ON NCP3231A High current synchronous buck converter Datasheet

NCP3231A
High Current Synchronous
Buck Converter
The NCP3231A is a high current, high efficiency, voltage−mode
synchronous buck converter which operates from 4.5 V to 18 V input
and generates output voltages down to 0.6 V at up to 25 A.
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Features
•
•
•
•
•
•
•
•
•
•
•
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Wide Input Voltage Range from 4.5 V to 18 V
0.6 V Internal Reference Voltage
500 kHz Switching Frequency
External Programmable Soft−start
Lossless Low−side FET Current Sensing
Output Over−voltage Protection and Under−voltage Protection
System Over−temperature Protection using a Thermistor or Sensor
Hiccup Mode Operation for All Faults
Pre−bias Start−up
Adjustable Output Voltage
Power Good Output
Internal Over−temperature Protection
This is a Pb−Free Device*
MARKING
DIAGRAM
1
1 40
NCP3231A
AWLYYWWG
QFN40 6x6, 0.5P
CASE 485CM
NCP3231A = Specific Device Code
A
= Assembly Location
WL
= Wafer Lot
YY
= Year
WW
= Work Week
G
= Pb−Free Package
AGND
ISET
SS
OTS
FB
PG
VIN
VIN
VIN
4
1
5
3
6
2
8
7
9
VIN
11
VIN
12
VIN
13
VIN
14
37 PGND
VSWH
15
36 BST
PGND
16
PGND
17
PGND
18
PGND
19
32 VSWH
PGND
20
31 VSWH
40 EN
VIN
EP42
39 VCC
GND
EP41
38 VB
35 VSW
34 VSWH
27
28
29
PGND
PGND
VSWH
30
26
PGND
VSWH
25
33 VSWH
PGND
23
PGND
24
22
PGND
21
PGND
VSWH
EP43
PGND
Cellular Base Stations
ASIC, FPGA, DSP and CPU Core and I/O Supplies
Telecom and Network Equipment
Server and Storage System
10
•
•
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COMP
PIN CONNECTIONS
Typical Applications
(TOP VIEW)
ORDERING INFORMATION
Device
Package
Shipping†
NCP3231AMNTXG
QFN40
(Pb−Free)
2500 /
Tape & Reel
†For information on tape and reel specifications,
including part orientation and tape sizes, please
refer to our Tape and Reel Packaging Specification
Brochure, BRD8011/D.
*For additional information on our Pb−Free strategy and soldering details, please
download the ON Semiconductor Soldering and Mounting Techniques
Reference Manual, SOLDERRM/D.
© Semiconductor Components Industries, LLC, 2015
May, 2015 − Rev. 3
1
Publication Order Number:
NCP3231A/D
NCP3231A
VB
VCC
LDO
VCC
VB
VB
VB
BST
VIN
OSC
COMP
VDD
VREF
FB
Control Logic
Ramp Generator
PWM Logic
+
E/A
−
− and −
VSWH
SS
Soft Start
VCC
2 mA
EN
VB
UVLO
OVP, UVP
Power Good
OCP, TSD
Protection
VSW
PVDD
Enable
Logic
1.2 V
POR
PGND
PG
VB
+
−
VREF
OTS
ISET
AGND
Figure 1. NCP3231A Block Diagram
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2
NCP3231A
PIN DESCRIPTION
Pin No.
Symbol
1
SS
A capacitor from this pin to GND allows the user to adjust the soft−start ramp time.
Description
2
FB
Output voltage feedback.
3
COMP
4
ISET
5
AGND
6
OTS
7
PG
Power good indicator of the output voltage. Open−drain output. Connect PG to VDD with an external resistor.
8−14,
EP42
VIN
The VIN pin is connected to the internal power NMOS switch. The VIN pin has high di/dt edges and must be
decoupled to ground close to the pin of the device.
15, 29−34,
EP43
VSWH
The VSWH pin is the connection of the drain and source of the internal NMOS switches. At switch off, the
inductor will drive this pin below ground as the body diode and the NMOS conducts with a high dv/dt.
16−28, 37
PGND
Ground reference and high−current return path for the bottom gate driver and low- side NMOS.
35
VSW
IC connection to the switch node between the top MOSFET and bottom MOSFET. Return path of the high−
side gate driver.
36
BST
Top gate driver input supply, a bootstrap capacitor connection between the switch node and this pin.
38
VB
39
VCC
40
EN
EP41
GND
Output of the error amplifier.
A resistor from this pin to ground sets the over−current protection (OCP) threshold.
Analog ground.
Negative input of internal thermal comparator. Tie this pin to ground if not in use.
The internal LDO output and input supply for the NCP3231A. Connect a minimum of 4.7 mF ceramic capacitor from this pin to ground.
Input Supply for IC. This pin must be connected to VIN.
Logic control for enabling the switcher. An internal pull−up enables the device automatically. The EN pin can
also be driven high to turn on the device, or low to turn off the device. A comparator and precision reference
allow the user to implement this pin as an adjustable UVLO circuit.
Exposed Pad. Connect GND to a large copper plane at ground potential to improve thermal dissipation.
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NCP3231A
VIN
VIN
BST
VCC
VOUT
VSWH
VSW
VB
PGND
NCP3231A
ISET
AGND
VPG
EN
FB
OTS
PG
COMP
SS
Figure 2. Typical Application Circuit
ABSOLUTE MAXIMUM RATINGS (measured vs. GND pad, unless otherwise noted)
Symbol
Value
Unit
VIN, VCC
20.5
−0.3
V
VSW to GND
VSWH, VSW
26
−0.6 (DC)
+35 (t < 50 ns)
−5 (t < 100 ns)
V
BST to GND
BST
30 (DC)
−0.6 (DC)
+40 (t < 50 ns)
V
6.0
−0.3
V
Rating
Power Supply to GND
All other pins
Operating Ambient Temperature Range (Note 1)
TA
−40 to +90
°C
Operating Junction Temperature Range (Note 1)
TJ
−40 to +150
°C
TJ(MAX)
+150
°C
Tstg
−55 to +150
°C
Electrostatic Discharge − Human Body Model
HBM
1.0
kV
Electrostatic Discharge − Charge Device Model
CDM
2.0
kV
HS FET Junction-to-Case Thermal Resistance (Note 2)
RqJC−HS
1.3
°C/W
LS FET Junction-to-Case Thermal Resistance (Note 2)
RqJC−LS
0.6
°C/W
Maximum Junction Temperature
Storage Temperature Range
THERMAL INFORMATION
Stresses exceeding those listed in the Maximum Ratings table may damage the device. If any of these limits are exceeded, device functionality
should not be assumed, damage may occur and reliability may be affected.
1. The maximum package power dissipation limit must not be exceeded.
PD +
T J(MAX) * T A
R qJA
2. RqJC thermal resistance is obtained by simulating a cold plate test on the exposed power pad. No specific JEDEC standard test exists, but
a close description can be found in the ANSI SEMI standard G30−88.
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4
NCP3231A
ELECTRICAL CHARACTERISTICS
(−40°C < TJ < +125°C, VCC = 12 V, for min/max values unless otherwise noted, TJ = +25°C for typical values)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Units
18
V
POWER SUPPLY
VIN/VCC Operation Voltage
VIN/VCC
4.5
VB UVLO Threshold (Rising)
4.1
4.2
4.3
V
VB UVLO Threshold (Falling)
3.4
3.66
3.8
V
4.9
5.15
5.45
V
IB = 25 mA, VCC = 4.5 V
36
110
mV
EN = H, COMP = H, no switching;
PG open; no switching
4.7
6.4
mA
NCP3231A; EN = 0; VCC = 18 V; PG open
100
140
mA
NCP3231A; EN = 0; VCC = 4.5 V; PG open
53
75
mA
mV
VB Output Voltage
VB
VB Dropout Voltage
VCC Quiescent Current
Shutdown Supply Current
VCC = 6 V, 0 ≤ IB ≤ 40 mA
FEEDBACK VOLTAGE
FB Input Voltage
VFB
Feedback Input Bias Current
IFB
TJ = 25°C, 4.5 V ≤ VCC ≤ 18 V
597
600
603
−40°C ≤ TJ ≤ 125°C; 4.5 V ≤ VCC ≤ 18 V
594
600
606
VFB = 0.6 V
75
nA
ERROR AMPLIFIER
Guaranteed by Design
Open Loop DC Gain
Open Loop Unity Gain Bandwidth
60
F0dB,EA
Open Loop Phase Margin
Slew Rate
COMP pin to GND = 10 pF
COMP Clamp Voltage, High
COMP Clamp Voltage, Low
85
dB
24
MHz
60
°
2.5
V/m
3.46
V
465
mV
Output Source Current
VFB = 0 V
15
mA
Output Sink Current
VFB = 1 V
20
mA
CURRENT LIMIT
Low−side ISET Current Source
LS_ISET
Low−side ISET Current Source
Temperature Coefficient
TC_LS_I−
SET
Low−side OCP Switch−over
Threshold
Low−side Fixed OCP Threshold
LS_OCPth
Low−side Programmable OCP Range
LS_OCPth
LS OCP Blanking time
LS_Tblnk
34
mA
+0.31
%/°C
Guaranteed by Design
600
mV
Guaranteed by Design
300
Sourced from ISET pin, before SS,
TJ = 25°C
mV
< 600
mV
Guaranteed by Design
150
ns
Maximum duty cycle
fsw = 500 kHz, VFB = 0 V
4.5 V < VCC < 18 V
92
%
Minimum duty cycle
VCOMP < PWM Ramp Offset Voltage
0
%
Minimum GH on−time
Guaranteed by Characterization
60
ns
PWM Ramp Amplitude
Guaranteed by Characterization
VCC/8.6 VCC/6.6 VCC/5.6
V
PWM Ramp Offset
Guaranteed by Characterization
0.64
V
PWM
OSCILLATOR
Oscillator Frequency Range
fsw
fsw = 500 kHz
4.5 V < VCC < 18 V
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5
450
500
550
kHz
NCP3231A
ELECTRICAL CHARACTERISTICS
(−40°C < TJ < +125°C, VCC = 12 V, for min/max values unless otherwise noted, TJ = +25°C for typical values)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Units
thiccup
tss < 1 ms, fsw = 500 kHz
4
ms
tss > 1 ms, fsw = 500 kHz
4 x tss
ms
OSCILLATOR
Hiccup Timer
ENABLE INPUT (EN)
EN Input Operating Range
Enable Threshold Voltage
V_EN
VEN rising
Enable Hysteresis
1.11
VEN falling
Deep Disable Threshold
1.2
5.5
V
1.29
V
144
0.7
Enable Pull−up Current
0.78
mV
0.9
V
2
mA
SOFTSTART INPUT (SS)
SS Startup Delay
tSSD
1.33
ms
SS End Threshold
SSEND
0.6
V
SS Source Current
ISS
2.15
2.5
2.8
mA
10
20
30
mA
665
675
685
mV
500
525
550
mV
VOLTAGE MONITOR
PG = 0.15 V
Power Good Sink Current
Output Overvoltage Rising Threshold
Overvoltage Fault Blanking Time
ms
20
Output Under−Voltage Trip Threshold
Under−voltage Protection Blanking
Time
20
ms
OVP and UVP Enable Delay
tSS
s
POWER STAGE
High−side On Resistance
RDSONH
VIN/VCC = 5 V, ID = 2 A
7
9.9
mW
Low−side On Resistance
RDSONL
VIN/VCC = VB, ID = 2 A
1.5
2.9
mW
IBOOT = 2 mA
0.28
VFBOOT
V
THERMAL MONITOR (OTS)
0.59
OTS comparator reference voltage
(Rising Threshold)
OTS comparator reference voltage
(Falling Hysteresis)
0.6
0.61
50
V
mV
THERMAL SHUTDOWN
Thermal Shutdown Threshold
Guaranteed by design
Thermal Shutdown Hysteresis
Guaranteed by design
135
150
25
165
°C
°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.
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NCP3231A
fSW, SWITCHING FREQUENCY (kHz)
0.602
0.601
0.600
0.599
0.598
0.597
0.596
−40 −25 −10
5
20
35
50
65
80
95 110 125
503
VCC = 12 V
502
501
500
VCC = 4.5 V
499
498
−40 −25 −10
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
Figure 3. Reference Voltage vs. Temperature
Figure 4. Switching Frequency vs.
Temperature
1.23
1.22
1.21
1.20
1.19
−40 −25 −10
5
20
35
50
65
80
95 110 125
1.10
1.09
1.08
1.07
1.06
−40 −25 −10
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 5. Rising Enable Threshold vs.
Temperature
Figure 6. Falling Enable Threshold vs.
Temperature
7
100
90
IQ, QUIESCENT CURRENT (mA)
ISD, SHUTDOWN CURRENT (mA)
504
TJ, JUNCTION TEMPERATURE (°C)
VEN, FALLING ENABLE THRESHOLD (V)
VEN, RISING ENABLE THRESHOLD (V)
VFB, FEEDBACK REFERENCE VOLTAGE (V)
TYPICAL CHARACTERISTICS
80
70
60
50
40
30
20
VCC = 12 V
10
0
−40 −25 −10
5
20
35
50
65
80
6
5
4
3
2
1
VCC = 12 V, No Switching
0
−40 −25 −10
95 110 125
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 7. Shutdown Current vs. Temperature
Figure 8. Quiescent Current vs. Temperature
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NCP3231A
TYPICAL CHARACTERISTICS
50
ISET, LSOCP SET CURRENT (mA)
ISS, SOFT−START CURRENT (mA)
2.70
2.65
2.60
2.55
2.50
2.45
2.40
2.35
2.30
−40 −25 −10
5
20
35
50
65
80
40
35
30
25
20
−40 −25 −10
95 110 125
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 9. Soft−start Current vs. Temperature
Figure 10. ISET Current vs. Temperature
9.5
2.5
9.0
LOW−SIDE FET RDS(on) (mW)
HIGH−SIDE FET RDS(on) (mW)
45
8.5
VIN/VCC = 4.5 V
8.0
7.5
VIN/VCC = 12 V
7.0
6.5
6.0
5.5
5.0
−40 −25 −10
5
20
35
50
65
80
95
2.0
VIN/VCC = 4.5 V
1.5
VIN/VCC = 12 V
1.0
0.5
0
−40 −25 −10
110 125
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 11. High−side RDS(on) vs. Temperature
Figure 12. Low−side RDS(on) vs. Temperature
100
100
VOUT = 3.3 V
95
VOUT = 3.3 V
95
85
EFFICIENCY (%)
EFFICIENCY (%)
90
VOUT = 1.0 V
80
VOUT = 2.5 V
75
VOUT = 1.8 V
70
VOUT = 1.2 V
65
90
85
VOUT = 2.5 V
VOUT = 1.0 V
VOUT = 1.8 V
80
VOUT = 1.2 V
75
70
60
VIN/VCC = 12 V
TA = 25°C
55
50
VIN/VCC = 5 V
TA = 25°C
65
60
0
2.5
5.0
0
7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0
2.5
5.0
7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0
IOUT, LOAD CURRENT (A)
IOUT, LOAD CURRENT (A)
Figure 13. Efficiency vs. Iout
(Vin = 12 V)
Figure 14. Efficiency vs. Iout
(Vin = 5 V)
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NCP3231A
TYPICAL CHARACTERISTICS
0.61
4.30
0.60
0.59
0.58
0.57
0.56
Falling Threshold
0.55
0.54
−40 −25 −10
5
20
35
50
65
80
4.26
4.22
4.18
4.14
4.10
−40 −25 −10
95 110 125
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 15. OTS Threshold vs. Temperature
Figure 16. VB UVLO Rising Threshold vs.
Junction Temperature
OVP, OVERVOLTAGE THRESHOLD (V)
3.80
3.76
3.72
3.68
3.64
3.60
−40 −25 −10
5
20
35
50
65
80
95 110 125
680
679
678
677
676
675
674
673
672
671
670
−40 −25 −10
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 17. VB UVLO Falling Threshold vs.
Junction Temperature
Figure 18. Output OVP vs. Junction
Temperature
UVP, UNDERVOLTAGE THRESHOLD (V)
VB UVLO, FALLING THRESHOLD VOLTAGE (V)
VB UVLO RISING THRESHOLD
VOLTAGE (V)
OTS, OVERTEMPERATURE
THRESHOLD VOLTAGE (V)
Rising Threshold
560
558
556
554
552
550
548
546
544
542
540
−40 −25 −10
5
20
35
50
65
80
95 110 125
TJ, JUNCTION TEMPERATURE (°C)
Figure 19. Output UVP vs. Junction
Temperature
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NCP3231A
TYPICAL CHARACTERISTICS
CH1 (Blue): EN
CH2 (Aqua): COMP
CH3 (Purple): Vout
CH4 (Green): SS
CH1 (Blue): VSW
CH2 (Aqua): COMP
CH3 (Purple): Vout
CH4 (Green): SS
Figure 20. Typical Startup Waveforms
(Vin = 12 V, Iout = 25 V, Vout = 1 V)
Figure 21. Typical Short Circuit Waveforms
(Vin = 12 V)
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NCP3231A
OPERATION DESCRIPTION
Overview
drive signal to avoid shoot through. During the dead time,
the body diode of the low side FET freewheels the current.
The body diode has much higher voltage drop than that of
the MOSFET, which reduces the efficiency significantly.
The longer the body diode conducts, the lower the
efficiency. NCP3231A implements adaptive dead time
control to minimize the dead time, as well as preventing
shoot through.
The NCP3231A is a 500 kHz, high efficiency, high current
PWM synchronous buck converter. It operates with a single
supply voltage from 4.5 to 18 V and can provide output
current as high as 25 A. NCP3231A utilizes voltage mode
with voltage feed−forward control to respond instantly to
Vin changes and provide for easier compensation over the
supply range of the converter. The device also includes
pre−bias startup capability to allow monotonic startup in the
event of a pre−biased output condition.
Protection features include overcurrent protection (OCP),
output over and under voltage protection (OVP, UVP), and
power good. The enable function is highly programmable to
allow for adjustable startup voltages at higher input
voltages. There is also an adjustable soft−start, an over−
temperature/over−voltage comparator, and internal thermal
shutdown.
Precision Enable (EN)
The ENABLE block allows the output to be toggled on
and off and is a precision analog input.
When the EN voltage exceeds V_EN, the controller will
initiate the soft-start sequence as long as the input voltage
and sub-regulated voltage have exceeded their UVLO
thresholds. V_EN_hyst helps to reject noise and allow the
pin to be resistively coupled to the input voltage or
sequenced with other rails.
If the EN voltage is held below typically 0.8 V, the
NCP3231A enters a deep disable state where the internal
bias circuitry is off. As the voltage at EN continues to rise,
the Enable comparator and reference are active and provide
a more accurate EN threshold. The drivers are held off until
the rising voltage at EN crosses V_EN.
An internal 2 mA pullup automatically enables the device
when the EN pin is left floating.
Reference Voltage
The NCP3231A incorporates an internal reference that
allows output voltages as low as 0.6 V. The tolerance of the
internal reference is guaranteed over the entire operating
temperature range of the controller. The reference voltage is
trimmed using a test configuration that accounts for error
amplifier offset and bias currents.
Oscillator Ramp
The ramp waveform is a saw tooth formed at the PWM
frequency with a peak−to−peak amplitude of VCC/6.6,
offset from GND by typically 0.64 V. The PWM duty cycle
is limited to a maximum of 92%, allowing the bootstrap
capacitor to charge during each cycle.
INPUT SUPPLY / VCC
VDD
2 mA
EN
Error Amplifier
The error amplifier’s primary function is to regulate the
converter’s output voltage using a resistor divider connected
from the converter’s output to the FB pin of the controller,
as shown in the Applications Schematic. A type III
compensation network must be connected around the error
amplifier to stabilize the converter. It has a bandwidth of
greater than 24 MHz, with open loop gain of at least 60 dB.
1.2 V
Figure 22. Enable Functional Block Diagram
Pre−bias Startup
In some applications the controller will be required to start
switching when its output capacitors are charged anywhere
from slightly above 0 V to just below the regulation voltage.
This situation occurs for a number of reasons: the
converter’s output capacitors may have residual charge on
them or the converter’s output may be held up by a low
current standby power supply. NCP3231A supports
pre−bias start up by holding off switching until the feedback
voltage and thus the output voltage rises above the set
regulated voltage. If the pre−bias voltage is higher than the
set regulated voltage, switching does not occur until the
output voltage drops back to the regulation point.
Programmable Soft−Start
An external capacitor connected from the SS pin to
ground sets up the soft start period, which can limit the
start−up inrush current. The soft start period can be
programmed based on the Equation 1.
t SS +
V ref
C SS
I SS
Enable
Logic
(eq. 1)
OCP is the only fault that is active during a soft−start.
Adaptive Non−Overlap Gate Driver
In a synchronous buck converter, a certain dead time is
required between the low side drive signal and high side
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NCP3231A
Power Good (PG) Operation
Power Good Pullup Voltage
LSOCP Trip Level
Inductor Current
Start
Reset/Start
Reset/Start
Backup Counter
Start
Hiccup
Hiccup Counter
1
2
3
tHiccup = 4xtSS
Skipped Pulses showing Skip Count
Figure 23. LSOCP Function with Counters and Power Good Shown (exaggerated for informational purposes)
PROTECTION FEATURES
Hiccup Mode
will turn on at that point as well, thus pulling PG low. Once
VFB has fallen below the Undervoltage Protection
Threshold (UVP), the device will enter hiccup mode.
The NCP3231A utilizes hiccup mode for all of its fault
conditions. Upon entering hiccup mode after a fault
detection, the NCP3231A turns off the high side and low
side FET’s and PG goes low. It waits for tHICCUP ms before
reinitiating a soft−start. tHiccup is defined as four soft start
timeouts (tss). And if the soft−start time tss is set to be less
than 1 ms, the hiccup time will be 4 ms. The equation for tss
is shown in Equation 1. OCP is the only active fault
detection during the hiccup mode soft start.
Under Voltage Protection (UVP)
A UVP circuit monitors the VFB voltage to detect an
under voltage event. If the VFB voltage is below this
threshold for more than 20 ms, a UVP fault is set and the
device will enter hiccup mode.
Over Current Protection (OCP)
The NCP3231A over current protection scheme senses
the peak freewheeling current in the low−side FET (LSOCP)
after a blanking time of 150 ns as shown in Figure 23. The
low−side FET drain−to−source voltage, VDS, is compared
against the voltage of a fixed, internal current source, ISET
and a user−selected resistor, RSET. Voltage across the
low−side FET is sensed from the VSW pin to GND.
After an OCP detection, the NCP3231A keeps the
high−side FET off until the Low−side FET current falls
below the trip point again and the next clock cycle occurs.
An internal OCP counter will count up to 3 consecutive
LSOCP events. After the third consecutive count, the device
enters hiccup mode.
Over Temperature Comparator (OTS)
The NCP3231A provides an over−temperature shutdown
(OTS) comparator with 50 mV hysteresis and a 0.6 V
reference in order to remotely sense an external temperature
detector or thermistor. When the voltage at the OTS pin rises
above 0.6 V, the drivers stop switching and both FET’s
remain off. When this voltage drops below typically 0.55 V,
a new soft−start cycle is generated automatically. Tie the
OTS pin to ground if this function is not required.
Over Voltage Protection (OVP)
When the voltage at the FB pin (VFB) is above the OVP
threshold for greater than 20 ms (typical), an OVP fault is set.
The high side FET (HSFET) will turn off and the low side
FET (LSFET) will turn on. The open-drain PG pull down
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12
NCP3231A
good pin is a high impedance output. If the NCP3231A
detects an OCP, OVP, UVP, OTS, TSD or is in soft start, it
pulls PG pin low. The PG pin is an open drain 10 mA pull
down output.
To prevent nuisance trips, there is a backup counter that
will reset the OCP counter after 7 consecutive cycles without
an LSOCP trigger. The backup counter is reset and then
started again after each OCP trip until the third OCP count
as stated above occurs.
Layout Guidelines
When laying out a power PCB for the NCP3231A there
are several general key points and special key points to
consider:
General Layout Guide: these are the common techniques
for high frequency high power board layout design.
Base component placement: High current path
components should be placed to keep the current path as
tight as possible. Placement of components on the bottom of
the board such as input or output decoupling can add loop
inductance.
Ground Return for Power and Signals: Solid,
uninterrupted ground planes must be present and adjacent to
the high current path.
Copper Shapes on Component Layers: Large copper
planes on one or multiple layers with adequate vias will
increase thermal transfer, reduce copper conduction losses,
and minimize loop inductance. Greater than 20 A designs
require 2~3 layer shapes or more, increasing the number of
layers will only improvement performance.
Via Placement for Power and Ground: Place enough vias
to adequately connect outer layers to inner layers for thermal
transfer and to minimize added inductance in layer
transition. Multiple vias should be placed near important
components like input ceramics and output ceramic
capacitors.
Key Signal Routes: Do not route sensitive signals, such as
FB near or under noisy nets such as the switch node VSW
and BST node, to reduce noise coupling effects on the
sensitive lines.
Special Layout Guide: please pay attention to the special
requirement of layout guide.
To improve the High-side OCP accuracy, users should
connect VCC and VIN directly and do not place any type of
filter or resistor between these two pins.
To improve the Low-side OCP accuracy, users should use
single ground connection instead of separate analog ground
and power ground. Make sure that the inner layers (at least
2nd layer, 3rd layer and 4th layer) are dedicated for ground
plane. Do not use other copper planes to cut or interrupt the
shape of ground plane, which may add more parasitic
components to affect the sensing accuracy.
Over Current Protection Threshold
The NCP3231A allows the user to adjust the LSOCP
threshold with an external resistor, RSET. This resistor,
along with an internal temperature compensated current
source, ISET, sets the current limit reference voltage for the
LSOCP comparator.
Internally, a current sense circuit samples the voltage from
VSW to GND. This voltage is then multiplied by a factor of
2 and compared against the ISET*RSET voltage threshold.
The basic design equation for LSOCP trip point selection is:
RSET +
2
+2
ǒiLoad ) 0.5iLpk*pkǓ
Rdson
ISET
67
(eq. 2)
ǒiLoad ) 0.5i Lpk*pkǓ
In this equation, iLoad is the over current protection point
of the load current, iLpk-pk is the peak to peak value of
inductor current, and for example, when input voltage is
12 V, output voltage is 3.3 V, switching frequency is 500 kHz
and inductor value is 330 nH, the peak to peak value of
inductor current is 14.5 A. ISET is temperature compensated
current source proportional to the on-resistance of LS
MOSFET, Rdson, the ratio of Rdson to ISET is about 67.
In case RSET is not connected, the device switches the
OCP threshold to a fixed 300 mV value: an internal safety
clamp on ISET is triggered as soon as the ISET voltage
reaches 600 mV, enabling the 300 mV fixed threshold.
Thermal Shutdown (TSD)
The NCP3231A protects itself from overheating with an
internal thermal monitoring circuit. If the junction
temperature exceeds the thermal shutdown threshold both
the upper and lower MOSFETs will be shut OFF. Once the
temperature drops below the falling hysteresis threshold, the
voltage at the COMP pin will be pulled below the ramp
valley voltage and a soft−start will be initiated.
Power Good Monitor (PG)
NCP3231A monitors the output voltage and signal when
the output is out of regulation or during a non−regulated
pre−bias condition, or fault condition. When the output
voltage is within the OVP and UVP thresholds, the power
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13
NCP3231A
PACKAGE DIMENSIONS
QFN40 6x6, 0.5P
CASE 485CM
ISSUE O
A B
D
PIN ONE
LOCATION
2X
ÉÉÉ
ÉÉÉ
ÉÉÉ
L1
DETAIL A
ALTERNATE
CONSTRUCTIONS
E
ÉÉ
ÉÉ
0.15 C
EXPOSED Cu
2X
TOP VIEW
0.15 C
(A3)
DETAIL B
0.10 C
DIM
A
A1
A3
b
D
D2
D3
E
E2
E3
E4
e
G
K
L
L1
MOLD CMPD
DETAIL B
ALTERNATE
CONSTRUCTION
A
43X
0.08 C
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ASME Y14.5M, 1994.
2. CONTROLLING DIMENSIONS: MILLIMETERS.
3. DIMENSION b APPLIES TO PLATED
TERMINAL AND IS MEASURED BETWEEN
0.15 AND 0.30mm FROM TERMINAL
4. COPLANARITY APPLIES TO THE EXPOSED
PAD AS WELL AS THE TERMINALS.
5. POSITIONAL TOLERANCE APPLIES TO ALL
THREE EXPOSED PADS.
L
L
SIDE VIEW A1
C
NOTE 4
SEATING
PLANE
0.10 C A B
D3
D2
NOTE 5
G
DETAIL A
40X
L
MILLIMETERS
MIN
MAX
0.80
1.00
−−−
0.05
0.20 REF
0.18
0.30
6.00 BSC
2.30
2.50
1.40
1.60
6.00 BSC
4.30
4.50
1.90
2.10
1.64
1.84
0.50 BSC
2.20 BSC
0.20
−−−
0.30
0.50
−−−
0.15
SOLDERING FOOTPRINT
E3
6.30
E2
4.56
E4
1.66
1
G
40
K
e
40X
e/2
G
BOTTOM VIEW
40X
0.63
2.56
1
b
0.10 C A B
0.05 C
2.16
NOTE 3
4.56
6.30
2.16
PKG
OUTLINE
40X
0.50
PITCH
0.30
DIMENSIONS: MILLIMETERS
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NCP3231A/D