NCV5171 D

NCV5171, NCV5173
1.5 A 280 kHz/560 kHz
Boost Regulators
The NCV5171/73 products are 280 kHz/560 kHz switching
regulators with a high efficiency, 1.5 A integrated switch. The parts
operate over a wide input voltage range, from 2.7 V to 30 V. The
flexibility of the design allows the chip to operate in most power
supply configurations, including boost, flyback, forward, inverting,
and SEPIC. The ICs utilize current mode architecture, which allows
excellent load and line regulation, as well as a practical means for
limiting current. Combining high frequency operation with a highly
integrated regulator circuit results in an extremely compact power
supply solution. The circuit design includes provisions for features
such as frequency synchronization, shutdown, and feedback controls
for positive voltage regulation. These parts are pin−to−pin compatible
with LT1372/1373.
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SOIC−8
D SUFFIX
CASE 751
MARKING DIAGRAM AND
PIN CONNECTIONS
1
Features
•
•
•
•
•
Integrated Power Switch: 1.5 A Guaranteed
Wide Input Range: 2.7 V to 30 V
High Frequency Allows for Small Components
Minimum External Components
Easy External Synchronization
Built in Overcurrent Protection
Frequency Foldback Reduces Component Stress During an
Overcurrent Condition
Thermal Shutdown with Hysteresis
Shut Down Current: 50 mA Maximum
Pin−to−Pin Compatible with LT1372/1373
NCV Prefix for Automotive and Other Applications Requiring
Unique Site and Control Change Requirements
♦ −40°C to 125°C
These are Pb−Free Devices
VSW
FB
Test
517xE
ALYW
G
•
•
•
•
•
•
•
8
VC
SS
517xE
x
A
L
Y
W
G
PGND
AGND
VCC
= Specific Device Code
= 1 or 3
= Assembly Location
= Wafer Lot
= Year
= Work Week
= Pb−Free Package
ORDERING INFORMATION
Device
Package
Shipping†
NCV5171EDR2G
SOIC−8
(Pb−Free)
2500 Units / Box
NCV5173EDR2G
SOIC−8
(Pb−Free)
2500 Units / Box
†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.
© Semiconductor Components Industries, LLC, 2016
February, 2016 − Rev. 6
1
Publication Order Number:
NCV5171/D
NCV5171, NCV5173
R2
3.72 k
2
C1
0.01 mF
3
4
SS
D1
FB
Test
SS
VOUT
8
VSW
VC
NCV5171/73
1
5V
MBRS120T3
7
PGND
6
AGND
L1
5
VCC
+
22 mH
C3
22 mF
3.3 V
R3
R1
5k
+
C2
22 mF
1.28 k
Figure 1. Applications Diagram
MAXIMUM RATINGS
Rating
Value
Unit
Junction Temperature Range, TJ
−40 to +150
°C
Storage Temperature Range, TSTORAGE
−65 to +150
°C
°C/W
Package Thermal Resistance
Junction−to−Case, RqJC
Junction−to−Ambient, RqJA
45
165
Lead Temperature Soldering: Reflow (Note 1)
ESD, Human Body Model
260 Peak
(Note 1)
°C
1.2
kV
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect
device reliability.
1. 60−180 seconds minimum above 237°C.
MAXIMUM RATINGS
Pin Name
Pin Symbol
VMAX
VMIN
ISOURCE
ISINK
IC Power Input
VCC
35 V
Shutdown/Sync
SS
30 V
−0.3 V
N/A
200 mA
−0.3 V
1.0 mA
1.0 mA
Loop Compensation
VC
6.0 V
−0.3 V
10 mA
10 mA
Voltage Feedback Input
FB
10 V
−0.3 V
1.0 mA
1.0 mA
Test Pin
Test
6.0 V
−0.3 V
1.0 mA
1.0 mA
Power Ground
PGND
0.3 V
−0.3 V
4A
10 mA
Analog Ground
AGND
0V
0V
N/A
10 mA
VSW
40 V
−0.3 V
10 mA
3.0 A
Switch Input
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NCV5171, NCV5173
ELECTRICAL CHARACTERISTICS (2.7 V< VCC < 30 V; −40°C < TJ < 125°C unless otherwise stated)
Test Conditions
Characteristic
Min
Typ
Max
Unit
1.276
1.300
V
Positive and Negative Error Amplifiers
FB Reference Voltage
VC tied to FB; measure at FB
1.246
FB Input Current
FB = VREF
−1.0
0.1
1.0
mA
FB Reference Voltage Line Regulation
VC = FB
−
0.01
0.03
%/V
Positive Error Amp Transconductance
IVC = ± 25 mA
300
550
800
mMho
Positive Error Amp Gain
(Note 2)
200
500
−
V/V
VC Source Current
FB = 1.0 V, VC = 1.25 V
25
50
90
mA
VC Sink Current
FB = 1.5 V, VC = 1.25 V
200
625
1500
mA
VC High Clamp Voltage
FB = 1.0 V; VC sources 25 mA
1.5
1.7
1.9
V
VC Low Clamp Voltage
FB = 1.5 V; VC sinks 25 mA
0.25
0.50
0.65
V
VC Threshold
Reduce VC from 1.5 V until switching stops
0.6
1.05
1.30
V
Base Operating Frequency
NCV5171, FB = 1 V
230
280
310
kHz
Base Operating Frequency
NCV5173, FB = 1 V
460
560
620
kHz
Reduced Operating Frequency
NCV5171, FB = 0 V
30
52
120
kHz
Reduced Operating Frequency
NCV5173, FB = 0 V
60
104
160
kHz
Maximum Duty Cycle
NCV5171
90
94
−
%
Maximum Duty Cycle
NCV5173
82
90
−
%
FB Frequency Shift Threshold
Frequency drops to reduced operating frequency
0.36
0.40
0.44
V
Sync Range
NCV5171
320
−
500
kHz
Sync Range
NCV5173
640
−
1000
kHz
Sync Pulse Transition Threshold
Rise time = 20 ns
2.5
−
−
V
SS Bias Current
SS = 0 V
SS = 3.0 V
−15
−
−3.0
3.0
−
8.0
mA
Oscillator
Sync/ Shutdown
Shutdown Threshold
0.40
0.85
1.20
V
2.7 V ≤ VCC ≤ 12 V
12 V < VCC ≤ 30 V
12
12
80
36
350
200
ms
Switch Saturation Voltage
ISWITCH = 1.5 A, (Note 2)
ISWITCH = 1.0 A, 0°C ≤ TJ ≤ 85°C
ISWITCH = 1.0 A, −40°C ≤ TJ ≤ 0°C
ISWITCH = 10 mA
−
−
−
−
0.8
0.55
0.75
0.09
1.4
−
−
0.45
V
Switch Current Limit
50% duty cycle, (Note 2)
80% duty cycle, (Note 2)
1.6
1.5
1.9
1.7
2.4
2.2
A
Minimum Pulse Width
FB = 0 V, ISW = 4.0 A, (Note 2)
200
250
300
ns
DICC/ DIVSW
2.7 V ≤ VCC ≤ 12 V, 10 mA ≤ ISW ≤ 1.0 A
12 V < VCC ≤ 30 V, 10 mA ≤ ISW ≤ 1.0 A
2.7 V ≤ VCC ≤ 12 V, 10 mA ≤ ISW ≤ 1.5 A, (Note 2)
12 V < VCC ≤ 30 V, 10 mA ≤ ISW ≤ 1.5 A, (Note 2)
−
−
−
−
10
−
17
−
30
100
30
100
mA/A
Switch Leakage
VSW = 40 V, VCC = 0V
−
2.0
100
mA
Shutdown Delay
−
Power Switch
Product parametric performance is indicated in the Electrical Characteristics for the listed test conditions, unless otherwise noted. Product
performance may not be indicated by the Electrical Characteristics if operated under different conditions.
2. Guaranteed by design, not 100% tested in production.
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NCV5171, NCV5173
ELECTRICAL CHARACTERISTICS (2.7 V< VCC < 30 V; −40°C < TJ < 125°C unless otherwise stated) (continued)
Characteristic
Test Conditions
Min
Typ
Max
Unit
General
Operating Current
ISW = 0
−
5.5
8.0
mA
Shutdown Mode Current
VC < 0.8 V, SS = 0 V, 2.7 V ≤ VCC ≤ 12 V
VC < 0.8 V, SS = 0 V, 12 V ≤ VCC ≤ 30 V
−
−
12
−
60
100
mA
Minimum Operation Input Voltage
VSW switching, maximum ISW = 10 mA
−
2.45
2.70
V
Thermal Shutdown
(Note 2)
150
180
210
°C
Thermal Hysteresis
(Note 2)
−
25
−
°C
Product parametric performance is indicated in the Electrical Characteristics for the listed test conditions, unless otherwise noted. Product
performance may not be indicated by the Electrical Characteristics if operated under different conditions.
2. Guaranteed by design, not 100% tested in production.
PACKAGE PIN DESCRIPTION
Package
Pin #
Pin
Symbol
1
VC
Loop compensation pin. The VC pin is the output of the error amplifier and is used for loop compensation,
current limit and soft start. Loop compensation can be implemented by a simple RC network as shown in the
application diagram on page 2 as R1 and C1.
2
FB
Positive regulator feedback pin. This pin senses a positive output voltage and is referenced to 1.276 V. When
the voltage at this pin falls below 0.4 V, chip switching frequency reduces to 20% of the nominal frequency.
3
Test
These pins are connected to internal test logic and should either be left floating or tied to ground. Connection
to a voltage between 2 V and 6 V shuts down the internal oscillator and leaves the power switch running.
4
SS
Synchronization and shutdown pin. This pin may be used to synchronize the part to nearly twice the base
frequency. A TTL low will shut the part down and put it into low current mode. If synchronization is not used,
this pin should be either tied high or left floating for normal operation.
5
VCC
Input power supply pin. This pin supplies power to the part and should have a bypass capacitor connected to
AGND.
6
AGND
Analog ground. This pin provides a clean ground for the controller circuitry and should not be in the path of
large currents. The output voltage sensing resistors should be connected to this ground pin. This pin is
connected to the IC substrate.
7
PGND
Power ground. This pin is the ground connection for the emitter of the power switching transistor. Connection
to a good ground plane is essential.
8
VSW
Function
High current switch pin. This pin connects internally to the collector of the power switch. The open voltage
across the power switch can be as high as 40 V. To minimize radiation, use a trace as short as practical.
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NCV5171, NCV5173
VCC
Thermal
Shutdown
Shutdown
2.0 V
Regulator
VSW
Oscillator
Delay
Timer
S PWM
Latch
R
Q
Switch
Driver
Sync
SS
Frequency
Shift 5:1
×5
Slope
Compensation
63 mW
Ramp
Summer
PWM Comparator
+
0.4 V Detector
FB
−
+
1.276 V
Positive
Error Amp
AGND
VC
Figure 2. Block Diagram
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5
−
PGND
NCV5171, NCV5173
TYPICAL PERFORMANCE CHARACTERISTICS
7.2
70
7.0
VCC = 12 V
40
30
6.2
VCC = 12 V
20
6.0
5.8
5.6
ISW = 1.5 A
50
(mA/A)
Current (mA)
6.6
6.4
VCC = 30 V
60
VCC = 30 V
6.8
0
50
Temperature (°C)
VCC = 2.7 V
10
VCC = 2.7 V
0
100
0
100
Figure 4. DICC/ DIVSW vs. Temperature
Figure 3. ICC (No Switching) vs. Temperature
1.9
1200
1000
−40 °C
800
1.8
85 °C
600
VIN (V)
VCE(SAT) (mV)
50
Temperature (°C)
1.7
25 °C
400
1.6
200
0
500
1.5
1000
0
ISW (mA)
285
fOSC (kHz)
fOSC (kHz)
280
275
270
265
260
0
50
Temperature (°C)
570
565
560
555
550
545
540
535
530
525
520
fOSC (% of Typical)
0
100
Figure 7. Switching Frequency vs. Temperature
(NCV5171)
100
−40°C
75
85°C
50
25°C
25
0
380
50
Temperature (°C)
100
Figure 8. Switching Frequency vs. Temperature
(NCV5173)
VCC = (12 V)
350
100
Figure 6. Minimum Input Voltage vs. Temperature
Figure 5. VCE(SAT) vs. ISW
255
50
Temperature (°C)
400
VFB (mV)
420
Figure 9. Switching Frequency vs. VFB
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450
NCV5171, NCV5173
TYPICAL PERFORMANCE CHARACTERISTICS
1.280
0.20
VCC = 12 V
0.18
1.276
IFB (mA)
Voltage (V)
1.278
1.274
VCC = 2.7 V
1.272
0.16
0.14
0.12
VCC = 30 V
0.10
1.270
0.08
1.268
0
50
Temperature (°C)
0
100
Figure 10. Reference Voltage vs. Temperature
99
VCC = 30 V
Duty Cycle (%)
98
Current (A)
2.50
2.40
VCC = 2.7 V
VCC = 12 V
2.20
0
100
Figure 11. IFB vs. Temperature
2.60
2.30
50
Temperature (°C)
VCC = 12 V
97
96
VCC = 2.7 V
95
94
VCC = 30 V
50
Temperature (°C)
93
0
100
50
Temperature (°C)
100
Figure 13. Maximum Duty Cycle vs. Temperature
Figure 12. Current Limit vs. Temperature
1.1
1.7
1.0
VC High Clamp Voltage
Voltage (V)
Voltage (V)
1.5
1.3
1.1
0.9
0.7
VC Threshold
0.9
0.8
0.7
0.6
0.5
0.4
0
50
Temperature (°C)
0
100
50
Temperature (°C)
Figure 15. Shutdown Threshold vs. Temperature
Figure 14. VC Threshold and High Clamp
Voltage vs. Temperature
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NCV5171, NCV5173
TYPICAL PERFORMANCE CHARACTERISTICS
40
160
25°C
VCC = 2.7 V
140
30
85°C
100
ISS (mA)
Delay (ms)
120
VCC = 12 V
80
20
−40°C
10
VCC = 30 V
60
0
40
0
50
Temperature (°C)
−10
100
1
20
85°C
gm (mmho)
ICC (mA)
25°C
7
9
600
−40°C
30
5
VSS (V)
Figure 17. ISS vs. VSS
Figure 16. Shutdown Delay vs. Temperature
40
3
550
500
10
0
450
0
10
VIN (V)
50
Temperature (°C)
100
Figure 19. Error Amplifier Transconductance
vs. Temperature
Figure 18. ICC vs. VIN During Shutdown
2.6
100
Current (mA)
2.5
IOUT (mA)
60
20
−20
−60
2.4
2.3
2.2
2.1
2.0
−255 −175 −125
−75
−25
VREF − VFB (mV)
0
0
25
50
Temperature (°C)
100
Figure 21. Switch Leakage vs. Temperature
Figure 20. Error Amplifier IOUT vs. VFB
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NCV5171, NCV5173
APPLICATIONS INFORMATION
THEORY OF OPERATION
The oscillator is trimmed to guarantee an 18% frequency
accuracy. The output of the oscillator turns on the power
switch at a frequency of 280 kHz (NCV5171) or 560 kHz
(NCV5173) as shown in Figure 22. The power switch is
turned off by the output of the PWM Comparator.
A TTL−compatible sync input at the SS pin is capable of
syncing up to 1.8 times the base oscillator frequency. As
shown in Figure 23, in order to sync to a higher frequency,
a positive transition turns on the power switch before the
output of the oscillator goes high, thereby resetting the
oscillator. The sync operation allows multiple power
supplies to operate at the same frequency.
A sustained logic low at the SS pin will shut down the IC
and reduce the supply current.
An additional feature includes frequency shift to 20% of
the nominal frequency when the FB pin triggers the
threshold. During power up, overload, or short circuit
conditions, the minimum switch on−time is limited by the
PWM comparator minimum pulse width. Extra switch
off−time reduces the minimum duty cycle to protect external
components and the IC itself.
As previously mentioned, this block also produces a ramp
for the slope compensation to improve regulator stability.
Current Mode Control
VCC
Oscillator
S
VC
−
+
Q
L
R
D1
Power Switch
PWM Comparator
VSW
In Out
X5
CO
Driver
RLOAD
SUMMER
Slope Compensation
63 mW
Figure 22. Current Mode Control Scheme
The NCV5171/73 boost regulator incorporates a current
mode control scheme, in which the PWM ramp signal is
derived from the power switch current. This ramp signal is
compared to the output of the error amplifier to control the
on−time of the power switch. The oscillator is used as a
fixed−frequency clock to ensure a constant operational
frequency. The resulting control scheme features several
advantages over conventional voltage mode control. First,
derived directly from the inductor, the ramp signal responds
immediately to line voltage changes. This eliminates the
delay caused by the output filter and error amplifier, which
is commonly found in voltage mode controllers. The second
benefit comes from inherent pulse−by−pulse current
limiting by merely clamping the peak switching current.
Finally, since current mode commands an output current
rather than voltage, the filter offers only a single pole to the
feedback loop. This allows both a simpler compensation and
a higher gain−bandwidth over a comparable voltage mode
circuit.
Without discrediting its apparent merits, current mode
control comes with its own peculiar problems, mainly,
subharmonic oscillation at duty cycles over 50%.
NCV5171/73 solves this problem by adopting a slope
compensation scheme in which a fixed ramp generated by
the oscillator is added to the current ramp. A proper slope
rate is provided to improve circuit stability without
sacrificing the advantages of current mode control.
Error Amplifier
VC
FB
1.276 V +
−
NCV5171/73
1MW
120 pF
Voltage
Clamp
C1
0.01 mF
R1
5 kW
positive error−amp
Figure 24. Error Amplifier Equivalent Circuit
The FB pin is directly connected to the inverting input of
the positive error amplifier, whose non−inverting input is
fed by the 1.276 V reference. It is a transconductance
amplifier with a high output impedance of approximately
1 MW, as shown in Figure 24. The VC pin is connected to the
output of the error amplifiers and is internally clamped
between 0.5 V and 1.7 V. A typical connection at the VC pin
includes a capacitor in series with a resistor to ground,
forming a pole/zero for loop compensation.
An external shunt can be connected between the VC pin
and ground to reduce its clamp voltage. Consequently, the
current limit of the internal power transistor current is
reduced from its nominal value.
Oscillator and Shutdown
Sync
Current
Ramp
VSW
Figure 23. Timing Diagram of Sync and Shutdown
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NCV5171, NCV5173
Switch Driver and Power Switch
approximately 1.5 V, the internal power switch briefly turns
on. This is a part of the NCV5171/73’s normal operation.
The turn−on of the power switch accounts for the initial
current swing.
When the VC pin voltage rises above the threshold, the
internal power switch starts to switch and a voltage pulse can
be seen at the VSW pin. Detecting a low output voltage at the
FB pin, the built−in frequency shift feature reduces the
switching frequency to a fraction of its nominal value,
reducing the minimum duty cycle, which is otherwise
limited by the minimum on−time of the switch. The peak
current during this phase is clamped by the internal current
limit.
When the FB pin voltage rises above 0.4 V, the frequency
increases to its nominal value, and the peak current begins
to decrease as the output approaches the regulation voltage.
The overshoot of the output voltage is prevented by the
active pull−on, by which the sink current of the error
amplifier is increased once an overvoltage condition is
detected. The overvoltage condition is defined as when the
FB pin voltage is 50 mV greater than the reference voltage.
The switch driver receives a control signal from the logic
section to drive the output power switch. The switch is
grounded through emitter resistors (63 mW total) to the
PGND pin. PGND is not connected to the IC substrate so that
switching noise can be isolated from the analog ground.
The peak switching current is clamped by an internal circuit.
The clamp current is guaranteed to be greater than 1.5 A and
varies with duty cycle due to slope compensation.
The power switch can withstand a maximum voltage of
40 V on the collector (VSW pin). The saturation voltage of
the switch is typically less than 1 V to minimize power
dissipation.
Short Circuit Condition
When a short circuit condition happens in a boost circuit,
the inductor current will increase during the whole
switching cycle, causing excessive current to be drawn from
the input power supply. Since control ICs don’t have the
means to limit load current, an external current limit circuit
(such as a fuse or relay) has to be implemented to protect the
load, power supply and ICs.
In other topologies, the frequency shift built into the IC
prevents damage to the chip and external components. This
feature reduces the minimum duty cycle and allows the
transformer secondary to absorb excess energy before the
switch turns back on.
COMPONENT SELECTION
Frequency Compensation
The goal of frequency compensation is to achieve
desirable transient response and DC regulation while
ensuring the stability of the system. A typical compensation
network, as shown in Figure 26, provides a frequency
response of two poles and one zero. This frequency response
is further illustrated in the Bode plot shown in Figure 27.
IL
VC
VOUT
R1
VCC
NCV5171/73
C2
VC
C1
GND
Figure 26. A Typical Compensation Network
Figure 25. Startup Waveforms of Circuit Shown in
the Application Diagram. Load = 400 mA.
The high DC gain in Figure 27 is desirable for achieving
DC accuracy over line and load variations. The DC gain of
a transconductance error amplifier can be calculated as
follows:
The NCV5171/73 can be activated by either connecting
the VCC pin to a voltage source or by enabling the SS pin.
Startup waveforms shown in Figure 25 are measured in the
boost converter demonstrated in the Application Diagram
on the page 2 of this document. Recorded after the input
voltage is turned on, this waveform shows the various
phases during the power up transition.
When the VCC voltage is below the minimum supply
voltage, the VSW pin is in high impedance. Therefore,
current conducts directly from the input power source to the
output through the inductor and diode. Once VCC reaches
GainDC + GM
RO
where:
GM = error amplifier transconductance;
RO = error amplifier output resistance ≈ 1 MW.
The low frequency pole, fP1, is determined by the error
amplifier output resistance and C1 as:
1
fP1 +
2pC1R
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10
O
NCV5171, NCV5173
The first zero generated by C1 and R1 is:
In the flyback topology, peak VSW voltage is governed by:
1
fZ1 +
2pC1R1
VSW(MAX) + VCC(MAX))(VOUT)VF)
The phase lead provided by this zero ensures that the loop
has at least a 45° phase margin at the crossover frequency.
Therefore, this zero should be placed close to the pole
generated in the power stage which can be identified at
frequency:
where:
N = transformer turns ratio, primary over secondary.
When the power switch turns off, there exists a voltage
spike superimposed on top of the steady−state voltage.
Usually this voltage spike is caused by transformer leakage
inductance charging stray capacitance between the VSW and
PGND pins. To prevent the voltage at the VSW pin from
exceeding the maximum rating, a transient voltage
suppressor in series with a diode is paralleled with the
primary windings. Another method of clamping switch
voltage is to connect a transient voltage suppressor between
the VSW pin and ground.
1
fP +
2pCORLOAD
where:
CO = equivalent output capacitance of the error amplifier
≈120 pF;
RLOAD= load resistance.
The high frequency pole, fP2, can be placed at the output
filter’s ESR zero or at half the switching frequency. Placing
the pole at this frequency will cut down on switching noise.
The frequency of this pole is determined by the value of C2
and R1:
Magnetic Component Selection
When choosing a magnetic component, one must consider
factors such as peak current, core and ferrite material, output
voltage ripple, EMI, temperature range, physical size and
cost. In boost circuits, the average inductor current is the
product of output current and voltage gain (VOUT/VCC),
assuming 100% energy transfer efficiency. In continuous
conduction mode, inductor ripple current is
1
fP2 +
2pC2R1
DC Gain
One simple method to ensure adequate phase margin is to
design the frequency response with a −20 dB per decade
slope, until unity−gain crossover. The crossover frequency
should be selected at the midpoint between fZ1 and fP2 where
the phase margin is maximized.
* VCC)
V (V
IRIPPLE + CC OUT
(f)(L)(VOUT)
where:
f = 280 kHz (NCV5171) or 560 kHz (NCV5173).
The peak inductor current is equal to average current plus
half of the ripple current, which should not cause inductor
saturation. The above equation can also be referenced when
selecting the value of the inductor based on the tolerance of
the ripple current in the circuits. Small ripple current
provides the benefits of small input capacitors and greater
output current capability. A core geometry like a rod or
barrel is prone to generating high magnetic field radiation,
but is relatively cheap and small. Other core geometries,
such as toroids, provide a closed magnetic loop to prevent
EMI.
fP1
fZ1
Gain (dB)
N
fP2
Frequency (LOG)
Figure 27. Bode Plot of the Compensation Network
Shown in Figure 26
Input Capacitor Selection
In boost circuits, the inductor becomes part of the input
filter, as shown in Figure 29. In continuous mode, the input
current waveform is triangular and does not contain a large
pulsed current, as shown in Figure 28. This reduces the
requirements imposed on the input capacitor selection.
During continuous conduction mode, the peak to peak
inductor ripple current is given in the previous section. As
we can see from Figure 28, the product of the inductor
current ripple and the input capacitor’s effective series
resistance (ESR) determine the VCC ripple. In most
applications, input capacitors in the range of 10 mF to 100 mF
with an ESR less than 0.3 W work well up to a full 1.5 A
switch current.
VSW Voltage Limit
In the boost topology, VSW pin maximum voltage is set by
the maximum output voltage plus the output diode forward
voltage. The diode forward voltage is typically 0.5 V for
Schottky diodes and 0.8 V for ultrafast recovery diodes
VSW(MAX) + VOUT(MAX))VF
where:
VF = output diode forward voltage.
www.onsemi.com
11
NCV5171, NCV5173
Output Capacitor Selection
VCC ripple
IIN
VOUT ripple
IL
IL
Figure 28. Boost Input Voltage and Current
Ripple Waveforms
Figure 30. Typical Output Voltage Ripple
IL
IIN
VCC
+
−
By examining the waveforms shown in Figure 30, we can
see that the output voltage ripple comes from two major
sources,
namely
capacitor
ESR
and
the
charging/discharging of the output capacitor. In boost
circuits, when the power switch turns off, IL flows into the
output capacitor causing an instant DV = IIN × ESR. At the
same time, current IL − IOUT charges the capacitor and
increases the output voltage gradually. When the power
switch is turned on, IL is shunted to ground and IOUT
discharges the output capacitor. When the IL ripple is small
enough, IL can be treated as a constant and is equal to input
current IIN.
CIN
RESR
Figure 29. Boost Circuit Effective Input Filter
Summing up, the output voltage peak−peak ripple can be
calculated by:
The situation is different in a flyback circuit. The input
current is discontinuous and a significant pulsed current is
seen by the input capacitors. Therefore, there are two
requirements for capacitors in a flyback regulator: energy
storage and filtering. To maintain a stable voltage supply to
the chip, a storage capacitor larger than 20 mF with low ESR
is required. To reduce the noise generated by the inductor,
insert a 1.0 mF ceramic capacitor between VCC and ground
as close as possible to the chip.
(I * IOUT)(1 * D)
VOUT(RIPPLE) + IN
(COUT)(f)
I
D
) OUT
) IIN ESR
(COUT)(f)
The equation can be expressed more conveniently in
terms of VCC, VOUT and IOUT for design purposes as
follows:
I
(V
* VCC)
VOUT(RIPPLE) + OUT OUT
(COUT)(f)
1
(COUT)(f)
(I
)(V
)(ESR)
) OUT OUT
VCC
The capacitor RMS ripple current is:
IRIPPLE + Ǹ(IIN * IOUT)2(1 * D))(IOUT)2(D)
+ IOUT
* VCC
ǸVOUTVCC
Although the above equations apply only for boost
circuits, similar equations can be derived for flyback
circuits.
www.onsemi.com
12
NCV5171, NCV5173
Reducing the Current Limit
Another solution to the current limiting problem is to
externally measure the current through the switch using
a sense resistor. Such a circuit is illustrated in Figure 32.
In some applications, the designer may prefer a lower
limit on the switch current than 1.5 A. An external shunt can
be connected between the VC pin and ground to reduce its
clamp voltage. Consequently, the current limit of the
internal power transistor current is reduced from its nominal
value.
The voltage on the VC pin can be evaluated with the
equation
VCC
PGND AGND
where:
RE = 0.063 W, the value of the internal emitter resistor;
AV = 5 V/V, the gain of the current sense amplifier.
Since RE and AV cannot be changed by the end user, the
only available method for limiting switch current below
1.5 A is to clamp the VC pin at a lower voltage. If the
maximum switch or inductor current is substituted into the
equation above, the desired clamp voltage will result.
A simple diode clamp, as shown in Figure 31, clamps the
VC voltage to a diode drop above the voltage on resistor R3.
Unfortunately, such a simple circuit is not generally
acceptable if VIN is loosely regulated.
R1
Q1
R2
C1
C2
C3
RSENSE
Output
Ground
Figure 32. Current Limiting using a Current Sense
Resistor
The switch current is limited to
VBE(Q1)
ISWITCH(PEAK) +
RSENSE
where:
VBE(Q1) = the base−emitter voltage drop of Q1, typically
0.65 V.
The improved circuit does not require a regulated voltage
to operate properly. Unfortunately, a price must be paid for
this convenience in the overall efficiency of the circuit. The
designer should note that the input and output grounds are
no longer common. Also, the addition of the current sense
resistor, RSENSE, results in a considerable power loss which
increases with the duty cycle. Resistor R2 and capacitor C3
form a low−pass filter to remove noise.
VIN
VCC
R2
D1
−
+
VIN
VC + ISWREAV
VC
VC
R3
R1
C1
C2
Figure 31. Current Limiting using a Diode Clamp
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13
NCV5171, NCV5173
Subharmonic Oscillation
pin. In normal operation, VSW looks similar to a square
wave, and is dependent on the converter topology. Formulas
for calculating VSW in the boost and flyback topologies are
given in the section “VSW Voltage Limit.” The voltage on
VSW charges capacitor C3 when the switch is off, causing
the voltage at the VC pin to shift upwards. When the switch
turns on, C3 discharges through R3, producing a negative
slope at the VC pin. This negative slope provides the slope
compensation.
The amount of slope compensation added by this circuit
is:
Subharmonic oscillation (SHM) is a problem found in
current−mode control systems, where instability results
when duty cycle exceeds 50%. SHM only occurs in
switching regulators with a continuous inductor current.
This instability is not harmful to the converter and usually
does not affect the output voltage regulation. SHM will
increase the radiated EM noise from the converter and can
cause, under certain circumstances, the inductor to emit
high−frequency audible noise.
SHM is an easily remedied problem. The rising slope of
the inductor current is supplemented with internal “slope
compensation” to prevent any duty cycle instability from
carrying through to the next switching cycle. In
NCV5171/73, the slope compensation is added during the
entire switch on−time, typically in the amount of 180 mA/ms.
In some cases, SHM can rear its ugly head despite the
presence of the onboard slope compensation. The simple
cure to this problem is more slope compensation to avoid the
unwanted oscillation. In that case, an external circuit, shown
in Figure 33, can be added to increase the amount of slope
compensation used. This circuit requires only a few
components and is “tacked on” to the compensation
network.
VSW
ǒ
VSW
R2
C1
SW
Ǔǒ(1 *fD)R
Ǔ
EAV
R3C3 t 1 * D
fSW
C2
C3
*(1*D)
R3C3fSW
where:
DI/DT = the amount of slope compensation added (A/s);
VSW = the voltage at the switch node when the transistor
is turned off (V);
fSW = the switching frequency, typically 280 kHz
(NCV5171) or 560 kHz (NCV5173)
D = the duty cycle;
RE = 0.063 W, the value of the internal emitter resistor;
AV = 5 V/V, the gain of the current sense amplifier.
In selecting appropriate values for the slope compensation
network, the designer is advised to choose a convenient
capacitor, then select values for R2 and R3 such that the
amount of slope compensation added is 100 mA/ms. Then
R2 may be increased or decreased as necessary. Of course,
the series combination of R2 and R3 should be large enough
to avoid drawing excessive current from VSW. Additionally,
to ensure that the control loop stability is improved, the time
constant formed by the additional components should be
chosen such that
VC
R1
Ǔ ǒ1 * e
R3
DI + V
SW R )R
DT
2
3
Finally, it is worth mentioning that the added slope
compensation is a tradeoff between duty cycle stability and
transient response. The more slope compensation a designer
adds, the slower the transient response will be, due to the
external circuitry interfering with the proper operation of the
error amplifier.
R3
Soft−Start
Figure 33. Technique for Increasing Slope
Compensation
Through the addition of an external circuit, a Soft−Start
function can be added to the NCV5171/73 family of
components. Soft−Start circuitry prevents the VC pin from
slamming high during startup, thereby inhibiting the
inductor current from rising at a high slope.
The dashed box contains the normal compensation
circuitry to limit the bandwidth of the error amplifier.
Resistors R2 and R3 form a voltage divider off of the VSW
www.onsemi.com
14
NCV5171, NCV5173
This circuit, shown in Figure 34, requires a minimum
number of components and allows the Soft−Start circuitry to
activate any time the SS pin is used to restart the converter.
this datasheet reveals that the typical operating current, IQ,
due to this circuitry is 5.5 mA. Additional guidance can be
found in the graph of operating current vs. temperature. This
graph shows that IQ is strongly dependent on input voltage,
VIN, and temperature. Then
VIN
PBIAS + VINIQ
VCC
SS
Since the onboard switch is an NPN transistor, the base
drive current must be factored in as well. This current is
drawn from the VIN pin, in addition to the control circuitry
current. The base drive current is listed in the specifications
as DICC/DISW, or switch transconductance. As before, the
designer will find additional guidance in the graphs. With
that information, the designer can calculate
SS
VC
D2
D1
R1
PDRIVER + VINISW
C1
C3
ICC
DISW
D
where:
ISW = the current through the switch;
D = the duty cycle or percentage of switch on−time.
ISW and D are dependent on the type of converter. In a
boost converter,
C2
ISW(AVG) ^ IL(AVG)
Figure 34. Soft Start
Resistor R1 and capacitors C1 and C2 form the
compensation network. At turn on, the voltage at the VC pin
starts to come up, charging capacitor C3 through Schottky
diode D2, clamping the voltage at the VC pin such that
switching begins when VC reaches the VC threshold,
typically 1.05 V (refer to graphs for detail over temperature).
1
Efficiency
D
V
* VIN
D ^ OUT
VOUT
In a flyback converter,
V
I
ISW(AVG) ^ OUT LOAD
VIN
VC + VF(D2))VC3
D^
Therefore, C3 slows the startup of the circuit by limiting
the voltage on the VC pin. The Soft−Start time increases with
the size of C3.
Diode D1 discharges C3 when SS is low. If the shutdown
function is not used with this part, the cathode of D1 should
be connected to VIN.
1
Efficiency
1
D
VOUT
N
VOUT ) NSP VIN
The switch saturation voltage, V(CE)SAT, is the last major
source of on−chip power loss. V(CE)SAT is the
collector−emitter voltage of the internal NPN transistor
when it is driven into saturation by its base drive current. The
value for V(CE)SAT can be obtained from the specifications
or from the graphs, as “Switch Saturation Voltage.” Thus,
Calculating Junction Temperature
PSAT ^ V(CE)SATISW
To ensure safe operation of NCV5171/73, the designer
must calculate the on−chip power dissipation and determine
its expected junction temperature. Internal thermal
protection circuitry will turn the part off once the junction
temperature exceeds 180°C ± 30°. However, repeated
operation at such high temperatures will ensure a reduced
operating life.
Calculation of the junction temperature is an imprecise
but simple task. First, the power losses must be quantified.
There are three major sources of power loss on the
NCV5171/73:
• biasing of internal control circuitry, PBIAS
• switch driver, PDRIVER
• switch saturation, PSAT
The internal control circuitry, including the oscillator and
linear regulator, requires a small amount of power even
when the switch is turned off. The specifications section of
D
Finally, the total on−chip power losses are
PD + PBIAS)PDRIVER)PSAT
Power dissipation in a semiconductor device results in the
generation of heat in the junctions at the surface of the chip.
This heat is transferred to the surface of the IC package, but
a thermal gradient exists due to the resistive properties of the
package molding compound. The magnitude of the thermal
gradient is expressed in manufacturers’ data sheets as qJA,
or junction−to−ambient thermal resistance. The on−chip
junction temperature can be calculated if qJA, the air
temperature near the surface of the IC, and the on−chip
power dissipation are known.
www.onsemi.com
15
NCV5171, NCV5173
TJ + TA)(PDqJA)
transitions that can cause problems. Therefore the following
guidelines should be followed in the layout.
where:
TJ = IC or FET junction temperature (°C);
TA = ambient temperature (°C);
PD = power dissipated by part in question (W);
qJA = junction−to−ambient thermal resistance (°C/W).
For the NCV5171/73, qJA=165°C/W.
Once the designer has calculated TJ, the question of
whether the NCV5171/73 can be used in an application is
settled. If TJ exceeds 150°C, the absolute maximum
allowable junction temperature, the NCV5171/73 is not
suitable for that application.
If TJ approaches 150°C, the designer should consider
possible means of reducing the junction temperature.
Perhaps another converter topology could be selected to
reduce the switch current. Increasing the airflow across the
surface of the chip might be considered to reduce TA.
1.
2.
3.
Circuit Layout Guidelines
In any switching power supply, circuit layout is very
important for proper operation. Rapidly switching currents
combined with trace inductance generates voltage
In boost circuits, high AC current circulates within the
loop composed of the diode, output capacitor, and
on−chip power transistor. The length of associated
traces and leads should be kept as short as possible. In
the flyback circuit, high AC current loops exist on both
sides of the transformer. On the primary side, the loop
consists of the input capacitor, transformer, and
on−chip power transistor, while the transformer,
rectifier diodes, and output capacitors form another
loop on the secondary side. Just as in the boost circuit,
all traces and leads containing large AC currents
should be kept short.
Separate the low current signal grounds from the
power grounds. Use single point grounding or ground
plane construction for the best results.
Locate the voltage feedback resistors as near the IC as
possible to keep the sensitive feedback wiring short.
Connect feedback resistors to the low current analog
ground.
www.onsemi.com
16
NCV5171, NCV5173
22 mH
3.3 VIN
MBRS120T3
5.0 VO
10 mF
3.6 k
VCC (5)
PGND (7)
GND
22 mF
GND
VSW (8)
AGND (6)
NCV5171/73
VC (1 )
FB (2)
0.1 mF
1.3 k
200 pF
5.0 k
Figure 35. Additional Application Diagram, 3.3 V Input, 5.0 V/ 400 mA Output Boost Converter
MBRS140T3
VCC
P6KE−15A
+
22 mF
−12 V
T1
1.0 mF
+
47 mF
GND
VCC (5)
GND
PGND (7)
1N4148
+
1:2
47 mF
VSW (8)
+12 V
AGND (6)
MBRS140T3
NCV5171/73
VC (1 )
FB (2)
47 nF
10.72 k
1.28 k
4.7 nF
2.0 k
Figure 36. Additional Application Diagram, 2.7 to 13 V Input, +12 V/ 200 mA Output Flyback Converter
GND
VCC (5)
VC (1 )
GND
5.0 k
2.2 mF
22 mF
NCV5171/73
200 pF
.01 mF
VIN
VSW (8)
AGND (6)
15 mH
1.1 k
Low
ESR
−5.0
VOUT
FB (2)
PGND (7)
300
Figure 37. Additional Application Diagram, −9.0 V to −28 V Input, −5.0 V/700 mA Output Inverted Buck Converter
www.onsemi.com
17
NCV5171, NCV5173
22 mH
VCC
22 mF
VCC (5)
PGND (7)
GND
VSW (8)
AGND (6)
5.0 V
+
22 mF
NCV5171/73
+
22 mH
VC (1 )
37.24 k
22 mF
Low
ESR
FB (2)
GND
.01 mF
200 pF
5.0 k
12.76 k
Figure 38. Additional Application Diagram, 2.7 V to 28 V Input, 5.0 V Output SEPIC Converter
R1
R2
1.245 k/0.1 W, 1%
99.755 k/0.1 W, 1%
GND
C1
D1
C10
.1 m
C11
.01 m
3 Test
R3
2.0 k
4 SS
VSW 8
PGND 7
NCV5171/73
1 V
C
2 FB
AGND
VCC
.1 m
50 V
D1
C3
.1 m
50 V
D1
D1
.1 m
50 V
D1
D1
D1
100 VO
1N4148 1N4148
6
5
C2
C8
10 m
C9
.1 m
C7
.1 m
50 V
1N4148 1N4148 1N4148 1N4148 1N4148
C4
.1 m
50 V
C5
.1 m
50 V
C6
.1 m
50 V
GND
4.0 V
Figure 39. Additional Application Diagram, 4.0 V Input, 100 V/ 10 mA Output Boost Converter with
Output Voltage Multiplier
200 pF
D1
C6 C1
0.01 mF
2
3
SS
FB
Test
VSW 8
NCV5171/73
5.0 k
1 V
C
+
R1
4 SS
PGND
7
L1
15 mH
22 mF
AGND 6
VCC
+5.0 V
−12 V
5
D2
D3
+
C3
22 mF
+
C5
22 mF
C4
0.1 mF
GND
GND
R2
R3
1.28 k
10.72 k
+12 V
Figure 40. Additional Application Diagram, 5.0 V Input, ± 12 V Output Dual Boost Converter
www.onsemi.com
18
NCV5171, NCV5173
PACKAGE DIMENSIONS
SOIC−8 NB
CASE 751−07
ISSUE AK
−X−
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.
A
8
5
S
B
0.25 (0.010)
M
Y
M
1
4
K
−Y−
G
C
N
DIM
A
B
C
D
G
H
J
K
M
N
S
X 45 _
SEATING
PLANE
−Z−
0.10 (0.004)
H
M
D
0.25 (0.010)
M
Z Y
S
X
J
S
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
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
mm Ǔ
ǒinches
*For additional information on our Pb−Free strategy and soldering
details, please download the ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
ON Semiconductor and the
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC) or its subsidiaries in the United States and/or other countries.
SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed
at www.onsemi.com/site/pdf/Patent−Marking.pdf. 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 time. All operating parameters, including “Typicals” must be validated for each
customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended,
or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which
the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or
unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and
expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim
alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable
copyright laws and is not for resale in any manner.
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19
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For additional information, please contact your local
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NCV5171/D