ONSEMI NCP1378P

NCP1378
PWM Current−Mode
Controller for Free−Running
Quasi−Resonant Operation
The NCP1378 combines a true current mode modulator and
a demagnetization detector to ensure full borderline/critical
Conduction Mode in any load/line conditions and minimum drain
voltage switching (Quasi−Resonant operation). Due to its inherent
skip cycle capability, the controller enters burst mode as soon as the
power demand falls below a predetermined level. As this happens at
low peak current, no audible noise can be heard. An internal 8.0 s
timer prevents the free−run frequency to exceed 100 kHz (therefore
below the 150 kHz CISPR−22 EMI starting limit), while the skip
adjustment capability lets the user select the frequency at which the
burst foldback takes place.
The transformer core reset detection is done through an auxiliary
winding which, brought via a dedicated pin, also enables fast Over
Voltage Protection (OVP). Once an OVP has been detected, the IC
permanently latches off.
The NCP1378 also features an efficient protective circuitry that, in
presence of an overcurrent condition, disables the output pulses and
enters a safe burst mode, trying to restart. Once the default has gone,
the device auto−recovers. Finally an internal 1.0 ms Soft−Start
eliminates the traditional startup stress.
The NCP1378 is tailored for low voltage applications having
UVLO thresholds of 8.4 V (on) and 7.5 V (off).
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MARKING
DIAGRAMS
8
SOIC−8
D SUFFIX
CASE 751
8
1
1
Pb−Free Package is Available*
Free−Running Borderline/Critical Mode Quasi−Resonant Operation
Latched Overvoltage Protection
Auto−Recovery Short−Circuit Protection Via UVLO Crossover
Current−Mode with Adjustable Skip Cycle Capability
Internal 1.0 ms Soft−Start
Internal Temperature Shutdown
Internal Leading Edge Blanking
500 mA Peak Current Source/Sink Capability
External Latch Triggering, e.g. Via Overtemperature Signal
Direct Optocoupler Connection
SPICE Models Available for TRANsient Analysis
Internal 8.0 s Minimum TOFF
Typical Applications
• Battery−Based Operations
8
1
1
A
WL, L
YY, Y
WW, W
 Semiconductor Components Industries, LLC, 2004
October, 2004 − Rev. 1
1
= Assembly Location
= Wafer Lot
= Year
= Work Week
PIN CONNECTIONS
Dmg 1
8 HV
FB 2
CS 3
6 VCC
GND 4
5 Drv
(Top View)
ORDERING INFORMATION
Package
Shipping†
NCP1378DR2
SOIC−8
2500 Tape & Reel
NCP1378P
PDIP−7
50 Units/Tube
PDIP−7
(Pb−Free)
50 Units/Tube
Device
NCP1378PG
*For additional information on our Pb−Free strategy and soldering details, please
download the ON Semiconductor Soldering and Mounting Techniques Reference
Manual, SOLDERRM/D.
1378P
AWL
YYWW
PDIP−7
P SUFFIX
CASE 626B
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
1378
ALYW
†For information on tape and reel specifications,
including part orientation and tape sizes, please
refer to our Tape and Reel Packaging Specifications
Brochure, BRD8011/D.
Publication Order Number:
NCP1378/D
NCP1378
R*
+
12 V @ 1 A
GND
OVP and
Demag
+
Universal
Network
NCP1378
1
8
2
7
3
6
4
5
+
*Please refer to the application information section.
Y1 Type
Figure 1. Typical Application Schematic
PIN FUNCTION DESCRIPTION
Pin
Symbol
Function
Description
1
Demag
Core reset detection and OVP
The auxiliary FLYBACK signal ensures discontinuous operation and offers a
fixed overvoltage detection level of 5.2 V.
2
FB
Sets the peak current setpoint
By connecting an optocoupler to this pin, the peak current setpoint is
adjusted accordingly to the output power demand. By bringing this pin below
the internal skip level, you shut off the device.
3
CS
Current sense input and skip
cycle level selection
This pin senses the primary current and routes it to the internal comparator
via an L.E.B. By inserting a resistor in series with the pin, you control the
level at which the skip operation takes place.
4
GND
The IC ground
−
5
Drv
Driving pulses
The driver’s output to an external MOSFET.
6
VCC
Supplies the IC
This pin is connected to an external bulk capacitor of typically 47 F.
7
NC
−
8
HV
High−voltage pin
This unconnected pin ensures adequate creepage distance.
Connected to the high−voltage rail, this pin injects a constant current into the
VCC bulk capacitor and ensures a clean lossless startup sequence.
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2
NCP1378
4.5 s
Delay
Demag
Resd
HV
Demag
+
8 s
Blanking
PON
4 mA
5.2 V
+
10 V
50 mV
+
+
S
+
OVP
VCC
+
Q
VCC
Drv
S*
Q
R* R
8.4 V
7.5 V
5.6 V (Fault)
To Internal
Supply
Driver src = 20 sink = 10
4.2 V
Fault
Mngt.
FB
Soft−Start = 1 ms
+
−
GND
/3
1V
200 A
when DRV
is OFF
Overload?
5 s
Timeout
340 ns
LEB
Timeout
Reset
CS
Demag
*S and R are level triggered whereas S is edge
triggered. R has priority over the other inputs.
Figure 2. Internal Circuit Architecture
MAXIMUM RATINGS
Rating
Symbol
Value
Unit
VCC, Drv
16
V
Maximum Voltage on all other pins except Pin 8 (HV), Pin 6 (VCC) and Pin 5 (Drv)
−
−0.3 to 10
V
Maximum Current into all pins except VCC (6), HV (8) and Demag (1) when 10 V ESD
diodes are activated
−
5.0
mA
Maximum Current in Pin 1
Idem
+3.0/−2.0
mA
Thermal Resistance, Junction−to−Case
RJC
57
°C/W
Thermal Resistance, Junction−to−Air, SOIC Version
RJA
178
°C/W
Thermal Resistance, Junction−to−Air, PDIP Version
RJA
100
°C/W
TJMAX
150
°C
Temperature Shutdown
−
155
°C
Hysteresis in Shutdown
−
30
°C
Storage Temperature Range
−
−60 to +150
°C
ESD Capability, HBM Model (All pins except VCC and HV)
−
2.0
kV
ESD Capability, Machine Model
−
200
V
VHV
500
V
Power Supply Voltage
Maximum Junction Temperature
Maximum Voltage on Pin 8 (HV), Pin 6 (VCC) Decoupled to Ground with 10 F
Maximum ratings are those values beyond which device damage can occur. Maximum ratings applied to the device are individual stress limit values
(not normal operating conditions) and are not valid simultaneously. If these limits are exceeded, device functional operation is not implied, damage
may occur and reliability may be affected.
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3
NCP1378
ELECTRICAL CHARACTERISTICS (For typical values Tj = 25°C, for min/max values Tj = 0°C to +125°C, Max Tj = 150°C, VCC = 11 V
unless otherwise noted.)
Characteristic
Pin
Symbol
Min
Typ
Max
Unit
VCC Increasing Level at which the Current Source Turns−Off
6
VCCON
7.8
8.4
9.0
V
VCC Decreasing Level at which the Current Source Turns−On
6
VCCmin
7.0
7.5
8.2
V
VCC Excursion between VCCON and VCCmin
6
VCChyst
0.8
−
−
−
VCC Decreasing Level at which the Latchoff Phase Ends
6
VCClatch
−
5.5
−
V
Internal IC Consumption, No Output Load on Pin 5, FSW = 60 kHz
6
ICC1
−
1.0
1.3
(Note 1)
mA
Internal IC Consumption, 1.0 nF Output Load on Pin 5, FSW = 60 kHz
6
ICC2
−
1.6
2.0
(Note 1)
mA
Internal IC Consumption, Latchoff Phase, VCC = 6.0 V
6
ICC3
−
220
−
A
High−Voltage Current Source, VCC = 7.8 V
8
IC1
2.4
4.0
6.0
mA
High−Voltage Current Source, VCC = 0
8
IC2
−
4.5
−
mA
Output Voltage Rise−Time @ CL = 1.0 nF, 10−90% of Output Signal
5
Tr
−
40
−
ns
Output Voltage Fall−Time @ CL = 1.0 nF, 10−90% of Output Signal
5
Tf
−
20
−
ns
Source Resistance
5
ROH
10
20
36
Sink Resistance
5
ROL
4.0
10
20
Input Bias Current @ 1.0 V Input Level on Pin 3
3
IIB
−
0.02
−
A
Maximum Internal Current Setpoint
3
ILimit
0.9
1.0
1.1
V
Propagation Delay from Current Detection to Gate OFF State
3
TDEL
−
110
160
ns
Leading Edge Blanking Duration
3
TLEB
−
340
−
ns
Internal Current Offset Injected on the CS Pin During OFF Time
3
Iskip
−
200
−
A
Sampling Delay After ON Time
1
Tsample
−
4.5
−
s
OVP Internal Reference Level
1
Vref
4.6
5.2
6.3
V
Internal Pullup Resistor
2
Rup
−
20
−
k
Pin 3 to Current Setpoint Division Ratio
−
Iratio
−
3.3
−
−
Internal Soft−Start
−
Tss
−
1.0
−
ms
Input Threshold Voltage (Vpin 1 Decreasing)
1
Vth
30
50
90
mV
Hysteresis (Vpin 1 Decreasing)
1
VH
−
20
−
mV
Input Clamp Voltage
High State (Ipin 1 = 3.0 mA)
Low State (Ipin 1 = −2.0 mA)
1
1
VCH
VCL
8.0
−0.9
10
−0.7
12
−0.5
Demag Propagation Delay
1
Tdem
−
240
−
ns
Internal Input Capacitance at Vpin 1 = 1.0 V
1
Cpar
−
10
−
pF
Internal Blanking Delay after TON
1
Tblank
−
8.0
−
s
SUPPLY SECTION
INTERNAL STARTUP CURRENT SOURCE (Tj 0°C)
DRIVE OUTPUT
CURRENT COMPARATOR (Pin 5 not loaded)
OVERVOLTAGE SECTION (VCC = 11 V)
FEEDBACK SECTION (VCC = 11 V, Pin 5 loaded by 1.0 k)
DEMAGNETIZATION DETECTION BLOCK
V
1. Max value at Tj = 0°C, please see characterization curves.
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4
NCP1378
8.8
8.0
8.6
7.8
VCCMIN, (V)
VCCON, (V)
TYPICAL CHARACTERISTICS
8.4
8.2
8.0
7.4
7.2
7.8
−25
0
25
50
75
100
7.0
−25
125
0
25
50
75
100
125
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 3. VCCON Threshold versus Temperature
Figure 4. VCCMIN Threshold versus Temperature
1.6
2.2
1.4
2.0
1.2
1.8
ICC2, (mA)
ICC1, (mA)
7.6
1.0
1.6
0.8
1.4
0.6
1.2
0.4
−25
0
25
50
75
100
1.0
−25
125
0
25
50
75
100
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 5. Current Consumption (No Load)
versus Temperature
Figure 6. Current Consumption (1.0 nF Load)
versus Temperature
125
1.10
8
7
1.05
5
Ilimit, (V)
IC1, (mA)
6
4
1.00
3
0.95
2
1
0
−25
0
25
50
75
100
0.90
−25
125
0
25
50
75
100
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 7. HV Current Source at VCC = 10 V
versus temperature
Figure 8. Maximum Current Setpoint versus
Temperature
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5
125
NCP1378
TYPICAL CHARACTERISTICS
40
20
35
18
16
30
ROL, ()
ROH, ()
14
25
20
15
12
10
8
6
10
4
5
2
0
−25
0
25
50
75
100
0
−25
125
0
TEMPERATURE (°C)
25
50
75
100
125
TEMPERATURE (°C)
Figure 9. Drive Source Resistance versus
Temperature
Figure 10. Drive Sink Resistance versus
Temperature
7.0
120
6.5
100
6.0
5.5
Vref, (V)
VTH, (mV)
80
60
5.0
4.5
40
4.0
20
3.5
0
−25
0
25
50
75
100
3.0
−25
125
0
TEMPERATURE (°C)
9.5
6.5
9.0
6.0
TOUT, (s)
TOFF, (s)
7.0
8.5
8.0
7.0
4.0
75
125
5.0
4.5
50
100
5.5
7.5
25
75
Figure 12. OVP Threshold versus Temperature
10.0
0
50
TEMPERATURE (°C)
Figure 11. Demagnetization Detection Threshold
versus Temperature
6.5
−25
25
100
125
3.5
−25
TEMPERATURE (°C)
0
25
50
75
100
125
TEMPERATURE (°C)
Figure 13. Minimum TOFF versus Temperature
Figure 14. Demagnetization Detection Timeout
versus Temperature
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6
NCP1378
APPLICATION INFORMATION
• Adjustable Skip Cycle Level: By offering the ability
INTRODUCTION
The NCP1378 implements a standard current mode
architecture where the switch−off time is dictated by the
peak current setpoint, whereas the core reset detection
triggers the turn−on event . This component represents the
ideal candidate where low part−count is the key parameter
in applications supplied by a battery. Due to its high−
performance High−Voltage technology, the NCP1378
incorporates all the necessary components/features needed
to build a rugged and reliable Switchmode Power Supply
(SMPS):
• Transformer Core Reset Detection: Borderline/critical
operation is ensured whatever the operating conditions
are. As a result, there are virtually no primary switch
turn−on losses and no secondary diode recovery
losses. The converter also stays a first−order system
and accordingly eases the feedback loop design.
• Quasi−Resonant Operation: By delaying the turn−on
event, it is possible to restart the MOSFET in the
minimum of the drain−source wave, ensuring reduced
EMI/video noise perturbations. In nominal power
conditions, the NCP1378 operates in Borderline
Conduction Mode (BCM) also called Critical
Conduction Mode.
• Undervoltage Lockout (UVLO): When VCC falls
below UVLO, all pulses are stopped and the IC
consumption drops down to a few hundreds of A
(ICC3 data). When VCC reaches the latchoff level
(5.5 V typical), the startup current source is activated
and brings VCC back to VCCON where the IC attempts
to startup.
• Overvoltage Protection (OVP): By sampling the
plateau voltage on the demagnetization winding, the
NCP1378 goes into latched fault condition whenever
an over−voltage condition is detected. The controller
stays fully latched in this position until the VCC is
cycled down to 4.0 V, e.g. when the user unplugs the
power supply from the mains outlet and replugs it.
• External LatchTrip Point: By externally forcing a
level on the OVP greater than the internal setpoint, it
is possible to latchoff the IC, e.g. with a signal coming
from a temperature sensor.
•
to tailor the level at which the skip cycle takes place,
the designer can make sure that the skip operation
only occurs at low peak current. This point guarantees
a noise−free operation with cheap transformer. This
option also offers the ability to fix the maximum
switching frequency when entering light load conditions.
Overcurrent Protection (OCP): NCP1378 enters burst
mode as soon as the power supply undergoes an
overload, which is detected through the sense of the
auxiliary voltage. As detailed above, as soon as VCC
crosses the UVLO level (called VCCmin in the
electrical table), all pulses are stopped and the device
enters a safe low power operation that prevents from
any lethal thermal runaway. By monitoring the VCC
level, the startup current source is activated ON and
OFF to create a kind of burst mode where the SMPS
tries to restart. If the fault has gone, the SMPS
resumes operation. On the other hand, if the fault is
still there, the burst sequence starts again.
Startup Sequence
When the power supply is first powered from the mains
outlet, the internal current source (typically 4.0 mA) is
biased and charges up the VCC capacitor. When the voltage
on this VCC capacitor reaches the VCCON level (typically
8.4 V), the current source turns off and no longer wastes
any power. At this time, the VCC capacitor only supplies the
controller and the auxiliary supply is supposed to take over
before VCC collapses below VCCmin. Figure 15 shows the
internal arrangement of this structure.
8
VCCON/VCC-
+
min
−
HV
IC1 or 0
6
SVCC
Aux
4
Figure 15. The Current Source Brings Vcc Above
VccON and Then Turns Off
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7
NCP1378
Skipping Cycle Mode
Once the power supply has started, the Vcc shall be
constrained below 16 V, which is the maximum rating on
pin 6. Figure 16 portrays a typical NCP1378 startup
sequence with a Vcc regulated at 8.0 V.
The NCP1378 automatically skips switching cycles
when the output power demand drops below a given
level. This is accomplished by monitoring the FB pin. In
normal operation, pin 2 imposes a peak current accordingly
to the load value. If the load demand decreases, the internal
loop asks for less peak current. When this setpoint reaches
a determined level, the IC prevents the current from
decreasing further down and starts to blank the output
pulses: the IC enters the so−called skip cycle mode, also
named controlled burst operation. The power transfer
now depends upon the width of the pulse bunches
(Figure 17) and follows the following formula:
9.0
8.4 V
Regulation
VCC
8.0
7.0
1 · Lp · Ip2 · Fsw · D
burst with:
2
6.0
Lp = Primary inductance
Fsw = Switching frequency within the burst
Ip = Peak current at which skip cycle occurs
5.0
3.00M
8.00M
13.0M
time in secs
18.0M
23.0M
Dburst = Burst width/burst recurrence
Figure 16. A Typical Startup Sequence
for the NCP1378
CURRENT SENSE SIGNAL (mV)
Normal Current Mode Operation
300
MAX PEAK
CURRENT
200
DRIVER = HIGH ? I = 0
DRIVER = LOW ? I = 200 A
SKIP CYCLE
CURRENT LIMIT
RESET
100
Rskip
−
+
3
Rsense
2
0
WIDTH
+
RECURRENCE
Figure 17. The Skip Cycle Takes Place at Low Peak
Currents which Guarantees Noise−Free Operation
The skip level selection is done through a simple resistor
inserted between the current sense input and the sense
element. Every time the NCP1378 output driver goes low,
a 200 A source forces a current to flow through the sense
pin (Figure 18): when the driver is high, the current source
is off and the current sense information is normally
processed. As soon as the driver goes low, the current
source delivers 200 A and develops a ground referenced
voltage across Rskip. If this voltage is below the feedback
voltage, the current sense comparator stays in the low state
and the internal latch can be triggered by the next clock
cycle. Now, if because of a low load mode the feedback
Figure 18. A Patented Method Allows for Skip
Level Selection via a Series Resistor Inserted in
Series with the Current
voltage is below Rskip level, then the current sense
comparator permanently resets the latch and the next clock
cycle (given by the demagnetization detection) is ignored:
we are skipping cycles as shown by Figure 17. As soon as
the feedback voltage goes up again, there can be two
situations: the recurrent period is small and a new
demagnetization detection (next wave) signal triggers the
NCP1378. To the opposite, in low output power conditions,
no more ringing waves are present on the drain and the
toggling of the current sense comparator alone initiates a
new cycle start. Figure 19 depicts these two different
situations.
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8
NCP1378
Drain
Signal
Timeout
Signal
Demag RESTART
Current Sense and Timeout Restart
Drain
Signal
Timeout
Signal
5 s
5 s
Figure 19. When the primary natural ringing becomes too low, the current
sense initiates a new cycle when FB passes the skip level.
Demagnetization Detection
The core reset detection is done by monitoring the
voltage activity on the auxiliary winding. This voltage
features a FLYBACK polarity. The typical detection level
is fixed at 50 mV as exemplified by Figure 20.
DEMAG SIGNAL (V)
7.0
5.0
POSSIBLE
RESTARTS
TO INTERNAL
COMPARATOR
3.0
Resd
2k
3
1.0
ESD
50 mV
0V
Rdem
4
1
ESD
4
−1.0
Figure 20. Core Reset Detection is Done through
a Dedicated Auxiliary Winding Monitoring
5
2
Aux
1
Figure 21. Internal Pad Implementation
An internal timer prevents any restart within 8.0 s
further to the driver going−low transition. This prevents the
switching frequency to exceed (1.0/TON + 8.0 s) but also
avoid false leakage inductance tripping at turn−off. In some
cases, the leakage inductance kick is so energetic, that a
slight filtering is necessary.
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9
NCP1378
Figure 23 shows where the sampling occurs on the
auxiliary winding.
The NCP1378 demagnetization detection pad features a
specific component arrangement as detailed by Figure 21.
In this picture, the zener diodes network protect the IC
against any potential ESD discharge that could appear on
the pins. The first ESD diode connected to the pad, exhibits
a parasitic capacitance. When this parasitic capacitance
(10 pF typically) is combined with Rdem, a restart delay is
created and the possibility to switch right in the
drain−source wave exists. This guarantees QR operation
with all the associated benefits (low EMI, no turn−on losses
etc.). Rdem should be calculated to limit the maximum
current flowing through pin 1 to less than
+3.0 mA/−2.0 mA: If during turn−on, the auxiliary
winding delivers 30 V (at the highest line level), then the
minimum Rdem value is defined by: 30 + 0.7/3.0 mA =
10.2 k. This value will be further increased e.g. to
introduce a restart delay and also a slight filtering in case
of high leakage energy.
Figure 22 portrays a typical VDS shot at nominal output
power.
SAMPLING HERE
DEMAG SIGNAL (V)
8.0
4.0
2.0
4.5 s
0
Figure 23. A Voltage Sample is Taken 4.5 s
After the Turn−Off Sequence
When an OVP condition has been detected, the
NCP1378 enters a latchoff phase and stops all switching
operations. The controller stays fully latched in this
position and the startup source being still active, it keeps
the VCC going up and down between 8.4 V and 5.5 V. This
state lasts until the VCC is cycled down to 4.0 V, e.g. when
the user unplugs the power supply from the mains outlet.
By default, the OVP comparator is biased to a 5.2 V
reference level and pin1 is directly routed to the
comparator. As a result, when Vpin1 reaches 5.2 V, the
OVP comparator is triggered. The threshold can thus be
adjusted by either modifying the power winding to
auxiliary winding turn ratios to match this 5.2 V level or
insert a resistor from pin1 to ground to cope with your
design requirement.
400
DRAIN VOLTAGE (V)
6.0
300
200
100
0
Figure 22. The NCP1378 Operates in
Borderline/Critical Operation
Latching Off the NCP1378
In certain cases, it can be very convenient to externally
shut down permanently the NCP1378 via a dedicated
signal, e.g. coming from a temperature sensor (Figure 24).
The reset occurs when the user unplugs the power supply
from the mains outlet. To trigger the latchoff by an external
signal, a simple PNP transistor can do the work, as
Figure 25 shows.
Overvoltage Protection
The overvoltage protection works by sampling the
plateau voltage 4.5 s after the turn−off sequence. This
delay guarantees a clean plateau, providing that the leakage
inductance ringing has been fully damped. If this would not
be the case, the designer should install a small RC damper
across the transformer primary inductance connections.
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NCP1378
CTN
NCP1378
Aux.
NCP1378
1
8
2
7
3
6
4
5
Aux.
ON/OFF
7
3
6
4
5
optocoupler LED. As a result, the auxiliary voltage also
decreases because it also operates in Flyback and thus
duplicates the output voltage, providing the leakage
inductance between windings is kept low. To account for
this situation and properly protect the power supply,
NCP1378 hosts a dedicated overload detection circuitry.
Once activated, this circuitry imposes to deliver pulses in
a burst manner with a low Duty Cycle. The system
auto−recovers when the fault condition disappears.
During the startup phase, the peak current is pushed to
the maximum until the output voltage reaches its target and
the feedback loop takes over. The auxiliary voltage takes
place after a few switching cycles and self−supplies the IC.
In presence of a short circuit on the output, the auxiliary
voltage will go down until it crosses the undervoltage
lockout level of typically 7.5 V. When this happens,
NCP1378 immediately stops the switching pulses and
unbiases all unnecessary logical blocks. The overall
consumption drops, while keeping the gate grounded, and
the VCC slowly falls down. As soon as VCC reaches
typically 5.5 V, the startup source turns−on again and a new
startup sequence occurs, bringing VCC toward 8.4 V as an
attempt to restart. If the default has gone, then the power
supply normally restarts. If not, a new protective burst is
initiated, shielding the SMPS from any runaway.
Figure 27 portrays the typical operating signals in short
circuit.
Shutdown can easily be implemented through a simple
NPN bipolar transistor as depicted by Figure 6. When OFF,
Q1 is transparent to the operation. When forward biased,
the transistor pulls the FB pin to ground (Vcesat 200 mV) and permanently disables the IC. A small time
constant on the transistor base will avoid false triggering
(Figure 26).
NCP1378
1
10 k
2
10 nF
2
Figure 25. A simple transistor arrangement allows
to trigger the latchoff by an external signal.
Shutting Off the NCP1378
3
8
VCCcap
Figure 24. A simple CTN triggers the latchoff as
soon as the temperature exceeds a given setpoint.
ON/OFF
1
Q1
1
8
2
7
3
6
4
5
Figure 26. A Simple Bipolar Transistor Totally
Disables the IC
Overload Operation
In applications where the output current is purposely not
controlled (e.g. wall adapters delivering raw DC level), it
is interesting to implement a true short−circuit protection.
A short−circuit actually forces the output voltage to be at
a low level, preventing a bias current to circulate in the
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NCP1378
VccON
Vccmin
Vcc
Vcclatch
Driving Pulses
Figure 27. Typical Waveforms in Short Circuit Conditions
Soft−Start
over. Suppose that this time was measured at around 10 ms.
The NCP1378 features an internal 1.0 ms Soft−Start to
soften the constraints occurring in the power supply during
startup. It is activated during the power on sequence. As
soon as VCC reaches VCCON, the peak current is gradually
increased from nearly zero up to the maximum clamping
level (e.g. 1.0 V). The Soft−Start is also activated during
the overcurrent burst (OCP) sequence. Every restart
attempt is followed by a Soft−Start activation. Generally
speaking, the Soft−Start will be activated when VCC ramps
up either from zero (fresh power–on sequence) or 5.5 V, the
latch–off voltage occurring during OCP.
CVCC is calculated using the equation C t · i or C V
31.1 F. Select a 47 F/25 V and this will fit. During the
latchoff phase, the current consumption drops to 220 A.
We can now calculate how long this latchoff phase will last:
(7.5–5.5) x 47 /220 u = 427 ms.
Protecting Pin 8 Against Negative Spikes
As any CMOS controller, NCP1378 is sensitive to
negative voltages that could appear on its pins. To avoid
any adverse latchup of the IC, we strongly recommend to
insert a resistor in series with pin8. This resistor prevents
from adversely latching the controller in case of negative
spikes appearing on the bulk capacitor during the
power−off sequence. A typical value of 6.8 k/0.5 W is
suitable. This resistor does not dissipate any power since it
only sees current during the startup sequence and during
overload.
Calculating the Vcc Capacitor
The VCC capacitor can be calculated knowing the IC
consumption as soon as VCC reaches VCCON. Suppose that
a NCP1378 is used and drives a MOSFET with a 30 nC
total gate charge (Qg). The total average current is thus
made of ICC1 (1.0 mA) plus the driver current, Fsw x Qg
or 1.8 mA. The total current is therefore 2.8 mA. The V
available to fully startup the circuit (e.g. never reach the
7.5 V UVLO during power on) is 8.4 – 7.5 = 0.9 V. We have
a capacitor that then needs to supply the NCP1378 with
2.8 mA during a given time until the auxiliary supply takes
Operating Shots
Below are some oscilloscope shots captured at Vin =
120 VDC with a transformer featuring a 800 H primary
inductance.
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NCP1378
Figure 28. This plot gathers waveforms captured at three different operating points:
1st Upper Plot: Free run, valley switching operation, Pout = 26 W.
2nd Middle Plot: Min Toff clamps the switching frequency and selects
the second valley.
3rd Lowest Plot: The skip slices the second valley pattern and will
further expand the burst as Pout goes low.
Vrsense (200 mV/div)
Vgate
200 A x Rskip
Current Sense Pin (200 mV/pin)
Figure 29. This picture explains how the 200 A internal offset
current creates the skip cycle level.
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NCP1378
PACKAGE DIMENSIONS
SOIC−8
D SUFFIX
CASE 751−07
ISSUE AC
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSION A AND B DO NOT INCLUDE
MOLD PROTRUSION.
4. MAXIMUM MOLD PROTRUSION 0.15 (0.006)
PER SIDE.
5. DIMENSION D DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBAR
PROTRUSION SHALL BE 0.127 (0.005) TOTAL
IN EXCESS OF THE D DIMENSION AT
MAXIMUM MATERIAL CONDITION.
6. 751−01 THRU 751−06 ARE OBSOLETE. NEW
STANDARD IS 751−07.
−X−
A
8
5
0.25 (0.010)
S
B
1
M
Y
M
4
K
−Y−
G
C
N
X 45 DIM
A
B
C
D
G
H
J
K
M
N
S
SEATING
PLANE
−Z−
0.10 (0.004)
H
D
0.25 (0.010)
M
Z Y
S
X
M
J
S
SOLDERING FOOTPRINT
1.52
0.060
7.0
0.275
4.0
0.155
0.6
0.024
1.270
0.050
SCALE 6:1
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14
mm inches
MILLIMETERS
MIN
MAX
4.80
5.00
3.80
4.00
1.35
1.75
0.33
0.51
1.27 BSC
0.10
0.25
0.19
0.25
0.40
1.27
0 8 0.25
0.50
5.80
6.20
INCHES
MIN
MAX
0.189
0.197
0.150
0.157
0.053
0.069
0.013
0.020
0.050 BSC
0.004
0.010
0.007
0.010
0.016
0.050
0 8 0.010
0.020
0.228
0.244
NCP1378
PACKAGE DIMENSIONS
PDIP−7
P SUFFIX
CASE 626B−01
ISSUE A
J
8
5
M
B
1
L
4
DIM
A
B
C
D
F
G
H
J
K
L
M
N
F
A
NOTE 2
C
−T−
N
SEATING
PLANE
H
D
K
G
0.13 (0.005)
M
T A
M
B
NOTES:
1. DIMENSIONS AND TOLERANCING PER
ASME Y14.5M, 1994.
2. DIMENSIONS IN MILLIMETERS.
3. DIMENSION L TO CENTER OF LEAD
WHEN FORMED PARALLEL.
4. PACKAGE CONTOUR OPTIONAL
(ROUND OR SQUARE CORNERS).
5. DIMENSIONS A AND B ARE DATUMS.
M
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MILLIMETERS
MIN
MAX
9.40 10.16
6.10
6.60
3.94
4.45
0.38
0.51
1.02
1.78
2.54 BSC
0.76
1.27
0.20
0.30
2.92
3.43
7.62 BSC
−−−
10 °
0.76
1.01
NCP1378
The product described herein (NCP1378), may be covered by one or more of the following U.S. patents: 6,362,067, 6,385,060, 6,385,061, 6,429,709,
6,587,357, 6,633,193. There may be other patents pending.
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any
liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental
damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over
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NCP1378/D