ON NCP3170BDR2G Synchronous pwm switching converter Datasheet

NCP3170
Synchronous PWM
Switching Converter
The NCP3170 is a flexible synchronous PWM Switching Buck
Regulator. The NCP3170 operates from 4.5 V to 18 V, sourcing up to
3 A and is capable of producing output voltages as low as 0.8 V. The
NCP3170 also incorporates current mode control. To reduce the
number of external components, a number of features are internally set
including soft start, power good detection, and switching frequency.
The NCP3170 is currently available in an SOIC−8 package.
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SOIC−8 NB
CASE 751
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
•
4.5 V to 18 V Operating Input Voltage Range
90 mW High-Side, 25 mW Low-Side Switch
FMEA Fault Tolerant During Pin Short Test
3 A Continuous Output Current
Fixed 500 kHz and 1 MHz PWM Operation
Cycle-by-Cycle Current Monitoring
1.5% Initial Output Accuracy
Internal 4.6 ms Soft-Start
Short-Circuit Protection
Turn on Into Pre-bias
Power Good Indication
Light Load Efficiency
Thermal Shutdown
These are Pb-Free Devices
MARKING DIAGRAM
8
3170x
ALYW
G
1
3170x
x
A
L
Y
W
G
PIN CONNECTIONS
Typical Applications
•
•
•
•
•
•
•
Set Top Boxes
DVD/Blu−rayt Drives and HDD
LCD Monitors and TVs
Cable Modems
PCIe Graphics Cards
Telecom/Networking/Datacom Equipment
Point of Load DC/DC Converters
VIN
C1
22 mF
VIN
EN
PG
CC
3.3 V
R1
FB1
AGND
PGND
VSW
AGND
FB
EN
COMP
PG
ORDERING INFORMATION
L1 4.7 mH
COMP
PGND
VIN
(Top View)
VSW
NCP3170
= Specific Device Code
= A or B
= Assembly Location
= Wafer Lot
= Year
= Work Week
= Pb-Free Package
C2, C3
22 mF
Device
Package
Shipping†
NCP3170ADR2G
SOIC−8
(Pb−Free)
2,500/Tape & Reel
NCP3170BDR2G
SOIC−8
(Pb−Free)
2,500/Tape & Reel
†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.
R2
RC
Figure 1. Typical Application Circuit
© Semiconductor Components Industries, LLC, 2013
April, 2013 − Rev. 3
1
Publication Order Number:
NCP3170/D
NCP3170
VIN
VDD
EN
Power
Control
(PC)
UVLO
POR
Driver
Voltage
Clamp
VCV
VCL
Soft Start
Reference
S
Slope
Compensation
0.030 V/A
Current
Sense
ORing
Circuit
Oscillator
+
FB
+
RCLRQ
−
−
Pulse by
Pulse
Current
Limit
SSETQ
VIN
COMP
Soft Start
Complete
998 mV
+
−
867 mV
+
−
728 mV
+
−
Logic
HS
PDRV
VSW
VCW
hs
VCL
LS
NDRV
PG
Over
Temperature
Protection
Zero
Current
Detection
VSW
AGND
PGND
Figure 2. NCP3170 Block Diagram
Table 1. PIN FUNCTION DESCRIPTION
Pin
Pin Name
1
PGND
The power ground pin is the high current path for the device. The pin should be soldered to a large copper
area to reduce thermal resistance. PGND needs to be electrically connected to AGND.
Description
2
VIN
The input voltage pin powers the internal control circuitry and is monitored by multiple voltage comparators.
The VIN pin is also connected to the internal power PMOS switch and linear regulator output. The VIN pin
has high di/dt edges and must be decoupled to ground close to the pin of the device.
3
AGND
4
FB
5
COMP
6
EN
Enable pin. Pull EN to logic high to enable the device. Pull EN to logic low to disable the device. Do not leave
it open.
7
PG
Power good is an open drain 500 mA pull down indicating output voltage is within the power good window. If
the power good function is not used, it can be connected to the VSW node to reduce thermal resistance. Do
not connect PG to the VSW node if the application is turning on into pre-bias.
8
VSW
The VSW pin is the connection of the drains of the internal N and P MOSFETS. At switch off, the inductor will
drive this pin below ground as the body diode and the NMOS conducts with a high dv/dt.
The analog ground pin serves as small-signal ground. All small-signal ground paths should connect to the
AGND pin and should also be electrically connected to power ground at a single point, avoiding any high
current ground returns.
Inverting input to the OTA error amplifier. The FB pin in conjunction with the external compensation serves to
stabilize and achieve the desired output voltage with current mode compensation.
The loop compensation pin is used to compensate the transconductance amplifier which stabilizes the
operation of the converter stage. Place compensation components as close to the converter as possible.
Connect a RC network between COMP and AGND to compensate the control loop.
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NCP3170
Table 2. ABSOLUTE MAXIMUM RATINGS (measured vs. GND pin 3, unless otherwise noted)
Symbol
Rating
Main Supply Voltage Input
Voltage between PGND and AGND
PWM Feedback Voltage
VMAX
VMIN
Unit
VIN
20
−0.3
V
VPAG
0.3
−0.3
V
FB
6
−0.3
V
COMP
6
−0.3
V
Enable Voltage
EN
VIN + 0.3 V
−0.3
V
PG Voltage
PG
VIN + 0.3 V
−0.3
V
VSW to AGND or PGND
VSW
VIN + 0.3 V
−0.7
V
VSWST
VIN + 10 V
Error Amplifier Voltage
VSW to AGND or PGND for 35ns
Junction Temperature (Note 1)
TJ
−5
V
+150
°C
Operating Ambient Temperature Range
TA
−40 to +85
°C
Storage Temperature Range
Tstg
− 55 to +150
°C
PD
RqJA
RqJC
1.15
87
37.8
W
°C/W
°C/W
RF
260 peak
°C
Thermal Characteristics (Note 2)
SOIC−8 Plastic Package
Maximum Power Dissipation @ TA = 25°C
Thermal Resistance Junction-to-Air
Thermal Resistance Junction-to-Case
Lead Temperature Soldering (10 sec):
Reflow (SMD Styles Only) Pb-Free (Note 3)
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. The maximum package power dissipation limit must not be exceeded.
PD +
T J(max) * T A
R qJA
2. The value of qJA is measured with the device mounted on 2in x 2in FR−4 board with 2oz. copper, in a still air environment with TA = 25°C.
The value in any given application depends on the user’s specific board design.
3. 60−180 seconds minimum above 237°C.
Table 3. RECOMMENDED OPERATING CONDITIONS
Rating
Symbol
Main Supply Voltage Input
Min
Max
Unit
4.5
18
V
Power Good Pin Voltage
PG
4.5
18
V
Switch Pin Voltage
VSW
−0.3
18
V
Enable Pin Voltage
EN
0
18
V
Comp Pin Voltage
COMP
−0.1
5.5
V
Feedback Pin Voltage
FB
−0.1
5.5
V
PGND
−0.1
−0.1
V
Junction Temperature Range
TJ
−40
125
°C
Operating Temperature Range
TA
−40
85
°C
Power Ground Pin Voltage
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NCP3170
Table 4. ELECTRICAL CHARACTERISTICS
(TA = 25°C, VIN = VEN = 12 V, VOUT = 3.3 V for min/max values unless otherwise noted (Note 7))
Conditions
Min
Typ
Max
Unit
(Note 5)
4.5
−
18
V
VIN = EN = 12 V VFB = 0.8 V
(Note 5)
−
−
1.7
1.7
2.0
2.0
mA
EN = 0 V (Note 5)
−
13
17
mA
VIN UVLO Threshold
VIN Rising Edge (Note 5)
−
4.41
−
V
VIN UVLO Threshold
VIN Falling Edge (Note 5)
−
4.13
−
V
Enable = VIN
450
900
500
1000
550
1100
kHz
91
90
−
−
96
96
%
VIN = 12 V
6.0
4.0
−
−
11
11.5
%
VFB = VCOMP
3.5
4.6
6.0
ms
(Note 4)
4.0
−
6.0
A
TA = 25°C
0.792
0.8
0.808
V
(Note 4)
−
1
−
%
−
201
−
mS
Characteristic
Input Voltage Range
SUPPLY CURRENT
Quiescent Supply Current
NCP3170A
NCP3170B
Shutdown Supply Current
UNDER VOLTAGE LOCKOUT
MODULATOR
Oscillator Frequency
NCP3170A
NCP3170B
Maximum Duty Ratio
NCP3170A
NCP3170B
Minimum Duty Ratio
NCP3170A
NCP3170B
VIN Soft Start Ramp Time
OVER CURRENT
Current Limit
PWM COMPENSATION
VFB Feedback Voltage
Line Regulation
GM
AOL DC gain
(Note 4)
40
55
−
dB
Unity Gain BW (COUT = 10 pF)
(Note 4)
2.0
−
−
MHz
Input Bias Current (Current Out of FB IB Pin)
(Note 4)
−
−
286
nA
IEAOP Output Source Current
VFB = 0 V
−
20.1
−
mA
IEAOM Output Sink Current
VFB = 2 V
−
21.3
−
mA
(Note 5)
−
1.41
−
V
Power Good High On Threshold
−
875
−
mV
Power Good High Off Threshold
−
859
−
mV
Power Good Low On Threshold
−
712
−
mV
Power Good Low Off Threshold
−
728
−
mV
Over Voltage Protection Threshold
−
998
−
mV
VIN = 12 V, IPG = 500 mA
−
0.195
−
V
High-Side Switch On-Resistance
VIN = 12 V
VIN = 4.5 V
−
−
90
100
130
150
mW
Low-Side Switch On-Resistance
VIN = 12 V
VIN = 4.5 V
−
−
25
29
35
39
mW
(Notes 4 and 6)
−
164
−
°C
−
43
−
°C
ENABLE
Enable Threshold
POWER GOOD
Power Good Low Voltage
PWM OUTPUT STAGE
THERMAL SHUTDOWN
Thermal Shutdown
Hysteresis
4.
5.
6.
7.
Guaranteed by design
Ambient temperature range of −40°C to +85°C.
This is not a protection feature.
The device is not guaranteed to operate beyond the maximum operating ratings.
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NCP3170
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit from Figure 1, TA = 25°C, VIN = VEN = 12 V, VOUT = 3.3 V unless otherwise specified)
Figure 3. Light Load (DCM) Operation 1 ms/DIV
Figure 4. Full Load (CCM) Operation 1 ms/DIV
Figure 5. Start−Up into Full Load 1 ms/DIV
Figure 6. Short−Circuit Protection 200 ms /DIV
Figure 7. 50% to 100% Load Transient 100 ms/DIV
Figure 8. 3.3 V Turn on into 1 V Pre−Bias 1 ms /DIV
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NCP3170
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit from Figure 1, TA = 25°C, VIN = VEN = 12 V, VOUT = 3.3 V unless otherwise specified)
30
2.1
24
21
18
15
Input Voltage = 12 V
12
9
6
3
0
−50 −30
10
30
50
70
90
110
Input Voltage = 4.5 V
1.5
−10
10
30
50
70
90
110 130
Figure 10. NCP3170 Enabled Current vs.
Temperature
503
Input Voltage = 18 V
SWITCHING FREQUENCY (kHz)
BANDGAP REFERENCE (mV)
1.6
Figure 9. ICC Shut Down Current vs.
Temperature
803
Input Voltage = 12 V
801
800
Input Voltage = 4.5 V
799
798
797
−50 −30
−10
10
30
50
70
90
110
502
Input Voltage = 18 V
Input Voltage = 4.5 V
501
Input Voltage = 12 V
500
499
498
497
496
−50 −30
130
−10
10
30
50
70
90
110 130
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 11. Bandgap Reference Voltage vs.
Temperature
Figure 12. Switching Frequency vs.
Temperature
880
TRIP VOLTAGE AT FB PIN (mV)
735
TRIP VOLTAGE AT FB PIN (mV)
1.7
TEMPERATURE (°C)
804
Under Voltage Protection Rising
725
720
715
1.8
TEMPERATURE (°C)
805
730
Input Voltage = 12 V
1.3
−50 −30
130
806
802
1.9
1.4
Input Voltage = 4.5 V
−10
Input Voltage = 18 V
2.0
Input Voltage = 18 V
CURRENT DRAW (mA)
CURRENT DRAW (mA)
27
Under Voltage Protection Falling
710
705
−50 −30
−10
10
30
50
70
90
110
875
Over Voltage Protection Falling
870
865
Over Voltage Protection Rising
860
855
−50 −30
130
−10
10
30
50
70
90
110 130
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 13. Input Under Voltage Protection at
12 V vs. Temperature
Figure 14. Input Over Voltage Protection at
12 V vs. Temperature
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NCP3170
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit from Figure 1, TA = 25°C, VIN = VEN = 12 V, VOUT = 3.3 V unless otherwise specified)
40
LOW SIDE MOSFET RDS(on) (mW)
HIGH SIDE MOSFET RDS(on) (mW)
130
120
110
Input Voltage = 4.5 V
100
90
Input Voltage = 12 V, 18 V
80
70
60
−50 −30
−10
10
30
50
70
90
110
Input Voltage = 4.5 V
25
Input Voltage = 12 V, 18 V
20
−10
10
30
50
70
90
110 130
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 15. High Side MOSFET RDS(on) vs.
Temperature
Figure 16. Low Side MOSFET RDS(on) vs.
Temperature
1001.5
TRIP VOLTAGE AT FB PIN (mV)
Input Voltage = 12 V
Input Voltage = 4.5 V
205
200
Input Voltage = 18 V
195
190
185
180
−50 −30
−10
10
30
50
70
90
110
130
1001.0
1000.5
1000.0
999.5
999.0
Input Voltage = 4.5 V
998.5
Input Voltage = 18 V
998.0
Input Voltage = 12 V
997.5
997.0
996.5
−50 −30
−10
10
30
50
70
90
110 130
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 17. Transconductance vs. Temperature
Figure 18. Over Voltage Protection vs.
Temperature
4.45
TRIP VOLTAGE AT FB PIN (mV)
TRANSCONDUCTANCE (mS)
30
15
−50 −30
130
215
210
35
4.40
Input Under Voltage Protection Rising
4.35
4.30
4.25
4.20
4.15
Input Under Voltage Protection Falling
4.10
4.05
−50 −30
−10
10
30
50
70
90
110
130
TEMPERATURE (°C)
Figure 19. Input Under Voltage Protection vs.
Temperature
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NCP3170
NCP3170A Efficiency and Thermal Derating
100
100
90
90
70
Vo = 1.8 V
60
80
Vo = 5 V
Vo = 3.3 V
EFFICIENCY (%)
EFFICIENCY (%)
80
Vo = 1.2 V
50
40
30
20
0
0
1
2
Vo = 1.8 V
Vo = 1.2 V
60
Vo = 3.3 V
50
40
30
20
12 V, 500 kHz
Efficiency
10
70
5 V, 500 kHz
Efficiency
10
0
3
0
1
2
3
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
Figure 20. Efficiency (VIN = 12 V) vs. Load
Current
Figure 21. Efficiency (VIN = 5 V) vs. Load Current
Thermal derating curves for the SOIC−8 package part under typical input and output conditions based on the evaluation board.
The ambient temperature is 25°C with natural convection (air speed < 50 LFM) unless otherwise specified.
5
IOUT, AMBIENT TEMPERATURE (°C)
IOUT, AMBIENT TEMPERATURE (°C)
5
4
1.2 V, 1.8 V,
3.3 V
3
2
1
0
25
35
45
55
65
75
TA, AMBIENT TEMPERATURE (°C)
85
4
1.2 V, 1.8 V,
3.3 V, 5.0 V
3
2
1
0
25
Figure 22. 500 kHz Derating Curves at 5 V
35
45
55
65
75
TA, AMBIENT TEMPERATURE (°C)
Figure 23. 500 kHz Derating Curves at 12 V
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85
NCP3170
NCP3170B Efficiency and Thermal Derating
100
100
90
90
80
Vo = 3.3 V
70
Vo = 5 V
EFFICIENCY (%)
EFFICIENCY (%)
80
Vo = 1.8 V
60
Vo = 1.2 V
50
40
30
20
0
0
1
2
Vo = 1.8 V
Vo = 1.2 V
60
Vo = 3.3 V
50
40
30
20
12 V, 1 MHz
Efficiency
10
70
5 V, 1 MHz
Efficiency
10
0
3
0
1
2
3
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
Figure 24. 12 V, 1 MHz Efficiency
Figure 25. 5 V, 1 MHz Efficiency
Thermal derating curves for the SOIC−8 package part under typical input and output conditions based on the evaluation board.
The ambient temperature is 25°C with natural convection (air speed < 50 LFM) unless otherwise specified.
5
IOUT, AMBIENT TEMPERATURE (°C)
IOUT, AMBIENT TEMPERATURE (°C)
5
4
1.2 V,
1.8 V
3
3.3 V
2
1
0
25
35
45
55
65
75
85
4
1.2 V,
1.8 V
3
3.3 V
2
5.0 V
1
0
25
TA, AMBIENT TEMPERATURE (°C)
35
45
55
65
75
85
TA, AMBIENT TEMPERATURE (°C)
Figure 26. 1 MHz Derating Curves at 5 V Input
Figure 27. 1 MHz Derating Curves at 12 V Input
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NCP3170
DETAILED DESCRIPTION
The NCP3170 is a current-mode, step down regulator
with an integrated high-side PMOS switch and a low-side
NMOS switch. It operates from a 4.5 V to 18 V input voltage
range and supplies up to 3 A of load current. The duty ratio
can be adjusted from 8% to 92% allowing a wide output
voltage range. Features include enable control, Power-On
Reset (POR), input under voltage lockout, fixed internal soft
start, power good indication, over voltage protection, and
thermal shutdown.
The enable pin can be used to delay a turn on by
connecting a capacitor as shown in Figure 30.
4.5 V−18 V
C1IN
Rbias
EN
AGND
An internal input voltage comparator not shown in
Figure 28 will force the part to disable below the minimum
input voltage of 4.13 V. The input under voltage disable
feature is used to prevent improper operation of the
converter due to insufficient voltages. The converter can be
turned on by tying the enable pin high and the part will
default to be input voltage enabled. The enable pin should
never be left floating.
Figure 30. Delay Enable
If the designer would like to add hysteresis to the enable
threshold it can be added by use of a bias resistor to the
output. The hysteresis is created once soft start has initiated.
With the output voltage rising, current flows into the enable
node, raising the voltage. The thresholds for enable as well
as hysteresis can be calculated using Equation 1.
VIN
VIN HYS + VIN Start * EN TH ) R1 UV
C1IN
EN
ƪ
NCP3170
R3 UV
where:
ENTH
VINSTART
R1UV
R2UV
R3UV
VOUT
Figure 28. Input Voltage Enable
If an adjustable Under Voltage Lockout (UVLO)
threshold is required, the EN pin can be used. The trip
voltage of the EN pin comparator is 1.38 V typical. Upon
application of an input voltage greater than 4.41 V, the VIN
UVLO will release and the enable will be checked to
determine if switching can commence. Once the 1.38 V trip
voltage is crossed, the part will enable and the soft start
sequence will initiate. If large resistor values are used, the
EN pin should be bypassed with a 1 nF capacitor to prevent
coupling problems from the switch node.
ƪ
1)
ƫ
(eq. 1)
EN TH
R2 UV
ǒR2 UV ) R3 UVǓ
R2 UV
R3 UV
ƫ
(eq. 2)
= Enable Threshold
= Input Voltage Start Threshold
= High Side Resistor
= Low Side Resistor
= Hysteresis Bias Resistor
= Regulated Output Voltage
VIN
C1IN
R1UV
EN
VIN
R3UV
VOUT
R1UV
EN
*
R1 UV
4.5 V−18 V
C1IN
C1UV
V OUT * EN TH
VIN Start + EN TH
AGND
4.5 V−18 V
NCP3170
C1DLY
Enable and Soft-Start
4.5 V−18 V
VIN
NCP3170
NCP3170
R2UV
AGND
Figure 31. Added Hysteresis to the Enable UVLO
R2UV
AGND
Figure 29. Input Under Voltage Lockout Enable
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NCP3170
slowly raises and the OTA regulates the output voltage to the
divided reference voltage. In a pre-biased condition, the
voltage at the FB pin is higher than the internal reference
voltage, so the OTA will keep the COMP voltage at ground
potential. As the internal reference is slewed up, the COMP
pin is held low until the FB pin voltage surpasses the internal
reference voltage, at which time the COMP pin is allowed
to respond to the OTA error signal. Since the bottom of the
PWM ramp is at 0.6 V there will be a slight delay between
the time the internal reference voltage passes the FB voltage
and when the part starts to switch. Once the COMP error
signal intersects with the bottom of the ramp, the high side
switch is turned on followed by the low side switch. After the
internal reference voltage has surpassed the FB voltage, soft
start proceeds normally without output voltage discharge.
The part can be enabled with standard TTL or high voltage
logic by using the configuration below.
4.5 V−18 V
VIN
C1IN
R1LOG
EN
C1LOG
R2LOG
NCP3170
AGND
Figure 32. Logic Turn-on
The enable can also be used for power sequencing in
conjunction with the Power Good (PG) pin as shown in
Figure 33. The enable pin can either be tied to the output
voltage of the master voltage or tied to the input voltage with
a resistor to the PG pin of the master regulator.
Power Good
The output voltage of the buck converter is monitored at
the feedback pin of the output power stage. Two
comparators are placed on the feedback node of the OTA to
monitor the operating window of the feedback voltage as
shown in Figure 34. All comparator outputs are ignored
during the soft start sequence as soft start is regulated by the
OTA since false trips would be generated. Further, the PG
pin is held low until the comparators are evaluated. PG state
does not affect the switching of the converter. After the soft
start period has ended, if the feedback is below the reference
voltage of comparator 1 (VFB < 0.726), the output is
considered operational undervoltage (OUV). The device
will indicate the under voltage situation by the PG pin
remaining low with a 100 kW pull-up resistance. When the
feedback pin voltage rises between the reference voltages of
comparator 1 and comparator 2 (0.726 < VFB < 0.862),
then the output voltage is considered power good and the PG
pin is released. Finally, if the feedback voltage is greater than
comparator 2 (VFB > 0.862), the output voltage is
considered operational overvoltage (OOV). The OOV will
be indicated by the PG pin remaining low. A block diagram
of the OOV and OUV functionality as well as a graphical
representation of the PG pin functionality is shown in
Figures 34 through 36.
4.5 V−18 V
VIN
VSW
Vo1
EN
PG
FB
AGND
Vo1
NCP3170
4.5 V−18 V
Vo2
VIN
VSW
Vo2
EN
FB
AGND
NCP3170
Figure 33. Enable Two Converter Power Sequencing
Once the part is enabled, the internal reference voltage is
slewed from ground to the set point of 800 mV. The slewing
process occurs over a 4.5 ms period, reducing the current
draw from the upstream power source, reducing stress on
internal MOSFETS, and ensuring the output inductor does
not saturate during start-up.
FB
800 mV
−
862 mV
726 mV
Pre-Bias Start-up
When starting into a pre-bias load, the NCP3170 will not
discharge the output capacitors. The soft start begins with
the internal reference at ground. Both the high side switch
and low side switches are turned off. The internal reference
12 V
+
Comp 2
+
−
SOFT
Start
Complete
+
−
Comp 1
Figure 34. OOV and OUV System
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11
100 kW
PG
NCP3170
Light Load Operation
Hysteresis = 14 mV
Light load operation is generally a load that is 1 mA to
300 mA where a load is in standby mode and requires very
little power. During light load operation, the regulator
emulates the operation of a non-synchronous buck converter
and the regulator is allowed to skip pulses. The
non-synchronous buck emulation is accomplished by
detecting the point at which the current flowing in the
inductor goes to zero and turning the low side switch off. At
the point when the current goes to zero, if the low side switch
is not turned off, current would reverse, discharging the
output capacitor. Since the low side switch is shutoff, the
only conduction path is through the body diode of the low
side MOSFET, which is back biased. Unlike traditional
synchronous buck converters, the current in the inductor
will become discontinuous. As a result, the switch node will
oscillate with the parasitic inductances and capacitances
connected to the switch node. The OTA will continue to
regulate the output voltage, but will skip pulses based on the
output load shown in Figure 37.
OOV
Hysteresis = 14 mV
VOOV = 862 mV
Power Good
OUV
VREF = 0.8 V
VOUV = 726 mV
Figure 35. OOV and OUV Window
0.862 V
0.8 V
0.726 V
FB Voltage
Soft Start Complete
6 ms = 166 kHz
Power Good
2 ms = 50 kHz
Figure 36. OOV and OUV Diagram
Switch
Node
If the power good function is not used, it can be connected
to the VSW node to reduce thermal resistance. Do not
connect PG to the VSW node if the application is turning on
into pre-bias.
0V
Zero Current Point
Inductor
Current
0A
Feedback
Voltage
Switching Frequency
COMP
Voltage
The NCP3170 switching frequency is fixed and set by an
internal oscillator. The practical switching frequency could
range from 450 kHz to 550 kHz for the NCP3170A and
900 kHz to 1.1 MHz for the NCP3170B due to device
variation.
Reference Votlage
Ramp Threshold
Figure 37. Light Load Operation
PROTECTION FEATURES
Over Current Protection
Current is limited to the load on a pulse by pulse basis.
During each high side on period, the current is compared
against an internally set limit. If the current limit is
exceeded, the high side and low side MOSFETS are shutoff
and no pulses are issued for 13.5 ms. During that time, the
output voltage will decay and the inductor current will
discharge. After the discharge period, the converter will
initiate a soft start. If the load is not released, the current will
build in the inductor until the current limit is exceeded, at
which time the high side and low side MOSFETS will be
shut off and the process will continue. If the load has been
released, a normal soft start will commence and the part will
continue switching normally until the current limit is
exceeded.
Switch
Node
13.5 ms Hold Time
Current Limit
Inductor
Current
Figure 38. Over Current Protection
Thermal Shutdown
The thermal limit, while not a protection feature, engages
at 150°C in case of thermal runaway. When the thermal
comparator is tripped at a die temperature of 150°C, the part
must cool to 120°C before a restart is allowed. When thermal
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12
NCP3170
operating in PWM mode. Figure 41 and 42 below shows the
safe operating area for the NCP3170A and B respectively.
While not shown in the safe operating area graph, the output
voltage is capable of increasing to the 93% duty ratio
limitation providing a high output voltage such as 16 V. If
the application requires a high duty ratio such as converting
from 14 V to 10 V the converter will operate normally until
the maximum duty ratio is reached. For example, if the input
voltage were 16 V and the user wanted to produce the
highest possible output voltage at full load, a good rule of
thumb is to use 80% duty ratio. The discrepancy between the
usable duty ratio and the actual duty ratio is due to the
voltage drops in the system, thus leading to a maximum
output voltage of 12.8 V rather than 14.8 V. The actual
achievable output to input voltage ratio is dependent on
layout, component selection, and acceptable output voltage
tolerance.
trip is engaged, switching ceases and high side and low side
MOSFETs are driven off. Further, the power good indicator
will pull low until the thermal trip has been released. Once
the die temperature reaches 120°C the part will reinitiate
soft-start and begin normal operation.
Switch
Node
Output
Voltage
Thermal
Comparator
150°C
120°C
IC
Temperature
Figure 39. Over Temperature Shutdown
Over Voltage Protection
Upon the completion of soft start, the output voltage of the
buck converter is monitored at the FB pin of the output
power stage. One comparator is placed on the feedback node
to provide over voltage protection. In the event an over
voltage is detected, the high side switch turns off and the low
side switch turns on until the feedback voltage falls below
the OOV threshold. Once the voltage has fallen below the
OOV threshold, switching continues normally as displayed
in Figure 40.
1.0 V
Figure 41. NCP3170A Safe Operating Area
0.862 V
0.800 V
0.726 V
FB Voltage
Softstart
Complete
Power
Good
Low Side
Switch
Figure 40. Over Voltage Low Side Switch Behavior
Figure 42. NCP3170B Safe Operating Area
Duty Ratio
Design Procedure
The duty ratio can be adjusted from 8% to 92% allowing
a wide output voltage range. The low 8% duty ratio limit will
restrict the PWM operation. For example if the application
is converting to 1.2 V the converter will perform normally
if the input voltage is below 15.5 V. If the input voltage
exceeds 15.5 V while supplying 1.2 V output voltage the
converter can skip pulses during operation. The skipping
pulse operation will result in higher ripple voltage than when
When starting the design of a buck regulator, it is important
to collect as much information as possible about the behavior
of the input and output before starting the design.
ON Semiconductor has a Microsoft Excel® based design
tool available online under the design tools section of the
NCP3170 product page. The tool allows you to capture your
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13
NCP3170
design point and optimize the performance of your regulator
based on your design criteria.
ra +
where:
ąDI
IOUT
ra
Table 5. DESIGN PARAMETERS
Design Parameter
Example Value
Input Voltage (VIN)
9 V to 16 V
Output Voltage (VOUT)
3.3 V
Input Ripple Voltage (VCCRIPPLE)
200 mV
Output Ripple Voltage (VOUTRIPPLE)
20 mV
Output Current Rating (IOUT)
3A
Operating Frequency (FSW)
500 kHz
D+
D+
where:
D
FSW
T
TOFF
TON
VIN
VHSD
VLSD
VOUT
T ON
T
(1 * D ) +
T
V IN * V HSD ) V LSD
V IN
³ 27.5% +
V OUT
I OUT
ra
F SW
(1 * D ) ³
(eq. 7)
12 V
3.0 A
where:
D
FSW
IOUT
LOUT
ra
34%
500 kHz
( 1 * 27.5% )
= Duty ratio
= Switching frequency
= Output current
= Output inductance
= Ripple current ratio
19
17
(eq. 4)
15
V OUT ) V LSD
V OUT
4.7 mH +
(eq. 3)
T OFF
= Ripple current
= Output current
= Ripple current ratio
L OUT +
[
3.3 V
(eq. 5)
INDUCTANCE (mH)
D+
1
T
(eq. 6)
Using the ripple current rule of thumb, the user can
establish acceptable values of inductance for a design using
Equation 6.
The buck converter produces input voltage (VIN) pulses
that are LC filtered to produce a lower DC output voltage
(VOUT). The output voltage can be changed by modifying
the on time relative to the switching period (T) or switching
frequency. The ratio of high side switch on time to the
switching period is called duty ratio (D). Duty ratio can also
be calculated using VOUT, VIN, the Low Side Switch Voltage
Drop (VLSD), and the High Side Switch Voltage Drop
(VHSD).
F SW +
DI
I OUT
12 V
= Duty ratio
= Switching frequency
= Switching period
= High side switch off time
= High side switch on time
= Input voltage
= High side switch voltage drop
= Low side switch voltage drop
= Output voltage
13
11
18 V
9
7V
7
4.7 mH
5
3
4.4 V
1
10
13
16
19
22
25
28
31
34
37
40
RIPPLE CURRENT RATIO (%)
Inductor Selection
Figure 43. Inductance vs. Current Ripple Ratio
When selecting an inductor, the designer may employ a
rule of thumb for the design where the percentage of ripple
current in the inductor should be between 10% and 40%.
When using ceramic output capacitors, the ripple current can
be greater because the ESR of the output capacitor is smaller,
thus a user might select a higher ripple current. However,
when using electrolytic capacitors, a lower ripple current
will result in lower output ripple due to the higher ESR of
electrolytic capacitors. The ratio of ripple current to
maximum output current is given in Equation 6.
When selecting an inductor, the designer must not exceed
the current rating of the part. To keep within the bounds of
the part’s maximum rating, a calculation of the RMS current
and peak current are required.
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NCP3170
Ǹ1 ) ra12 ³
2
I RMS + I OUT
3.01 A + 3 A
where:
IOUT
IRMS
ra
Ǹ1 ) 34%
12
(eq. 8)
³
1.02 A +
where:
D
FSW
IPP
LOUT
VOUT
= Output current
= Inductor RMS current
= Ripple current ratio
I PK + I OUT
3.51 A + 3 A
where:
IOUT
IPK
ra
2
I PP +
ǒ1 ) raǓ ³
2
ǒ
Ǔ
34%
1)
2
(eq. 9)
A standard inductor should be found so the inductor will
be rounded to 4.7 mH. The inductor should support an RMS
current of 3.01 A and a peak current of 3.51 A. A good
design practice is to select an inductor that has a saturation
current that exceeds the maximum current limit with some
margin.
The final selection of an output inductor has both
mechanical and electrical considerations. From a
mechanical perspective, smaller inductor values generally
correspond to smaller physical size. Since the inductor is
often one of the largest components in the regulation system,
a minimum inductor value is particularly important in space
constrained applications. From an electrical perspective, the
maximum current slew rate through the output inductor for
a buck regulator is given by Equation 10.
V IN * V OUT
L OUT
where:
LOUT
VIN
VOUT
F SW
L OUT
3.3 V
³
(eq. 11)
( 1 * 27.5% )
4.7 mH
500 kHz
= Duty ratio
= Switching frequency
= Peak-to-peak current of the inductor
= Output inductance
= Output voltage
2
DCR ³
61 mW + 3.01 2
6.73 mW
LP CU_DC + I RMS
where:
DCR
IRMS
LPCU_DC
(eq. 12)
= Inductor DC resistance
= Inductor RMS current
= Inductor DC power dissipation
The core losses and AC copper losses will depend on the
geometry of the selected core, core material, and wire used.
Most vendors will provide the appropriate information to
make accurate calculations of the power dissipation at which
point the total inductor losses can be captured by the
equation below:
LP tot + LP CU_DC ) LP CU_AC ) LP Core ³
³
12 V * 3.3 V
A
1.85
+
ms
4.7 mH
(1 * D )
From Equation 11, it is clear that the ripple current
increases as LOUT decreases, emphasizing the trade-off
between dynamic response and ripple current.
The power dissipation of an inductor falls into two
categories: copper and core losses. Copper losses can be
further categorized into DC losses and AC losses. A good
first order approximation of the inductor losses can be made
using the DC resistance as shown below:
= Output current
= Inductor peak current
= Ripple current ratio
SlewRate LOUT +
V OUT
67 mW + 61 mW ) 5 mW ) 1 mW
(eq. 10)
where:
LPCore
LPCU_AC
LPCU_DC
LPtot
= Output inductance
= Input voltage
= Output voltage
(eq. 13)
= Inductor core power dissipation
= Inductor AC power dissipation
= Inductor DC power dissipation
= Total inductor losses
Output Capacitor Selection
The important factors to consider when selecting an
output capacitor are DC voltage rating, ripple current rating,
output ripple voltage requirements, and transient response
requirements.
The output capacitor must be able to operate properly for
the life time of a product. When selecting a capacitor it is
important to select a voltage rating that is de-rated to the
guaranteed operating life time of a product. Further, it is
important to note that when using ceramic capacitors, the
capacitance decreases as the voltage applied increases; thus
a ceramic capacitor rated at 100 mF 6.3 V may measure
Equation 10 implies that larger inductor values limit the
regulator’s ability to slew current through the output
inductor in response to output load transients. Consequently,
output capacitors must supply the load current until the
inductor current reaches the output load current level.
Reduced inductance to increase slew rates results in larger
values of output capacitance to maintain tight output voltage
regulation. In contrast, smaller values of inductance increase
the regulator’s maximum achievable slew rate and decrease
the necessary capacitance at the expense of higher ripple
current. The peak-to-peak ripple current for NCP3170 is
given by the following equation:
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NCP3170
100 mF at 0 V but measure 20 mF with an applied voltage of
3.3 V depending on the type of capacitor selected.
The output capacitor must be rated to handle the ripple
current at full load with proper derating. The capacitor RMS
ratings given in datasheets are generally for lower switching
frequencies than used in switch mode power supplies, but a
multiplier is given for higher frequency operation. The RMS
current for the output capacitor can be calculated below:
CO RMS + I OUT
1.84 mV +
V ESLOFF +
0.7 mV +
ra
³
Ǹ12
where:
D
ESL
FSW
IPP
(eq. 14)
34%
0.294 A + 3.0 A
Ǹ12
where:
CoRMS
IOUT
ra
V ESLON +
= Output capacitor RMS current
= Output current
= Ripple current ratio
ra
ǒ
CO ESR )
1
8
F SW
C OUT
Ǔ
where:
CoESR
COUT
FSW
IOUT
ra
VESR_C
34%
ǒ
5 mW )
1
8
500 kHz
F SW
³
D
(eq. 16)
1 nH @ 1.01 A @ 500 kHz
27.5%
ESL
I PP
F SW
(1 * D )
1 nH
1.1 A
³
(eq. 17)
500 kHz
( 1 * 27.5% )
= Duty ratio
= Capacitor inductance
= Switching frequency
= Peak-to-peak current
DV OUT−ESR + I TRAN
³
7.5 mV + 1.5 A
where:
CoESR
(eq. 15)
10.89 mV + 3
I PP
The output capacitor is a basic component for fast
response of the power supply. For the first few microseconds
of a load transient, the output capacitor supplies current to
the load. Once the regulator recognizes a load transient, it
adjusts the duty ratio, but the current slope is limited by the
inductor value.
During a load step transient, the output voltage initially
drops due to the current variation inside the capacitor and the
ESR (neglecting the effect of the ESL).
The maximum allowable output voltage ripple is a
combination of the ripple current selected, the output
capacitance selected, the Equivalent Series Inductance
(ESL), and Equivalent Series Resistance (ESR).
The main component of the ripple voltage is usually due
to the ESR of the output capacitor and the capacitance
selected, which can be calculated as shown in Equation 14:
V ESR_C + I OUT
ESL
Ǔ
44 mF
ITRAN
ąDVOUT_ESR
= Output capacitor ESR
= Output capacitance
= Switching frequency
= Output current
= Ripple current ratio
= Ripple voltage from the capacitor
CO ESR ³
(eq. 18)
5 mW
= Output capacitor Equivalent Series
Resistance
= Output transient current
= Voltage deviation of VOUT due to the
effects of ESR
A minimum capacitor value is required to sustain the
current during the load transient without discharging it. The
voltage drop due to output capacitor discharge is given by
the following equation:
The impedance of a capacitor is a function of the
frequency of operation. When using ceramic capacitors, the
ESR of the capacitor decreases until the resonant frequency
is reached, at which point the ESR increases; therefore the
ripple voltage might not be what one expected due to the
switching frequency. Further, the method of layout can add
resistance in series with the capacitance, increasing ripple
voltage.
The ESL of capacitors depends on the technology chosen,
but tends to range from 1 nH to 20 nH, where ceramic
capacitors have the lowest inductance and electrolytic
capacitors have the highest. The calculated contributing
voltage ripple from ESL is shown for the switch on and
switch off below:
DV OUT−DIS +
138.1 mV +
ǒI TRANǓ
2
(1.5)
2
where:
COUT
D
FSW
FCROSS
ITRAN
LOUT
VIN
VOUT
ąDVOUT_DIS
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16
F CROSS
2
50 kHz
2
L OUT
C OUT
F SW
ǒVIN * V OUTǓ
³
(eq. 19)
4.7 mH
500 kHz
44 mF
ǒ 12 V * 3.3 V Ǔ
= Output capacitance
= Duty ratio
= Switching frequency
= Loop cross over frequency
= Output transient current
= Output inductor value
= Input voltage
= Output voltage
= Voltage deviation of VOUT due to the
effects of capacitor discharge
NCP3170
The equation reaches its maximum value with D = 0.5 at
which point the input capacitance RMS current is half the
output current. Loss in the input capacitors can be calculated
with the following equation:
In a typical converter design, the ESR of the output
capacitor bank dominates the transient response. Please note
that DVOUT_DIS and DVOUT_ESR are out of phase with each
other, and the larger of these two voltages will determine the
maximum deviation of the output voltage (neglecting the
effect of the ESL). It is important to note that the converters
frequency response will change when the NCP3170 is
operating in synchronous mode or non-synchronous mode
due to the change in plant response from CCM to DCM. The
effect will be a larger transient voltage excursion when
transitioning from no load to full load quickly.
18 mW + 10 mW
where:
CINESR
The input capacitor has to sustain the ripple current
produced during the on time of the upper MOSFET, so it
must have a low ESR to minimize losses and input voltage
ripple. The RMS value of the input ripple current is:
Iin RMS + I OUT ǸD (1 * D) ³
where:
D
IinRMS
IOUT
Ǹ27.5%
2
(eq. 21)
ǒ 1.34 A Ǔ 2
= Input capacitance Equivalent Series
Resistance
= Input capacitance RMS current
= Power loss in the input capacitor
IinRMS
PCIN
Input Capacitor Selection
1.34 A + 3 A
ǒIin RMSǓ
P CIN + CIN ESR
Due to large di/dt through the input capacitors, electrolytic
or ceramics should be used. If a tantalum capacitor must be
used, it must be surge protected, otherwise capacitor failure
could occur.
(eq. 20)
( 1 * 27.5% )
= Duty ratio
= Input capacitance RMS current
= Load current
POWER MOSFET DISSIPATION
Power dissipation, package size, and the thermal
environment drive power supply design. Once the
dissipation is known, the thermal impedance can be
calculated to prevent the specified maximum junction
temperatures from being exceeded at the highest ambient
temperature.
Power dissipation has two primary contributors:
conduction losses and switching losses. The high-side
MOSFET will display both switching and conduction
losses. The switching losses of the low side MOSFET will
not be calculated as it switches into nearly zero voltage and
the losses are insignificant. However, the body diode in the
low-side MOSFET will suffer diode losses during the
non-overlap time of the gate drivers.
Starting with the high-side MOSFET, the power
dissipation can be approximated from:
P D_HS + P COND ) P SW_TOT
where:
PCOND
PD_HS
PSW_TOT
where:
IRMS_HS
RDS(ON)_HS
PCOND
Using the ra term from Equation 6, IRMS becomes:
I RMS_HS + I OUT
where:
D
ra
IOUT
IRMS_HS
(eq. 22)
R DS(on)_HS
D
ǒ1 ) ra12 Ǔ
2
(eq. 24)
= Duty ratio
= Ripple current ratio
= Output current
= High side MOSFET RMS current
P SW_TOT + P SW ) P DS ) P RR
where:
PDS
PRR
PSW
PSW_TOT
The first term in Equation 21 is the conduction loss of the
high-side MOSFET while it is on.
2
Ǹ
The second term from Equation 22 is the total switching
loss and can be approximated from the following equations.
= Conduction losses
= Power losses in the high side MOSFET
= Total switching losses
P COND + ǒI RMS_HSǓ
= RMS current in the high side MOSFET
= On resistance of the high side MOSFET
= Conduction power losses
(eq. 23)
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(eq. 25)
= High side MOSFET drain to source losses
= High side MOSFET reverse recovery
losses
= High side MOSFET switching losses
= High side MOSFET total switching losses
NCP3170
QGD
RG
RHSPD
tFALL
VCL
VTH
The first term for total switching losses from Equation 25
are the losses associated with turning the high-side
MOSFET on and off and the corresponding overlap in drain
voltage and current.
P SW + P TON ) P TOFF +
+
1
2
ǒIOUT
where:
FSW
IOUT
PSW
PTON
PTOFF
tFALL
tRISE
VIN
V IN
F SWǓ
ǒtRISE ) t FALLǓ
(eq. 26)
= MOSFET gate to drain gate charge
= MOSFET gate resistance
= Drive pull down resistance
= MOSFET fall time
= Clamp voltage
= MOSFET gate threshold voltage
Next, the MOSFET output capacitance losses are caused
by both the high-side and low-side MOSFETs, but are
dissipated only in the high-side MOSFET.
= Switching frequency
= Load current
= High side MOSFET switching losses
= Turn on power losses
= Turn off power losses
= MOSFET fall time
= MOSFET rise time
= Input voltage
P DS +
where:
COSS
FSW
PDS
VIN
When calculating the rise time and fall time of the high
side MOSFET, it is important to know the charge
characteristic shown in Figure 44.
1
2
C OSS
V IN
2
(eq. 29)
F SW
= MOSFET output capacitance at 0 V
= Switching frequency
= MOSFET drain to source charge losses
= Input voltage
Finally, the loss due to the reverse recovery time of the
body diode in the low−side MOSFET is shown as follows:
P RR + Q RR
where:
FSW
PRR
V IN
F SW
(eq. 30)
= Switching frequency
= High side MOSFET reverse recovery
losses
= Reverse recovery charge
= Input voltage
QRR
VIN
The low-side MOSFET turns on into small negative
voltages so switching losses are negligible. The low-side
MOSFET’s power dissipation only consists of conduction
loss due to RDS(on) and body diode loss during non-overlap
periods.
Vth
P D_LS + P COND ) P BODY
where:
PBODY
PCOND
PD_LS
Figure 44. High Side MOSFET Total Charge
t RISE +
where:
IG1
QGD
RHSPU
RG
tRISE
VCL
VTH
t FALL +
where:
IG2
Q GD
I G1
+
Q GD
ǒVCL * VTHǓńǒRHSPU ) R GǓ
(eq. 27)
I G2
+
Q GD
ǒVCL * VTHǓńǒRHSPD ) R GǓ
= Low side MOSFET body diode losses
= Low side MOSFET conduction losses
= Low side MOSFET losses
Conduction loss in the low-side MOSFET is described as
follows:
P COND + ǒI RMS_LSǓ
= Output current from the high-side gate
drive
= MOSFET gate to drain gate charge
= Drive pull up resistance
= MOSFET gate resistance
= MOSFET rise time
= Clamp voltage
= MOSFET gate threshold voltage
Q GD
(eq. 31)
where:
IRMS_LS
RDS(ON)_LS
PCOND
where:
D
IOUT
IRMS_LS
ra
= Output current from the low-side gate
drive
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18
R DS(on)_LS
(eq. 32)
= RMS current in the low side
= Low-side MOSFET on resistance
= High side MOSFET conduction losses
I RMS_LS + I OUT
(eq. 28)
2
Ǹ
(1 * D )
ǒ1 ) ra12 Ǔ
2
= Duty ratio
= Load current
= RMS current in the low side
= Ripple current ratio
(eq. 33)
NCP3170
The body diode losses can be approximated as:
P BODY + V FD I OUT F SW ǒNOL LH ) NOL HLǓ
where:
FSW
IOUT
NOLHL
NOLLH
PBODY
VFD
Compensation Network
To create a stable power supply, the compensation
network around the transconductance amplifier must be
used in conjunction with the PWM generator and the power
stage. Since the power stage design criteria is set by the
application, the compensation network must correct the
overall output to ensure stability. The NCP3170 is a current
mode regulator and as such there exists a voltage loop and
a current loop. The current loop causes the inductor to act
like a current source which governs most of the
characteristics of current mode control. The output inductor
and capacitor of the power stage form a double pole but
because the inductor is treated like a current source in closed
loop, it becomes a single pole system. Since the feedback
loop is controlling the inductor current, it is effectively like
having a current source feeding a capacitor; therefore the
pole is controlled by the load and the output capacitance. A
table of compensation values for 500 kHz and 1 MHz is
provided below for two 22 mF ceramic capacitors. The table
also provides the resistor value for CompCalc at the defined
operating point.
(eq. 34)
= Switching frequency
= Load current
= Dead time between the high-side
MOSFET turning off and the low-side
MOSFET turning on, typically 30 ns
= Dead time between the low-side
MOSFET turning off and the high-side
MOSFET turning on, typically 30 ns
= Low-side MOSFET body diode losses
= Body diode forward voltage drop
typically 0.92 V
Table 6. COMPENSATION VALUES
NCP3170A
VIN
Vout
Lout
R1
R2
Rf
Cf
Cc
Rc
Cp
Resistance for
(V)
(V)
(mF)
(kW)
(kW)
(kW)
(pF)
(nF)
(kW)
(pF)
Current Gain
12
0.8
1.8
24.9
NI
NI
NI
NI
NI
15
3.6
12
1.0
2.5
24.9
100
1
150
15
0.825
NI
4
12
1.1
2.5
24.9
66.5
1
150
10
2
NI
20
12
1.2
2.5
24.9
49.9
1
150
10
2
NI
20
12
1.5
3.6
24.9
28.7
1
150
10
2.49
NI
20
12
1.8
3.6
24.9
20
1
150
10
2.49
NI
20
12
2.5
4.7
24.9
11.8
1
150
8.2
3.74
NI
25
12
3.3
4.7
24.9
7.87
1
150
6.8
4.99
NI
27
12
5.0
7.2
24.9
4.75
1
150
3.9
10
NI
27
12
10.68
7.2
24.9
2.05
1
150
3.9
10
NI
30
18
14.8
7.2
24.9
1.43
1
150
6.8
6.98
NI
30
5
0.8
1.8
24.9
NI
NI
NI
NI
NI
15
15
5
1.0
2.5
24.9
100
1
150
15
0.825
NI
28
5
1.1
2.5
24.9
66.5
1
150
10
2
NI
30
5
1.2
2.5
24.9
49.9
1
150
10
2
NI
30
5
1.5
3.6
24.9
28.7
1
150
10
2.49
NI
30
5
1.8
3.6
24.9
20
1
150
10
2.49
NI
30
5
2.5
3.6
24.9
11.8
1
150
6.8
4.99
NI
50
5
3.3
3.6
24.9
7.87
1
150
6.8
4.99
NI
50
http://onsemi.com
19
NCP3170
Table 6. COMPENSATION VALUES (continued)
NCP3170B
VIN
Vout
Lout
R1
R2
Rf
Cf
Cc
Rc
Cp
Resistance for
(V)
(V)
(mF)
(kW)
(kW)
(kW)
(pF)
(nF)
(kW)
(pF)
Current Gain
12
1.2
1.5
24.9
49.9
1
82
2.7
6.04
NI
20
12
1.5
1.8
24.9
28.7
1
82
2.7
6.04
NI
22
12
1.8
1.8
24.9
20
1
82
2.7
6.04
NI
22
12
2.5
2.7
24.9
11.8
1
82
1.8
10
NI
32
12
3.3
3.3
24.9
7.87
1
82
1.5
12.1
NI
52
12
5.0
3.3
24.9
4.75
1
82
2.2
8.25
NI
52
12
10.68
1.5
24.9
2.05
1
82
2.2
5.1
NI
52
18
14.8
3.3
24.9
1.43
1
82
2.2
5.1
NI
52
5
0.8
1.0
24.9
NI
NI
NI
15
0.499
NI
20
5
1.0
1.0
24.9
100
NI
NI
6.8
1.69
NI
28
5
1.1
1.0
24.9
66.5
NI
NI
3.9
3.61
NI
42
5
1.2
1.5
24.9
49.9
1
82
2.7
6.04
NI
55
5
1.5
1.5
24.9
28.7
1
82
2.7
6.04
NI
55
5
1.8
1.5
24.9
20
1
82
1.8
10
NI
55
5
2.5
1.8
24.9
11.8
1
82
1.8
10
NI
55
5
3.3
1.8
24.9
7.87
1
82
1.8
10
NI
55
To compensate the converter we must first calculate the
current feedback
M+
L OUT
F SW
R MAP
7.299 +
V RAMP
VIN
500 kHz
ǒ
32
4.7 mH
3.3 V
)1.46
12 V
1000
where:
FSW
LOUT
M
Vin
VOUT
VRAMP
RMAP
)1³
Ǔ
0.33 V
where:
A
FSW
IOUT
LOUT
M
VIN
VOUT
(eq. 35)
)1
12 V
W
Next the DC gain of the plant must be calculated.
G+
= Switching Frequency
= Output inductor value
= Current feedback
= Input Voltage
= Output Voltage
= Slope Compensation Ramp
= Current Sense Resistance
33.016 +
where:
G
A
V OUT
M*0.5*M
)
V OUT
V IN
FSW
L OUT
3.0 A
3.3 V
7.299*0.5*7.299
)
4.7 mH
32
3.3 V
)1.46
12 V
(eq. 37)
Ǔ
W
= DC gain of the plant
= Un−scaled gain
Y+
(eq. 36)
1
0.339 W +
ǒ
0.339 W
The amplitude ratio can be calculated using the following
equation:
1
I OUT
A ³
R MAP
1000
The un-scaled gain of the converter also needs to be
calculated as follows:
A+
= Un-scaled gain
= Switching Frequency
= Output Current
= Output inductor value
= Current feedback
= Input Voltage
= Output Voltage
where:
Vo
VREF
Y
3.3 V
12 V
500 kHz
http://onsemi.com
20
0.8 V
VREF
³ 0.242 +
V OUT
3.3 V
= Output voltage
= Regulator reference voltage
= Amplitude ratio
(eq. 38)
NCP3170
determining phase margin. To start the design, a resistor
value should be chosen for R1 from which all other
components can be chosen. A good starting value is 24.9 kW.
The NCP3170 allows the output of the DC−DC regulator
to be adjusted down to 0.8 V via an external resistor divider
network. The regulator will maintain 0.8 V at the feedback
pin. Thus, if a resistor divider circuit was placed across the
feedback pin to VOUT, the regulator will regulate the output
voltage proportional to the resistor divider network in order
to maintain 0.8 V at the FB pin.
The ESR of the output capacitor creates a “zero” at the
frequency as shown in Equation 39:
FZ ESR +
723 kHz +
where:
COESR
COUT
FZESR
1
2p
C OUT
CO ESR
³
(eq. 39)
1
2p
5 mW
44 mF
= Output capacitor ESR
= Output capacitor
= Output capacitor zero ESR frequency
FP +
10.664 kHz +
where:
A
COUT
FP
1
2p
A
C OUT
³
R1
0.339 W
FB
(eq. 40)
1
2p
VOUT
44 mF
R2
= Un-scaled gain
= Output capacitor
= Current mode pole frequency
Figure 46. Feedback Resistor Divider
The relationship between the resistor divider network
above and the output voltage is shown in Equation 41:
The two equations above define the bode plot that the
power stage has created or open loop response of the system.
The next step is to close the loop by considering the feedback
values. The closed loop crossover frequency should be less
than 1/10 of the switching frequency, which would place the
maximum crossover frequency at 50 kHz.
Figure 45 shows a pseudo Type III transconductance error
amplifier.
R2 + R1
where:
R1
R2
VOUT
VREF
CF
IEA
RC
(eq. 41)
= Top resistor divider
= Bottom resistor divider
= Output voltage
= Regulator reference voltage
VO (V)
R1 (kW)
R2 (kW)
0.8
24.9
Open
1.0
24.9
100
+
1.1
24.9
66.5
−
1.2
24.9
49.9
1.5
24.9
28.7
1.8
24.9
20
2.5
24.9
11.8
3.3
24.9
8.06
5.0
24.9
4.64
R2
CP
VREF
Ǔ
Table 7. OUTPUT VOLTAGE SETTINGS
ZFB
CC
V REF
V OUT * V REF
The most frequently used output voltages and their
associated standard R1 and R2 values are listed in the table
below.
ZIN
R1
ǒ
Figure 45. Pseudo Type III Transconductance Error
Amplifier
The compensation network consists of the internal error
amplifier and the impedance networks ZIN (R1, R2, and CF)
and external ZFB (RC, CC, and CP). The compensation
network has to provide a closed loop transfer function with
the highest 0 dB crossing frequency to have fast response
and the highest gain in DC conditions, so as to minimize load
regulation issues. A stable control loop has a gain crossing
with −20 dB/decade slope and a phase margin greater than
45°. Include worst-case component variations when
The compensation components for the Pseudo Type III
Transconductance Error Amplifier can be calculated using
the method described below. The method serves to provide
a good starting place for compensation of a power supply.
The values can be adjusted in real time using the
compensation tool CompCalc
http://www.onsemi.com/pub/Collateral/COMPCALC.ZIP
http://onsemi.com
21
NCP3170
The first pole to crossover at the desired frequency should
be setup at FPO to decrease at −20 dB per decade:
F PO +
1.512 kHz +
where:
Fcross
FPO
F CROSS
G
50 kHz
33.061
RC +
2.925 kW +
³
(eq. 42)
where:
CC
COUT
FP
RC
³
= Cross over frequency
= Pole frequency to meet crossover
frequency
= DC gain of the plant
G
CC +
5.12 nF +
where:
CC
FPO
gm
y
CF +
456 pF +
where:
CF
Fcross
gm
R1
R2
RF
y
2
gm
F PO
p
0.242
2p
75.2 pF +
where:
CP
FESR
RC
³
(eq. 43)
200 ms
R1 ) R2
(R1 * RF ) R2 * RF ) R2 * R1)
F cross
³
(eq. 44)
1
2p
5.12 nF
1.512 kHz
1
RC
2p
F ESR
³
(eq. 45)
1
2p
2.925 kW
723 kHz
= Compensation pole capacitor
= Capacitor ESR zero frequency
= Compensation resistor
If the ESR frequency is greater than the switching
frequency, a CF compensation capacitor may be needed for
stability as the output LC filter is considered high Q and thus
will not give the phase boost at the crossover frequency.
Further at low duty cycles due to some blanking and filtering
of the current signal the current gain of the converter is not
constant and the current gain is small. Thus adding CF and
RF can give the needed phase boost.
1.512 kHz
= Compensation capacitor
= Pole frequency
= Transconductance of amplifier
= Amplitude ratio
2p
FP
CC
= Compensation capacitance
= Output capacitance
= Current mode pole frequency
= Compensation resistor
CP +
The crossover combined compensation network can be
used to calculate the transconductance output compensation
network as follows:
1
2p
³
(eq. 46)
24.9 kW ) 7.87 kW
2p
(24.9 kW * 1 kW ) 7.87 kW * 1 kW ) 7.87 kW * 24.9 kW)
50 kHz
IPK
= Compensation pole capacitor
= Cross over frequency
= Transconductance of amplifier
= Top resistor divider
= Bottom resistor divider
= Feed through resistor
Figure 47. Input Charge Inrush Current
Calculating Input Inrush Current
I ICinrush_PK1 +
The input inrush current has two distinct stages: input
charging and output charging. The input charging of a buck
stage is usually controlled, but there are times when it is not
and is limited only by the input RC network, and the output
impedance of the upstream power stage. If the upstream
power stage is a perfect voltage source and switches on
instantaneously, then the input inrush current can be
depicted as shown in Figure 47 and calculated as:
V IN
CIN ESR
(eq. 47)
12
1.2 kA +
0.01
I ICinrush_RMS1 +
V IN
CIN ESR
0.316
Ǹ
5
CIN ESR
C IN
t DELAY_TOTAL
(eq. 48)
12.58 A +
http://onsemi.com
22
12 V
0.01
0.316
Ǹ
5
0.01 W
1 ms
22 mF
NCP3170
where:
CIN
= Output capacitor
CINESR
= Output capacitor ESR
tDELAY_TOTAL = Total delay interval
VIN
= Input Voltage
3.3 V
Output
Voltage
Once the tDELAY_TOTAL has expired, the buck converter
starts to switch and a second inrush current can be
calculated:
I OCinrush_RMS +
ǒC OUT ) C LOADǓ
where:
COUT
CLOAD
D
ICL
IOCinrush_RMS
tSS
VOUT
t SS
V OUT D
) I CL
Ǹ3
Output
Current
D (eq. 49)
tss
= Total converter output capacitance
= Total load capacitance
= Duty ratio of the load
= Applied load at the output
= RMS inrush current during start-up
= Soft start interval
= Output voltage
Figure 49. Resistive Load Current
Alternatively, if the output load has an under voltage
lockout, turns on at a defined voltage level, and draws a
constant current, then the RMS connected load current is:
I CL1 +
From the above equation, it is clear that the inrush current
is dependent on the type of load that is connected to the
output. Two types of load are considered in Figure 48: a
resistive load and a stepped current load.
Inrush
Current
492 mA +
where:
IOUT
VOUT
VOUT_TO
XCP3170
Load
OR
Ǹ
V OUT * V OUT_TO
Ǹ
V OUT
I OUT
(eq. 51)
3.3 V * 2.5 V
3.3 V
1A
= Output current
= Output voltage
= Output voltage load turn on
1.0 V
3.3 V
Output
Voltage
Figure 48. Load Connected to the Output Stage
If the load is resistive in nature, the output current will
increase with soft start linearly which can be quantified in
Equation 50.
I CLR_RMS +
191 mA +
where:
ICLR_RMS
ICR_PK
ROUT
VOUT
1
Ǹ3
1
Ǹ3
V OUT
R OUT
3.3 V
10 W
I CR_PK +
300 mA +
Output
Current
V OUT
R OUT
t
tss
(eq. 50)
3.3 V
Figure 50. Voltage Enable Load Current
10 W
If the inrush current is higher than the steady state input
current during max load, then an input fuse should be rated
accordingly using I2t methodology.
= RMS resistor current
= Peak resistor current
= Output resistance
= Output voltage
http://onsemi.com
23
NCP3170
THERMAL MANAGEMENT AND LAYOUT
Consideration
In the NCP3170 buck regulator high pulsing current flows
through two loops as shown in the figure below.
VIN
VIN
VSW
Input
Current
L1 4.7 mH
3.3 V
EN
C1
22 mF
Cbypass
0.1 mF
R1
DRIVE
PG
FB1
COMP
CC
AGND
PGND
C2, C3
22 mF
R2
RC
Figure 51. Buck Converter Current Paths
important so the designer can see where the currents are
flowing.
The first loop shown in blue activates when the high side
switch turns on. When the switch turns on, the edge of the
current waveform is provided by the bypass capacitor. The
remainder of the current is provided by the input capacitor.
Slower currents are provided by the upstream power supply
which fills up the input capacitor when the high side switch
is off. The current flows through the high side MOSFET and
to the output, charging the output capacitors and providing
current to the load. The current returns through a PCB
ground trace where the output capacitors are connected, the
regulator is grounded, and the input capacitors are grounded.
The second loop starts from the inductor to the output
capacitors and load, and returns through the low side
MOSFET. Current flows in the second loop when the low
side NMOSFET is on. The designer should note that there
are locations where the red line and the blue line overlap;
these areas are considered to have DC current. Areas
containing a single blue line indicate that AC currents flow
and transition very quickly. The key to power supply layout
is to focus on the connections where the AC current flows.
A good rule of thumb is that for every inch of PCB trace,
20 nH of inductance exists. When laying out a PCB,
minimizing the AC loop area reduces the noise of the circuit
and improves efficiency. A ground plane is strongly
recommended to connect the input capacitor, output
capacitor, and PGND pin of the NCP3170. Drawing the real
high power current flow lines on the recommended layout is
Figure 52. Recommended Signal Layout
http://onsemi.com
24
NCP3170
5. Create copper planes as short as possible from the
VSW pin to the output inductor, from the output
inductor to the output capacitor, and from the load
to PGND.
6. Create a copper plane on all of the unused PCB
area and connect it to stable DC nodes such as:
VIN, GND, or VOUT.
7. Keep sensitive signal traces far away from the
VSW pins or shield them.
The NCP3170 is the major source of power dissipation in
the system for which the equations above detailed the loss
mechanisms. The control portion of the IC power
dissipation is determined by the formula below:
PC + IC
where:
ICC
PC
VIN
V IN
(eq. 52)
= Control circuitry current draw
= Control power dissipation
= Input voltage
Once the IC power dissipations are determined, the
designer can calculate the required thermal impedance to
maintain a specified junction temperature at the worst case
ambient temperature. The formula for calculating the
junction temperature with the package in free air is:
TJ + TA ) PD
where:
PD
RqJA
TA
TJ
R qJA
(eq. 53)
= Power dissipation of the IC
= Thermal resistance junction to ambient
of the regulator package
= Ambient temperature
= Junction temperature
The thermal performance of the NCP3170 is strongly
affected by the PCB layout. Extra care should be taken by
users during the design process to ensure that the IC will
operate under the recommended environmental conditions.
As with any power design, proper laboratory testing should
be performed to ensure the design will dissipate the required
power under worst case operating conditions. Variables
considered during testing should include maximum ambient
temperature, minimum airflow, maximum input voltage,
maximum loading, and component variations (i.e., worst
case MOSFET RDS(on)). Several layout tips are listed below
for the best electric and thermal performance. Figure 53
illustrates a PCB layout example of the NCP3170.
1. The VSW pin is connected to the internal PFET
and NFET drains, which are a low resistance
thermal path. Connect a large copper plane to the
VSW pin to help thermal dissipation. If the PG pin
is not used in the design, it can be connected to the
VSW plane, further reducing the thermal
impedance. The designer should ensure that the
VSW thermal plane is rounded at the corners to
reduce noise.
2. The user should not use thermal relief connections
to the VIN and the PGND pins. Construct a large
plane around the PGND and VIN pins to help
thermal dissipation.
3. The input capacitor should be connected to the
VIN and PGND pins as close as possible to the IC.
4. A ground plane on the bottom and top layers of the
PBC board is preferred. If a ground plane is not
used, separate PGND from AGND and connect
them only at one point to avoid the PGND pin
noise coupling to the AGND pin.
Figure 53. Recommend Thermal Layout
http://onsemi.com
25
NCP3170
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
−Y−
K
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
D
0.25 (0.010)
M
Z Y
S
X
S
M
J
SOLDERING FOOTPRINT*
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
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.
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ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). 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
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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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NCP3170/D