES: 1. Does not include current for timing resistors or capacitors. -55 to +125 0 to 70 -65 to +150 C C C 40 VCC+1.4V +5 mA V V +18 30 V mA PARAMETER RATING UNIT
1994 Aug 31
1
853-0125 13721
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
BLOCK DIAGRAM
FEED FORWARD 16 EXTERNAL RT CT SYNC INPUT 9 78 DEMAGNETIZATION OVER-VOLTAGE PROTECTION 13 0.6V
REFERENCE VOLTAGE 0.48V FEEDBACK VOLTAGE GAIN ADJUST MODULATOR INPUT 3 4 + - 0.6V
SAWTOOTH GENERATOR
+
- VCC
PULSE WIDTH MODULATOR + - - S LATCH R Q 14 + 15 OUTPUTS
5 + 8
CUTY CYCLE CONTROL
- 0.6V + Q1 100 1k 0.48V CURRENT 11 LIMITING + 0.6V - + 1 VCC - 12 0.6V R S START STOP
- + OC1 STABILIZED SUPPLY 2 VZ
10 REMOTE ON/OFF NOTE: 1. See Voltage/Current fed supply characteristic curve.
SL00361
Figure 2. Block Diagram
1994 Aug 31
2
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
DC ELECTRICAL CHARACTERISTICS
TA=25C, VCC=12V, unless otherwise specified. SYMBOL Reference sections VREF Internal reference voltage Temperature coefficient of VREF VZ Internal Zener reference Temperature coefficient of VZ Oscillator section Frequency range Initial accuracy oscillator Duty cycle range Modulator Modulation input current Housekeeping function IIN` Pin 6, input current At 2V Over temperature For 50% max duty cycle 15kHz to 50kHz/41% of VZ Pin 1, low supply voltage protection thresholds Pin 3, feedback loop protection trip threshold At 2V Pin 3, pull-up current Pin 13, demagnetization/over-voltage protection trip on threshold IIN Pin 13, input current Pin 16, feed-forward duty cycle control *Pin 16, feed-forward input current External synchronization Pin 9 Off On Sink current Remote Pin 10 Off On At 0V Sink current 25C Over temperature -85 -100 -125 -85 -125 -125 A A 0 2 0.8 VZ 0 2 0.8 VZ V V Voltage at Pin 9=0V, 25C Over temperature 0 2 -65 0.8 VZ -100 -125 0 2 -65 0.8 VZ -125 -125 V V A A Over temperature At 0.25V 25C Over temperature Voltage at Pin 16=2VZ At 16V, VCC=18V 25C Over temperature 0.2 5 10 0.2 5 10 A A 30 40 -0.6 -10 -20 50 30 40 -0.6 -10 -20 50 % original duty cycle A -7 470 -15 600 -35 720 -7 470 -15 600 -35 720 A mV 40 8 400 50 9.0 600 60 10.5 720 40 8 400 50 9.0 600 60 10.5 720 % of duty cycle V mV 0.2 20 0.2 20 A Voltage at Pin 5=2V Over temperature 0.2 20 0.2 20 A Over temperature R=5k fO=20kHz 0 50 5 98 0 100k 50 5 98 100k Hz % % IL=-7mA 7.8 25C Over temperature 3.69 3.65 -100 8.4 200 8.8 7.8 3.72 3.81 3.85 3.57 3.53 -100 8.4 200 8.8 3.72 3.95 4.00 V V ppm/C V ppm/C PARAMETER TEST CONDITIONS SE5560 Min Typ Max Min NE5560 Typ Max UNIT
Pin 6, duty cycle limit control
1994 Aug 31
3
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
DC ELECTRICAL CHARACTERISTICS (Continued)
SYMBOL Current limiting IIN Pin 11 Input current Voltage at Pin 11=250mV 25C Over temperature Single pulse inhibit delay OC2 OC1 OC Trip Levels: Shut down, slow start, low level Current limit, high level Low Level in terms of high level, OC2 Output voltage swing Output voltage swing Open-loop gain RF BW Output stage VCE(SAT) IC=40mA Output current (Pin 15) Max. emitter voltage (Pin 14) Supply voltage/current1 ICC Supply current IZ=0, voltage-forced, VCC=12V, 25C Over temp. VCC VCC Supply voltage Supply voltage ICC=10mA current-fed ICC=30mA current-fed 20 20 10 15 23 30 19 20 10 15 24 30 mA mA V V 40 5 6 0.5 40 5 6 0.5 V mA V Feedback resistor Small-signal bandwidth 54 10k 3 60 Inhibit delay time for 20% overdrive at 40mA IOUT 0.500 0.400 0.750 0.7 0.600 0.480 0.800 -40 0.8 0.700 0.560 0.850 0.500 0.400 0.750 0.7 0.600 0.560 0.800 -40 0.8 0.700 0.500 0.850 A s V V V -2 -20 -2 -20 A PARAMETER TEST CONDITIONS SE5560 Min Typ Max Min NE5560 Typ Max UNIT
Error amplifier VOH VOL 6.2 9.5 0.7 54 10k 3 60 6.2 9.5 0.7 V V dB MHz
NOTES: 1. Does not include current for timing resistors or capacitors.
1994 Aug 31
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Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
MAXIMUM PIN VOLTAGES
NE5560 Pin No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 VCC VZ Feedback Gain Modulator Duty Cycle Control RT CT External Sync Remote On/Off Current Limiting GND Demagnetization/Overvoltage Output (Emit) Output (Collector) Feed-forward VZ VZ VCC GND VCC VZ VCC+2VBE VCC VZ VZ Current force mode Function See Note 1 Do not force (8.4V) VZ Maximum Voltage
NOTES: 1. When voltage-forced, maximum is 18V; when current-fed, maximum is 30mA. See voltage-/current-fed supply characteristic curve.
TYPICAL PERFORMANCE CHARACTERISTICS
Open-Loop Gain
60 0
Error Amplifier Open-Loop Phase
50
-30
PHASE ANGLE (DEG)
40 GAIN (dB)
-60
30
-90
20
-120
10
-150
0 1k 10k 100k FREQUENCY (Hz) 1M 10M
-180 1k 10k 100k 1M FREQUENCY (Hz) 10M
SL00362
Figure 3. Typical Performance Characteristics
1994 Aug 31
5
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
Graph for Determining MAX
MAX (%) 100 90 MAXIMUM DUTY CYCLE (%) 80 70 60 50 40 30 20 10 R2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 R1 * R2 103 2 3 4 5 6 7 8 9 10 12 R2 R1 DUTY CYCLE 6 CONTROL 2 SOFT START DUTY CYCLE % 80 70 60 50 40 30 20 10 MAX 50% MAX 30% MAX 90% MAX 70% (%)
Soft-Start Min. Duty Cycle vs R1 + R2
4
R1 + R2 2 ()
Power Derating Curve
mA
NE5560 Voltage-/Current-Fed Supply Characteristics
Current-Fed Dropping Resistor
1.0 20
VS
Pd MAX (W)
SE VCC 50 NE
RVCC 1 10 VCC V R VCC + *V S CC (10 20mA)
24
GND 12 0 -60C 25C 70C 125C 0 10 20 VCC 30 TA OPERATING CURVE V
SEE DC ELECTRICAL CHARACTERISTICS FOR CURRENT FED VCC RANGE
VO/VREF (%) 7 6 5 4 3 2 RS 3 VREF(3.72V) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2
Regulation vs Error Amp Closed Gain
R 4 R (%) f + 20 100 90 - + R R f + 100 80 70 60 50 R R f + 500 40 30 20 10
Transfer Curve of Pulse-Width Modular Duty Cycle vs Input Voltage
R1
S
S
S
0.1 10 20 30 40 50 60 70 80 90
0
1
2
3
4
5
6
V4,5,6 (V) SL00363
Figure 4. Typical Performance Characteristics
1994 Aug 31
6
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
Typical Frequency Plot vs RT and CT
100 90 R=5k 80 R=10k 70 DUTY CYCLE R=20k 60 50 40 30 20 10 V 16 2 2.5 3 3.5 4 4.5 CnF) 1 1.5 2 2.5 VZ
f(kHz) 1000
100 90 80 70 60 50 40 30 20
R=40k 10 9 8 7 6 5 4 3 2 1
SL00364
Figure 5. Typical Performance Characteristics
THEORY OF OPERATION
The following functions are incorporated: - A temperature-compensated reference source. - An error amplifier with Pin 3 as input. The output is connected to Pin 4 so that the gain is adjustable with external resistors. - A sawtooth generator with a TTL-compatible synchronization input (Pins 7, 8, 9). - A pulse-width modulator with a duty cycle range from 0 to 95%. The PWM has two additional inputs: Pin 6 can be used for a precise setting of MAX Pin 5 gives a direct access to the modulator, allowing for real constant-current operation: - A gate at the output of the PWM provides a simple dynamic current limit. - A latch that is set by the flyback of the sawtooth and reset by the output pulse of the above mentioned gate prohibits double pulsing. - Another latch functions as a start-stop circuit; it provides a fast switch-off and a slow start. - A current protection circuit that operates via the start-stop circuit. This is a combined function with the current limit circuit, therefore Pin 11 has two trip-on levels; the lower one for cycle-by-cycle current limiting, the upper one for current protection by means of switch-off and slow-start. - A TTL-compatible remote on/off input at Pin 10, also operating via the start-stop circuit. - An inhibit input at Pin 13. The output pulse can be inhibited immediately. - An output gate that is commanded by the latches and the inhibit circuit.
- An output transistor of which both the collector (Pin 15) and the emitter (Pin 14) are externally available. This allows for normal or inverse output pulses. - A power supply that can be either voltage- or current-driven (Pins 1 and 12). The internally-generated stabilized output voltage VZ is connected to Pin 2. - A special function is the so-called feed-forward at Pin 16. The amplitude of the sawtooth generator is modulated in such a way that the duty cycle becomes inversely proportional to the voltage on this pin: ~ 1/V16. - Loop fault protection circuits assure that the duty cycle is reduced to zero or a low value for open- or short-circuited feedback loops.
Stabilized Power Supply (Pins 1, 2, 12)
The power supply of the NE5560 is of the well known series regulation type and provides a stabilized output voltage of typically 8.5V. This voltage VZ is also present at Pin 2 and can be used for precise setting of MAX and to supply external circuitry. Its max. current capability is 5mA. The circuit can be fed directly from a DC voltage source between 10.5V and 18V or can be current-driven via a limiting resistor. In the latter case, internal pinch-off resistors will limit the maximum supply voltage: typical 23V for 10mA and max. 30V for 30mA. The low supply voltage protection is active when V(1-12) is below 10.5V and inhibits the output pulse (no hysteresis). When the supply voltage surpasses the 10.5V level, the IC starts delivering output pulses via the slow-start function.
1994 Aug 31
7
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
The current consumption at 12V is less than 10mA, provided that no current is drawn from VZ and R(7-12)20k.
The Sawtooth Generator
Figure 6 shows the principal circuitry of the oscillator. A resistor between Pin 7 and Pin 12 (GND) determines the constant current that charges the timing capacitor C(8-12). This causes a linear increasing voltage on Pin 8 until the upper level of 5.6V is reached. Comparator H sets the RS flip-flop and Q1 discharges C(8-12) down to 1.1V, where comparator L resets the flip-flop. During this flyback time, Q2 inhibits the output. Synchronization at a frequency lower than the free-running frequency is accomplished via the TTL gate on Pin 9. By activating this gate (V9<2V), the setting of the sawtooth is prevented. This is indicated in Figure 7. Figure 8 shows a typical plot of the oscillator frequency against the timing capacitor. The frequency range of the NE5560 goes from <50Hz up to >100kHz.
the voltage drop over R(3-4). As a result, the duty cycle will become zero, provided that R(3-4)>100k. When the feedback loop is short-circuited, the duty cycle would jump to the adjusted maximum duty cycle. Therefore, an additional comparator is active for feedback voltages at Pin 3 below 0.6V. Now an internal resistor of typically 1k is shunted to the impedance on the MAX setting Pin 6. Depending on this impedance, will be reduced to a value 0. This will be discussed further.
The Pulse-Width Modulator
The function of the PWM circuit is to translate a feedback voltage into a periodical pulse of which the duty cycle depends on that feedback voltage. As can be seen in Figure 10, the PWM circuit in the NE5560 is a long-tailed pair in which the sawtooth on Pin 8 is compared with the LOWEST voltage on either Pin 4 (error amplifier), Pin 5, or Pin 6 (MAX and slow-start). The transfer graph is given in Figure 11. The output of the PWM causes the resetting of the output bi-stable.
Limitation of the Maximum Duty Cycle
With Pins 5 and 6 not connected and with a rather low feedback voltage on Pin 3, the NE5560 will deliver output pulses with a duty cycle of 95%. In many SMPS applications, however, this high will cause problems. Especially in forward converters, where the transformer will saturate when exceeds 50%, a limitation of the maximum duty cycle is a must. A DC voltage applied to Pin 6 (PWM input) will set MAX at a value in accordance with Figure 11. For low tolerances of MAX, this voltage on Pin 6 should be set with a resistor divider from VZ (Pin 2). The upper and lower sawtooth levels are also set by means of an internal resistor divider from VZ, so forming a bridge configuration with the MAX setting is low because tolerances in VZ are compensated and the sawtooth levels are determined by internal resistor matching rather than by absolute resistor tolerance. Figure 12 can be used for determining the tap on the bleeder for a certain MAX setting. As already mentioned, Figure 13 gives a graphical representation of this. The value o is limited to the lower and the higher side; mum input voltage the resulting feedback voltage on Pin 3 exceeds 0.6V.
Reference Voltage Source
The internal reference voltage source is based on the bandgap voltage of silicon. Good design practice assures a temperature dependency typically 100ppm/C. The reference voltage is connected to the positive input of the error amplifier and has a typical value of 3.72V.
Error Amplifier Compensation
For closed-loop gains less than 40dB, it is necessary to add a simple compensation capacitor as shown in Figures 8 and 9.
Error Amplifier with Loop-Fault Protection Circuits
This operational amplifier is of a generally used concept and has an open-loop gain of typically 60dB. As can be seen in Figure 9, the inverting input is connected to Pin 3 for a feedback information proportional to VO. The output goes to the PWM circuit, but is also connected to Pin 4, so that the required gain can be set with RS and R(3-4). This is indicated in Figure 9, showing the relative change of the feedback voltage as a function of the duty cycle. Additionally, Pin 4 can be used for phase shift networks that improve the loop stability. When the SMPS feedback loop is interrupted, the error amplifier would settle in the middle of its active region because of the feedback via R(3-4). This would result in a large duty cycle. A current source on Pin 3 prevents this by pushing the input voltage high via
* It must be large enough to ensure that at maximum load and mini* It must be small enough to limit the amount of energy in the SMPS
when a loop fault occurs. In practice, a value of 10-15% will be a good compromise.
VZ
TO PWM
5.6V
- N + SET
Q1 TO OUTPUT LATCH
7 8 RT CT 1.1V
- L + RESET
Q2
9 SYN
SL00365
Figure 6. Sawtooth Generator 1994 Aug 31 8
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
5.6V
SET VS
1.1V INHIBIT `SET'
RESET
>2V VS <0.8V
SL00366
Figure 7. Sawtooth Oscillator Synchronization
60dB
SLOPE 20dB/DECADE
1kHz
10kHz
1MHz
SL00367
Figure 8. Error Amplifier Compensation Open-Loop Gain
3.72V
START/ STOP + ERROR AMP -
3 (+)
PWM
OUT RESET (-) (-) O.C. 4 5 (-) 6
8
SL00368
Figure 9. Error Amplifier
1994 Aug 31
9
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
VO/VREF (%) 7 6 5 4 3 2 RS 3 VREF(3.72V) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 - + R R f + 100 R 4 R1 R f
+ 20
S
S
R R
f
+ 500
S
0.1 10 20 30 40 50 60 70 80 90
a. Duty Cycle -- -- % Regulation
3.72V START/ STOP + ERROR AMP -
3 (+)
PWM
OUT RESET (-) (-) O.C. 4 5 (-) 6
8
b. Pulse-Width Modulation Figure 10.
(%) 100 90 80 70 60 50 40 30 20 10 0 1 2 3 4 5 6 V4,5,6 (V)
SL00369
Extra PWM Input (Pin 5)
The PWM has an additional inverting input: Pin 5. It allows for attacking the duty cycle via the PWM circuit, independently from the feedback and the MAX information. This is necessary when the SMPS must have a real constant-current behavior, possibly with a fold-back characteristic. However, the realization of this feature must be done with additional external components. When not used, Pin 5 should be tied to Pin 6.
SL00370
Figure 11. Transfer Curve of Pulse-Width Modulator Duty Cycle vs Input Voltage
1994 Aug 31
10
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
MAX (%) 100 90 80 MAXIMUM DUTY CYCLE (%) 70 60 50 40 30 20 10 R2 R1 * R2 12 R2 R1 DUTY CYCLE 6 CONTROL 2
0
0.1 0.2
0.3
0.4
0.5 0.6
0.7
0.8
0.9
1
SL00371
Figure 12. Graph for Determining MAX
(%)
SOFT START DUTY CYCLE %
80 70 60 50 40 30 20 10 103 6 7 8 9 10 MAX 50% MAX 30% MAX 90% MAX 70%
2
3
4
5
4
R1 + R2 2 ()
SL00372
Figure 13. Soft-Start Minimum Duty Cycle vs R1 + R2
0.6V
0.48V from PWM
START STOP
RESET OF OUTPUT BISTABLE
11
SL00373
Figure 14. Current Protection Input
1994 Aug 31
11
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
OC1
NORMAL OPERATING POINT RELATIVE I OUT (mA)
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.
OC2
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 (MAX setting).
SL00374
V11 (CURRENT LIMITING)
LEVEL 1 LEVEL 2 .48 .60 (V)
Figure 15. Output Characteristics
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
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 VZ will charge the capacitor via R1 to the normal MAX voltage. The output starts 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 MAX setting on Pin 6.
PWM 2 VZ LATCH 15
R1 6 Q1 R2 CSS RESET 0.6V 12 Q2 100 START/ STOP SET LOW SUPPLY VOLTAGE PROTECTION 14
0.48V
0.6V
11
10
SL00375
Figure 16. Start-Stop Circuit 1994 Aug 31 12
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
5.6V V6 SAWTOOTH 1.1V DISCHARGE DEAD TIME V15 CHARGE .6V
INCREASES SET RESET
SL00376
Figure 17. Start-Stop Circuit
Remote On/Off Circuit (Pin 10)
In systems where two or more power supplies are used, it is often necessary to switch these supplies on and off in a sequential way. Furthermore, there are many applications in which a supply must be switched by a logical signal. This can be done via the TTL-compatible remote on/off input on Pin 10. The output pulse is inhibited for levels below 0.8V. The output of the IC is no longer blocked when the remote on/off input is left floating or when a voltage >2V is applied. Start-up occurs via the slow-start circuit.
VZ
V1
FLYBACK SET 15 RESET 14
The Output Stage
The output stage of the NE5560 contains a flip-flop, a push-pull driven output transistor, and a gate, as indicated in Figure 18. The flip-flop is set by the flyback of the sawtooth. Resetting occurs by a signal either from the PWM or the current limit circuit. With this configuration, it is assured that the output is switched only once per period, thus prohibiting double pulsing. The collector and emitter of the output transistor are connected to respectively Pin 15 and Pin 14, allowing for normal or inverted output pulses. An internally-grounded emitter would cause intolerable voltage spikes over the bonding wire, especially at high output currents. This current capability of the output transistor is 40mA peak for VCE 0.4V. An internal clamping diode to the supply voltage protects the collector against overvoltages. The max. voltage at the emitter (Pin 14) must not exceed +5V. A gate, activated by one of the set or reset pulses, or by a command from the start-stop circuit will immediately switch-off the output transistor by short-circuiting its base. The external inhibitor (Pin 13) operates also via this base. Demagnetization Sense As indicated in Figure 18, the output of this NPN comparator will block the output pulse, when a voltage above 0.6V is applied to Pin 13. A specific application for this function is to prevent saturation of forward-converter transformers. This is indicated in Figure 19. Feed-Forward (Pin 16) The basic formula for a forward converter is V OUT + dV IN n (n + transformer ratio)
H VZ + - 13 0.6V
FROM START STOP NOTES: The signal V13 can be derived from the demagnetizing winding in a forward converter as shown below.
B + P1 P2 - "I" "I" "n" S1
SL00377
Figure 18. Output Stage This loop now only has to regulate for load variations which require only a low feedback gain in the normal operation area. The transformer of a forward converter must be designed in such a way
This means that in order to keep VOUT at a constant value, the duty cycle must be made inversely proportional to the input voltage. A pre-regulation (feed-forward) with the function ~1/VIN can ease the feedback-loop design.
1994 Aug 31
13
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
that it does not saturate, even under transient conditions, where the max. inductance is determined by MAXxVIN max. A regulation of MAX~1/VIN will allow for a considerable reduction or simplification of the transformer. The function of ~1/VIN can be realized by using Pin 16 of the NE5560.
FEED-FORWARD INPUT
16
DO NOT EXCEED VCC
2
R1
6 CSS R2 ON ON ON
MAX
7
RT
8 CT
1
1 T
2 (50) T
2
3 T
3
SL00379
Figure 20. External Maximum Control
SL00378
Figure 19. Output Stage Inhibit
V16 2XVZ
Figure 20 shows the electrical realization. When the voltage at Pin 16 exceeds the stabilized voltage VZ (Pin 2), it will increase the charging current for the timing capacitor on Pin 8. The operating frequency is not affected, because the upper trip level for sawtooth increases also. Note that the MAX voltage on Pin 6 remains constant because it is set via VZ. Figure 21 visualizes the effect on MAX and the normal operating duty cycle . For V16=2xVZ, these duty cycles have halved. The graph for =f(V16) is given in Figure 22.
NOTE: V16 must be less than Pin 1 voltage. VZ d MAX d MAX
1
2
WORKING LEVEL T T
MAX LEVEL
APPLICATIONS NE/SE5560 Push-Pull Regulator
This application describes the use of the Philips Semiconductors NE/SE5560 adapted to function as a push-pull switched mode regulator, as shown in Figures 23 and 24.
100
SL00380
Figure 21. Feed-Forward Circuitry
Input voltage range is +12V to +18V for a nominal output of +30V and -30V at a maximum load current of 1A with an average efficiency of 81%. Features include feed-forward input compensation, cycle-to-cycle drive current protection and other voltage sensing, line (to positive output) regulation <1% for an input range of +13V to +18V and load regulation to positive output of <3% for IL(+) of 0.1 to 1A. The main pulse-width modulator operates to 48kHz with power switching at 24kHz.
DUTY CYCLE
90 80 70 60 50 40 30 20 10 1 1.5 2 2.5 V 16 VZ
SL00381
Figure 22. Feed-Forward Regulation
1994 Aug 31
14
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
NOTES: Power ground and signal ground must be kept separated T1, Primary = 130T (C.T.) #26 Secondary = 18T (C.T.) #22 Core = Ferroxcube 3622 3C8 material C.T. = 50T #26 0n Ferroxcube 2616 core (3C8) F2D bobbin T2, Primary = 16T (C.T.) #18 Secondaries (each) 52T (C.T.) #22 Core = Ferroxcube 4229 3C8 material L1, L2 = 120T #20 on single gapped EC35 Ferroxcube core. 3C8 material.
SL00382
Figure 23. NE/SE5560 Push-Pull Switched-Mode Regulated Supply with CMOS Drive Conversion Logic
1994 Aug 31
15
Philips Semiconductors
Product specification
Switched-mode power supply control circuit
NE/SE5560
NOTES: Power ground and signal ground must be kept separated T1, Primary = 130T (C.T.) #26 Secondary = 18T (C.T.) #22 Core = Ferroxcube 3622 3C8 material C.T. = 50T #26 0n Ferroxcube 2616 core (3C8) F2D bobbin T2, Primary = 16T (C.T.) #18 Secondaries (each) 52T (C.T.) #22 Core = Ferroxcube 4229 3C8 material L1, L2 = 120T #20 on single gapped EC35 Ferroxcube core. 3C8 material.
SL00383
Figure 24. NE/SE5560 Push-Pull Switched-Mode Regulated With TTL Drive Conversion Logic
1994 Aug 31
16

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