ON NCV8853 Automotive grade non-synchronous buck controller Datasheet

NCV8853
Automotive Grade
Non-Synchronous Buck
Controller
The NCV8853 is an adjustable−output non−synchronous buck
controller which drives an external P−channel MOSFET. The device
uses peak current mode control with internal slope compensation. The
IC incorporates an internal regulator that supplies charge to the gate
driver.
Protection features include internal soft−start, undervoltage lockout,
cycle−by−cycle current limit, hiccup−mode overcurrent protection,
hiccup−mode short−circuit protection.
Additional features include: power good signal, low quiescent
current sleep mode and externally synchronizable switching
frequency.
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Ultra Low Iq Sleep Mode
Adjustable Output with 800 mV ±2.0% Reference Voltage
Wide Input of 3.1 to 44 V with Undervoltage Lockout (UVLO)
Power Good (PG)
Internal Soft−Start (SS)
Fixed−Frequency Peak Current Mode Control
Internal Slope Compensating Artificial Ramp
Internal High−Side PMOS Gate Driver
Regulated Gate Driver Current Source
External Frequency Synchronization (SYNC)
Programmable Cycle−by−Cycle Current Limit (CL)
Hiccup Overcurrent Protection (OCP)
Output Short Circuit Hiccup Protection (SCP)
Space−Saving 8−PIN SOIC Package
NCV Prefix for Automotive and Other Applications Requiring
Unique Site and Control Change Requirements; AEC−Q100
Qualified and PPAP Capable
These Devices are Pb−Free and are RoHS Compliant
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MARKING
DIAGRAM
8
V8853xx
ALYW
G
SOIC−8
SUFFIX D
CASE 751
8
1
1
V8853xx = Specific Device Code
x = 00, 01
A
= Assembly Location
L
= Wafer Lot
Y
= Year
W
= Work Week
G
= Pb−Free Package
PINOUT DIAGRAM
1 PG
VIN 8
2 EN/SYNC
ISNS 7
3 COMP
GDRV 6
4 FB
GND 5
ORDERING INFORMATION
Device
Package
Shipping†
NCV885300D1R2G
SOIC−8 2500/Tape & Reel
(Pb−Free)
NCV885301D1R2G
SOIC−8 2500/Tape & Reel
(Pb−Free)
†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.
© Semiconductor Components Industries, LLC, 2015
August, 2015 − Rev. 4
1
Publication Order Number:
NCV8853/D
NCV8853
VIN
PG
EN/SYNC
VIN
EN/SYNC
ISNS
COMP
GDRV
FB
GND
VO
Figure 1. NCV8853 Application Diagram
UVLO
OSC
PG
DRIVE
LOGIC
+
SCP
1
7
ISNS
6
GDRV
4
FB
5
GND
CL
FAULT
LOGIC
2
VIN
CSA
OCP
EN/SYNC
8
PWM
CLAMP
PG
SS
COMP 3
VEA
VREF
Figure 2. NCV8853 Simple Block Diagram
PIN DESCRIPTIONS
No
Pin Symbol
Function
1
PG
2
EN/SYNC
Enable and synchronization input. The falling edge synchronizes the internal oscillator. The part is disabled
into sleep mode when this pin is brought low for longer than the enable time−out period.
3
COMP
Output of the voltage error amplifier. An external compensator network from COMP to GND is used to stabilize the converter and tailor transient performance.
4
FB
5
GND
6
GDRV
Gate driver output. Connect to gate of the external P−channel MOSFET. A series resistance can be added
from GDRV to the gate to tailor EMC performance.
7
ISNS
Current sense input. Connect this pin to the source of the external P−channel MOSFET, through a current−
sense resistor to VIN to sense the switching current for regulation and current limiting.
8
VIN
Power good output. Use a pull−up resistor to 5.0 V. PG is pulled low when power is not good.
Output voltage feedback. A resistor from the output voltage to FB with another resistor from FB to GND
creates a voltage divider for regulation and programming of the output voltage.
Ground reference.
Main power input for the IC.
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NCV8853
MAXIMUM RATINGS (Voltages are with respect to GND unless otherwise indicated.)
Rating
DC Voltage (VIN, ISNS, GDRV)
Peak Transient Voltage (Load Dump on VIN)
Value
Unit
−0.3 to 44
V
44
V
DC Voltage (EN/SYNC, PG)
−0.3 to 6.0
V
DC Voltage (COMP, FB)
−0.3 to 3.6
V
DC Voltage Stress (VIN − GDRV)
−0.7 to 12
V
Operating Junction Temperature Range
−40 to 150
°C
Storage Temperature Range
−65 to 150
°C
265
°C
Peak Reflow Soldering Temperature: Pb−Free 60 to 150 seconds at 217°C
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.
PACKAGE ATTRIBUTES
Characteristic
Value
ESD Capability
Human Body Model
Machine Model
Charge Device Model
2.0 kV
200 V
>1.0 kV
Moisture Sensitivity Level
MSL 1 260°C
Package Thermal Resistance
Junction–to–Ambient, RqJA
100°C/W
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3
NCV8853
ELECTRICAL CHARACTERISTICS (VIN = 3.4 V to 36 V, EN = 5 V. Min/Max values are valid for the temperature range
−40°C ≤ TJ ≤ 150°C unless noted otherwise, and are guaranteed by test, design or statistical correlation.)
Characteristic
Symbol
Conditions
Min
Typ
Max
Unit
GENERAL
VIN = 13.2 V, EN = 0 V, Sleep Mode
2.5
6.0
mA
Iq,off
VIN = 13.2 V, EN = 5 V or toggled, VFB = 1 V,
No Switching
2.0
3.0
mA
Iq,on
VIN = 13.2 V, EN = 5 V or toggled, VFB = 0 V,
Switching
3.0
5.0
mA
Undervoltage Lockout
Vuvlo
VIN decreasing
2.9
3.1
3.3
V
Undervoltage Lockout
Hysteresis
Vuvlo,hys
50
150
300
mV
Overvoltage Lockout
Vovlo
36.9
38
39.3
V
Switching Frequency
fSW
306
340
374
Slope Compensation
ma
Quiescent Current
Iq,sleep
OSCILLATOR
Minimum On Time
Max Duty Cycle − Switching
51
tonmin
Dmax,sw
90
Maximum duty cycle when switching
110
kHz
mV/ms
140
ns
93
%
100
%
Max Duty Cycle
Dmax
Soft−Start Time
tss
1.0
1.5
2.0
ms
Soft−Start Delay
tss,dlly
200
300
400
ms
0.8
V
EN/SYNC
Low Threshold
Vs,il
High Threshold
Vs,ih
Input Current
Isync
SYNC Frequency Range
fsync
Relative to Nominal Switching Frequency
SYNC Delay
ts,dly
From SYNC falling edge to GDRV falling edge
SYNC Duty Cycle
Dsync
Disable Delay Time
2.0
V
5.0
80
50
25
10
mA
300
%
100
ns
75
%
ten
% of fSW
300
%
Rising Threshold
PGrise
% of Vref
87
90
93
%
Falling Threshold
PGfall
% of Vref
90
93
96
%
0.40
V
POWER GOOD
Power Good Pulldown
Vpg
VOLTAGE ERROR AMP
DC Gain
Gain−Bandwidth Product
FB Bias Current
Charge Currents
Av
55
80
91
dB
GBW
1.7
2.4
3.1
MHz
0.1
1.0
mA
mA
Ivfb,bias
Isrc,vea
Source, VFB = 0.9 V, VCOMP = 1.2 V
1.2
1.8
2.5
Isnk,vea
Sink, VFB = 0.7 V, VCOMP = 1.2 V
0.5
0.8
1.0
Vref
784
800
816
High Saturation Voltage
Vc,max
2.2
Low Saturation Voltage
Vc,min
Reference Voltage
2.3
0.001
mV
V
0.3
V
40
V
CURRENT SENSE AMP
Common−Mode Range
CMR
3.1
Differential Mode Range
DMR
300
Amplifier Gain
Acsa
mV
2.0
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V/V
NCV8853
ELECTRICAL CHARACTERISTICS (VIN = 3.4 V to 36 V, EN = 5 V. Min/Max values are valid for the temperature range
−40°C ≤ TJ ≤ 150°C unless noted otherwise, and are guaranteed by test, design or statistical correlation.)
Characteristic
Symbol
Conditions
Min
Typ
Max
Unit
30
70
50
120
mA
100
115
mV
200
nsec
175
%
CURRENT SENSE AMP
Input Bias Current
Isns,bias
NCV885300
NCV885301
CURRENT LIMIT / OVER CURRENT PROTECTION
Cycle−by−Cycle Current
Limit Threshold
Vcl
Cycle−by−Cycle Current
Limit Response Time
tcl
Over Current Protection
Threshold
Vocp
Over Current Protection
Response Time
tocp
200
ns
ton,min
100
ns
85
% of Vcl
125
150
GATE DRIVERS
Leading Edge Blanking
Time
Gate Driver Pull Up Current
Isink
VIN − VGDRV = 4 V
200
300
mA
Gate Driver Pull Down
Current
Isrc
VIN − VGDRV = 4 V
200
300
mA
Gate Driver Clamp Voltage
(VIN – VGDRV)
Vdrv
8.0
10
V
Power Switch Gate to
Source Voltage
Vgs
6.0
VIN = 4 V
3.8
From start of soft−start, % of soft−start time
105
% of Feedback Voltage (Vref)
65
V
SHORT CIRCUIT PROTECTION
Startup Blanking Time
Short−Circuit Threshold
Voltage
Hiccup Time
tscp,dly
Vscp
%
75
%
% of Soft−Start Time
135
tscp
Switcher Running
60
200
ns
Thermal Shutdown
Threshold
Tsd
TJ rising
160
170
180
°C
Thermal Shutdown
Hysteresis
Tsd,hys
TJ Shutdown – TJ Startup
10
15
20
°C
200
ns
SC Response Time
thcp,dly
70
300
%
THERMAL SHUTDOWN
Thermal Shutdown Delay
ttsd
TJ > Thermal Shutdown Threshold to stop
switching
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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5
NCV8853
4.5
4.5
4
4
Iq, QUIESCENT CURRENT (mA)
Iq, QUIESCENT CURRENT, SLEEP
(mA)
TYPICAL CHARACTERISTICS CURVES
3.5
3
2.5
2
1.5
1
0.5
0
−50
−25
0
25
50
75
100
125
3.5
Switching
3
2.5
No Switching
2
1.5
1
0.5
0
−50
150
−25
0
25
50
75
100
125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 3. Quiescent Current (Sleep) vs.
Junction Temperature
Figure 4. Quiescent Current vs. Junction
Temperature
150
70.27
0.79926
0.79924
70.26
0.79922
70.25
0.79918
SCP (%)
VREF, (V)
0.7992
0.79916
0.79914
70.24
70.23
0.79912
0.7991
70.22
0.79908
0.79906
−50
−25
0
25
50
75
100
125
150
−25
0
25
50
75
100
125 150
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 5. Reference Voltage vs. Junction
Temperature
Figure 6. Short−Circuit Protection Threshold
vs. Junction Temperature
111.2
230
GATE DRIVE CURRENT (mA)
111
MINIMUM ON TIME (ns)
70.21
−50
110.8
110.6
110.4
110.2
110
109.8
109.6
225
Source
220
215
Sink
210
109.4
109.2
−50
−25
0
25
50
75
100
125
150
205
−50
−25
0
25
50
75
100
125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 7. Minimum On Time vs. Junction
Temperature
Figure 8. Gate Drive Current vs. Junction
Temperature
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6
150
NCV8853
OVER CURRENT PROTECTION (% Vd)
CYCLE−BY−CYCLE CURRENT LIMIT (mV)
TYPICAL CHARACTERISTICS CURVES
111.2
111
110.8
110.6
110.4
110.2
110
109.8
109.6
109.4
109.2
−50
−25
0
25
50
75
100
125
150
150.2
150
149.8
149.6
149.4
149.2
149
148.8
148.6
148.4
−50
−25
0
25
50
75
100
125
TJ, JUNCTION TEMPERATURE (°C)
TJ, JUNCTION TEMPERATURE (°C)
Figure 9. Cycle−by−Cycle Limit vs. Junction
Temperature
Figure 10. Over Current Protection vs.
Junction Temperature
3.4
150
1.62
3.35
Rising
1.6
1.58
3.25
tss (ms)
UVLO (V)
3.3
3.2
3.15
1.56
1.54
Falling
3.1
1.52
3.05
1.5
3
−50
−25
0
25
50
75
100
125
150
1.48
−50
TJ, JUNCTION TEMPERATURE (°C)
−25
0
25
50
75
100
125
TJ, JUNCTION TEMPERATURE (°C)
Figure 11. UVLO Threshold vs. Junction
Temperature
Figure 12. Soft−Start Time vs. Junction
Temperature
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150
NCV8853
THEORY OF OPERATION
VIN
Current Information
RSNS
ISNS
CSA
Oscillator
GDRV
S
R
Q
+
Slope
Compensation
L
GATE
DRIVE
+
-
CO
PWM
COMPARATOR
RLOAD
VEA
Voltage Error
Vout
FB
−
+
VREF
NCV885300
COMP
Compensation
Figure 13. Current Mode Control Schematic
Current Mode Control
current sense amplifier detects a voltage above the peak
current limit between VIN and ISNS after the current limit
leading edge blanking time, the peak current limit causes the
power switch to turn off for the remainder of the cycle. Set
the current limit with a resistor from VIN to ISNS, with R =
0.100 / Ilimit.
If the voltage across the current sense resistor exceeds the
overcurrent threshold voltage the part enters overcurrent
hiccup mode. The part will remain off for the hiccup time
and then go through the power on reset procedure.
The NCV8853 SMPS 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 from the
resistor in the power path, the 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 for a simpler compensation.
The NCV8853 also includes 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.
Short Circuit Hiccup Protection
When the output voltage falls below the short circuit trip
voltage the part enters short circuit latch off. When a short
is detected the NCV8853 disables the outputs and attempts
to re−enable the outputs after the short circuit hiccup time.
The part remains off for the hiccup time and then goes
through the power on reset procedure. If the short has been
removed then the output stage re−enables and operates
normally; however, if the short is still present the cycle
begins again. Internal heat dissipation is kept to a minimum
as current will only flow during the reset time of the
protection circuitry. The hiccup mode is continuous until the
short is removed.
Overcurrent Protection
The NCV8853 features two current limit protections:
peak current mode and overcurrent hiccup mode. When the
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NCV8853
Gate Drive
B: One off−time is skipped in period 3, while the minimum
off−time is maintained in periods 1, 2, and 4.
C: An off−time is skipped in period 1 and in period 3, while
the minimum off−time is maintained in periods 2 and 4.
D: Low input voltage causes the IC to regulate at continuous
100% duty cycle (dropout).
To turn on the P−Channel MOSFET, the gate driver turns
on a current source to ground. A clamp ensures that the gate
drive voltage does not exceed 10 V. When the clamp starts
conducting the current source starts to turn off. To turn off
the external MOSFET, the gate driver turns on a current
source to VIN.
EN/SYNC
100% Duty Cycle Operation
This pin has three modes. When a dc logic high
(CMOS/TTL compatible) voltage is applied to this pin the
NCV8853 operates at the default frequency. When a dc logic
low voltage is applied to this pin the NCV8853 enters a low
quiescent current sleep mode. When a square wave of at least
40% of the switching frequency is applied to this pin the
switcher operates at the same frequency as the square wave.
If the signal is slower than 40% of the switching frequency,
it will be interpreted as enabling and disabling the part. The
falling edge of the square wave corresponds to the start of the
switching cycle.
Each cycle, the oscillator allows either a maximum duty
cycle up to 93% or 100% duty cycle operation. The
oscillator does not allow duty cycles between 93% and
100%.
Every cycle, the oscillator determines whether an
off−time is necessary. If so, the oscillator creates a duty cycle
up to 93%. If an off−time is not required, it can be skipped
and 100% duty cycle is allowed for the cycle.
Below are a few examples of what this could look like on
the switching node:
1
Power Good
≤93%
≤93%
≤93%
Overvoltage Lockout
VBAT
To protect the IC, if the voltage on the VIN pin the exceeds
Vovlo the NCV8853 will shutdown. When the voltage drops
below this voltage the part will go through the normal
soft−start procedure.
93%
100%
93%
Undervoltage Lockout
93%
Undervoltage lockout protection is engaged when the
input voltage drops below the Vuvlo signal. The part will
remain off until the input voltage rises above the Vuvlo value
plus hysteresis. Depending on the desired output voltage, it
is possible to engage the short-circuit hiccup mode before
undervoltage lockout occurs.
VBAT
93%
C
4
The power good pin is high when the reference voltage
reaches 90% of its target of 800 mV. The pin should be pulled
up with a 10 kW resistor to 5 V. The output voltage of the
controller may be used in this case.
≤93%
B
3
VBAT
A
2
100%
93%
100%
Soft−Start
To ensure moderate inrush current and reduce output
overshoot, the NCV8853 features a soft start which
periodically adds charge to a capacitor until the final
reference voltage is achieved. When using an external
SYNC signal, charging is based on the switching frequency.
If, for example, the NCV8853 is synchronized to twice the
free running (not synced) frequency, the soft−start will be
half as long.
VBAT
D
100%
100%
100%
100%
Figure 14. Duty Cycle Timing
A: Continuous operation. Each period has a duty cycle that
is less than or equal to 93%.
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NCV8853
DESIGN METHODOLOGY
leading to decreased efficiency, especially noticeable at light
loads.
Typically, the switching frequency is selected to avoid
interfering with signals of known frequencies. Using the
EN/SYNC pin, the NCV8853 can be synced to frequencies
from 80% to 300% of the nominal frequency (340 kHz).
Choosing external components for the NCV8853
encompasses the following design process:
1. Define operational parameters
2. Select switching frequency
3. Select current sensor
4. Select a MOSFET
5. Select a diode
6. Select output inductor
7. Select output capacitors
8. Select compensator components
(3) Current Sensor Selection
Current sensing for peak current mode control relies on
the inductor current signal. This is translated into a voltage
via a current sense resistor, which is then measured
differentially by the current sense amplifier, generating a
single−ended output to use as a signal. The easiest means of
implementing this transresistance is through the use of a
sense resistor in series with the source of the MOSFET and
VIN. A sense resistor should be calculated as follows:
(1) Operating Parameter Definition
First, select feedback resistors to choose the output
voltage as follows:
V OUT + V REF @
R1 ) R2
R2
where:
VOUT: desired output voltage
R1: upper feedback resistor (between VOUT and FB) [W]
R2: lower feedback resistor (between FB and GND) [W]
R SNS +
where:
RSNS: sense resistor [W]
VCL: current limit threshold voltage [V]
ICL: desired cycle−by−cycle current limit [A]
For a 5.0 V output, set R1 to 42.2 kW and R2 to 8.06 kW.
Certain operating parameters must be defined before
proceeding with the rest of the design. These are application
dependent and include the following:
VIN: input voltage, range from minimum to maximum
with a typical value [V]
IOUT: output current, range from minimum to maximum
with an initial startup value
ICL: desired typical current limit
A number of basic calculations must be performed up front
to use in the design process, as follows:
D MIN +
(4) MOSFET Selection
The NCV8853 has been designed to work with a
P−channel MOSFET in a non−synchronous buck
configuration. The MOSFET needs to be capable of
handling the maximum allowable current in the system, ICL.
Keep in mind that, depending on your minimum VIN signal,
it is possible to achieve 100% duty cycle. The power
dissipated through the MOSFET during conduction is as
follows:
V OUT
V IN(max)
P MOS,on + I CL 2 @ D MAX @ r DS,on
where:
PMOS,on: power through MOSFET [W]
ICL: cycle−by−cycle current limit [A]
rDS,on: on−resistance of the MOSFET [W]
To calculate the switching losses through the MOSFET, use
the following equation:
V
D + OUT
V IN(typ)
D MAX +
V CL
I CL
V OUT
V IN(min)
where:
DMIN: minimum duty cycle (ideal) [%]
VIN(max): maximum input voltage [V]
D: typical duty cycle (ideal) [%]
VIN(typ): typical input voltage [V]
DMAX: maximum duty cycle (ideal) [%]
VIN(min): minimum input voltage [V]
P MOS,sw + 1 V IN @ I OUT @ ǒt on ) t offǓ @ F SW
2
Q
t on + t off + Gate
I drv
where:
PMOS, sw: MOSFET switching losses [W]
ton: time to turn on the MOSFET [s]
toff: time to turn off the MOSFET [s]
QGate: gate charge [C]
Idrv: gate drive current [A]
(2) Switching Frequency Selection
Selecting the switching frequency is a trade−off between
component size and power losses. Operation at higher
switching frequencies allows the use of smaller inductor and
capacitor values to achieve the same inductor current ripple
and output voltage ripple. However, increasing the
frequency increases the switching losses of the MOSFETs,
(5) Diode Selection
The diode must be chosen according to its maximum
current and voltage ratings, and to thermal considerations.
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NCV8853
with peak−to−peak ripple current. Core losses also increase
as switching frequency increases.
Ac winding losses are based on the ac resistance of the
winding and the RMS ripple current through the inductor,
which is much lower than the dc current. The ac winding
losses are due to skin and proximity effects and are typically
much less than dc losses, but increase with frequency. Dc
winding losses account for a large percentage of output
inductor losses and are the dominant factor at switching
frequencies at or below 500 kHz. The dc winding losses in
the inductor can be calculated with the following equation:
The maximum reverse voltage the diode sees is the
maximum input voltage (with some margin in case of
ringing on the switch node). The maximum forward current
is the peak current limit of the NCV8853, or 150% of ICL.
(6) Output Inductor Selection
Both mechanical and electrical considerations influence
the selection of an output inductor. 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 power supply, a minimum
inductor value is particularly important in space−
constrained applications. From an electrical perspective, an
inductor is chosen for a set amount of current ripple and to
assure adequate transient response.
The output inductor controls the current ripple that occurs
over a switching period. A high current ripple will result in
excessive power loss and ripple current requirements. A low
current ripple will result in a poor control signal and a slow
current slew rate in the event of a load transient. A good
starting point for peak−to−peak ripple is around 10% of the
inductor current.To choose the inductor value based on the
peak−to−peak ripple current, use the following equation:
iL +
P L(dc) + I OUT 2 @ R dc
where:
PL(dc): dc winding losses in the output inductor
Rdc: dc resistance of the output inductor (DCR)
(7) Output Capacitor Selection
The output capacitor is a basic component for the fast
response of a power supply. In fact, for the first few
microseconds of a load transient, they supply the current to
the load. The controller recognizes the load transient and
proceeds to increase the duty cycle to its maximum.
Neglecting the effect of the ESL, the output voltage has a
first drop due to ESR of the bulk capacitor(s).
V OUT @ (1 * D MIN)
L @ F SW
DV OUT(ESR) + DI OUT @ ESR
where:
iL: peak−to−peak output current ripple [Ap−p]
From this equation it is clear that the ripple current increases
as L decreases, emphasizing the trade−off between dynamic
response and ripple current. The peak and valley values of
the triangular current waveform are as follows:
A lower ESR produces a lower ΔV during load transient.
In addition, a lower ESR produces a lower output voltage
ripple.
In the case of stepping into a short, the inductor current
approaches zero with the worst case initial current at the
current limit and the initial voltage at the output voltage set
point, calculating the voltage overshoot as follows:
iL
2
i
I L(vly) + I OUT * L
2
I L(pk) + I OUT )
DV OS +
ǸL @CI
CL
2
) V OUT 2 * V OUT
Accordingly, a minimum amount of capacitance can be
chosen for maximum allowed output voltage overshoot:
where:
IL(pk): peak (maximum) value of ripple current [A]
IL(vly): valley (minimum) value of ripple current [A]
Saturation current is specified by inductor manufacturers as
the current at which the inductance value has dropped from
the nominal value, typically 10%. For stable operation, the
output inductor must be chosen so that the inductance is
close to the nominal value even at the peak output current,
IL(pk), it is recommended to choose an inductor with
saturation current sufficiently higher than the peak output
current, such that the inductance is very close to the nominal
value at the peak output current. This allows for a safety
factor and allows for more optimized compensation.
Inductor efficiency is another consideration when
selecting an output inductor. Inductor losses include dc and
ac winding losses, which are very low due to high core
resistance, and magnetic hysteresis losses, which increase
C MIN +
L @ I CL 2
ǒVOUT ) DVOS(max)Ǔ
2
* V OUT 2
where:
CMIN: minimum amount of capacitance to minimize
voltage overshoot to ΔVOS(max) [F]
ΔVOS(max): maximum allowed voltage overshoot during
a short [V]
(8) Select Compensator Components
The Current Mode control method employed by the
NCV8853 allows the use of a simple, Type II compensation
to optimize the dynamic response according to system
requirements. Using a simulation tool such as CompCalc
can assist in the selection of these components.
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11
NCV8853
PACKAGE DIMENSIONS
SOIC−8 NB
CASE 751−07
ISSUE AK
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSION A AND B DO NOT INCLUDE
MOLD PROTRUSION.
4. MAXIMUM MOLD PROTRUSION 0.15 (0.006)
PER SIDE.
5. DIMENSION D DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBAR
PROTRUSION SHALL BE 0.127 (0.005) TOTAL
IN EXCESS OF THE D DIMENSION AT
MAXIMUM MATERIAL CONDITION.
6. 751−01 THRU 751−06 ARE OBSOLETE. NEW
STANDARD IS 751−07.
−X−
A
8
5
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
S
J
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
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NCV8853/D