SM72239 [NSC]
SM72239;
| 型号： | SM72239 |
| 厂家： | 
National Semiconductor |
| 描述： | SM72239 |
| 文件： | 总17页 (文件大小：412K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
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April 21, 2009
LM5008
High Voltage (100V) Step Down Switching Regulator
General Description
Features
The LM5008 Step Down Switching Regulator features all of
the functions needed to implement a low cost, efficient, Buck
bias regulator. This high voltage regulator contains an 100 V
N-Channel Buck Switch. The device is easy to implement and
is provided in the MSOP-8 and the thermally enhanced LLP-8
packages. The regulator is based on a hysteretic control
scheme using an ON time inversely proportional to VIN. This
feature allows the operating frequency to remain relatively
constant. The hysteretic control requires no loop compensa-
tion. An intelligent current limit is implemented with forced
OFF time, which is inversely proportional to Vout. This
scheme ensures short circuit protection while providing min-
imum foldback. Other protection features include: Thermal
Shutdown, VCC under-voltage lockout, Gate drive under-volt-
age lockout, and Max Duty Cycle limiter
Integrated 100V, N-Channel buck switch
■
■
■
■
■
■
Internal VCC regulator
No loop compensation required
Ultra-Fast transient response
On time varies inversely with line voltage
Operating frequency remains constant with varying line
voltage and load current
Adjustable output voltage
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■
■
■
■
■
Highly efficient operation
Precision internal reference
Low bias current
Intelligent current limit protection
Thermal shutdown
Typical Applications
Non-Isolated Telecommunication Buck Regulator
■
■
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Secondary High Voltage Post Regulator
+42V Automotive Systems
Package
MSOP - 8
■
■
LLP - 8 (4mm x 4mm)
Connection Diagram
20097902
8-Lead MSOP, LLP
Ordering Information
Order Number
LM5008MM
Package Type
NSC Package Drawing
Supplied As
1000 Units on Tape and Reel
3500 Units on Tape and Reel
1000 Units on Tape and Reel
4500 Units on Tape and Reel
1000 Units on Tape and Reel
4500 Units on Tape and Reel
MSOP-8
MUA08A
LM5008MMX
LM5008SD
SDC08A
SDC08B
LM5008SDX
LM5008SDC
LM5008SDCX
LLP-8
© 2009 National Semiconductor Corporation
200979
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Typical Application Circuit and Block Diagram
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2
Pin Descriptions
Pin
Name
Description
Application Information
1
SW
Switching Node
Power switching node. Connect to the output inductor,
re-circulating diode, and bootstrap capacitor.
2
3
BST
RCL
Boost Pin (Boot–strap capacitor input)
An external capacitor is required between the BST
and the SW pins. A 0.01µF ceramic capacitor is
recommended. An internal diode charges the
capacitor from VCC
.
Current Limit OFF time set pin
Toff = 10-5 / (0.285 + (FB / 6.35 x 10− 6 x RCL))
A resistor between this pin and RTN sets the off-time
when current limit is detected. The off-time is preset
to 35µs if FB = 0V.
4
5
RTN
FB
Ground pin
Ground for the entire circuit.
Feedback input from Regulated Output
This pin is connected to the inverting input of the
internal regulation comparator. The regulation
threshold is 2.5V.
6
7
RON/SD
On time set pin
Ton = 1.25 x 10-10 RON / VIN
A resistor between this pin and VIN sets the switch on
time as a function of VIN. The minimum recommended
on time is 400ns at the maximum input voltage. This
pin can be used for remote shutdown.
VCC
Output from the internal high voltage series pass If an auxiliary voltage is available to raise the voltage
regulator. Regulated at 7.0V.
on this pin, above the regulation setpoint (7V), the
internal series pass regulator will shutdown, reducing
the IC power dissipation. Do not exceed 14V. This
voltage provides gate drive power for the internal Buck
switch. An internal diode is provided between this pin
and the BST pin. A local 0.1µF decoupling capacitor
is recommended. Series pass regulator is current
limited to 10mA.
8
VIN
EP
Input voltage
Exposed Pad
Recommended operating range: 9.5V to 95V.
The exposed pad has no electrical contact. Connect
to system ground plane for reduced thermal
resistance.
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BST to SW
VCC to GND
14V
14V
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
All Other Inputs to GND
Lead Temperature (Soldering 4 sec)
Storage Temperature Range
-0.3 to 7V
260°C
-55°C to +150°C
VIN to GND
-0.3V to 100V
-0.3V to 114V
-1V
BST to GND
Operating Ratings (Note 1)
VIN
SW to GND (Steady State)
ESD Rating (Note 5)
Human Body Model
BST to VCC
9.5V to 95V
−40°C to + 125°C
Operating Junction Temperature
2kV
100V
Electrical Characteristics
Specifications with standard typeface are for TJ = 25°C, and those with boldface type apply over full Operating Junction Tem-
perature range. VIN = 48V, unless otherwise stated (Note 3).
Symbol
VCC Supply
VCC Reg
Parameter
Conditions
Min
Typ
Max
Units
VCC Regulator Output
VCC Current Limit
6.6
7
7.4
V
mA
V
(Note 4)
9.5
6.3
VCC undervoltage Lockout
Voltage (VCC increasing)
VCC Undervoltage Hysteresis
VCC UVLO Delay (filter)
IIN Operating Current
200
10
mV
µs
100mV overdrive
Non-Switching, FB = 3V
RON/SD = 0V
485
76
675
150
µA
µA
IIN Shutdown Current
Switch Characteristics
Buck Switch Rds(on)
ITEST = 200mA, (Note 6)
VBST − VSW Rising
1.15
4.5
2.47
5.5
Ω
V
Gate Drive UVLO
3.4
Gate Drive UVLO Hysteresis
430
mV
Current Limit
Current Limit Threshold
0.41
0.51
400
0.61
A
Current Limit Response Time
Iswitch Overdrive = 0.1A Time
to Switch Off
ns
OFF time generator (test 1)
OFF time generator (test 2)
FB=0V, RCL = 100K
35
µs
µs
FB=2.3V, RCL = 100K
2.56
On Time Generator
TON - 1
Vin = 10V
Ron = 200K
2.15
200
2.77
300
3.5
420
1.05
µs
ns
TON - 2
Vin = 95V
Ron = 200K
Remote Shutdown Threshold
Remote Shutdown Hysteresis
Rising
0.40
0.70
35
V
mV
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Symbol
Parameter
Conditions
FB = 0V
Min
Typ
300
2.5
Max
Units
ns
Minimum Off Time
Minimum Off Timer
Regulation and OV Comparators
FB Reference Threshold
Internal reference
2.445
2.550
V
Trip point for switch ON
FB Over-Voltage Threshold
FB Bias Current
Trip point for switch OFF
2.875
100
V
nA
Thermal Shutdown
Tsd
Thermal Shutdown Temp.
165
25
°C
°C
Thermal Shutdown Hysteresis
Thermal Resistance
Junction to Ambient
MUA Package
SDC Package
°C/W
°C/W
θJA
200
40
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: For detailed information on soldering plastic MSOP and LLP packages, refer to the Packaging Data Book available from National Semiconductor
Corporation.
Note 3: All limits are guaranteed. All electrical characteristics having room temperature limits are tested during production with TA = TJ = 25°C. All hot and cold
limits are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control.
Note 4: The VCC output is intended as a self bias for the internal gate drive power and control circuits. Device thermal limitations limit external loading.
Note 5: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 6: For devices procured in the LLP-8 package the Rds(on) limits are guaranteed by design characterization data only.
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Typical Performance Characteristics
20097912
20097909
FIGURE 5. Current Limit Off-Time vs VFB and RCL
FIGURE 2. ICC Current vs Applied VCC Voltage
20097923
20097910
FIGURE 6. Efficiency vs VIN
FIGURE 3. ON-Time vs Input Voltage and RON
(Circuit of Figure 13)
20097911
FIGURE 4. Maximum Frequency vs VOUT and VIN
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20097927
20097924
FIGURE 7. Efficiency vs Load Current vs VIN
FIGURE 8. Output Voltage vs Load Current
(Circuit of Figure 13)
(Circuit of Figure 13)
voltage at FB falls below the internal reference - until then the
Functional Description
inductor current remains zero. In this mode the operating fre-
quency is lower than in continuous conduction mode, and
varies with load current. Therefore at light loads the conver-
sion efficiency is maintained, since the switching losses re-
duce with the reduction in load and frequency. The discon-
tinuous operating frequency can be calculated as follows:
The LM5008 Step Down Switching Regulator features all the
functions needed to implement a low cost, efficient, Buck bias
power converter. This high voltage regulator contains a 100
V N-Channel Buck Switch, is easy to implement and is pro-
vided in the MSOP-8 and the thermally enhanced LLP-8
packages. The regulator is based on a hysteretic control
scheme using an on-time inversely proportional to VIN. The
hysteretic control requires no loop compensation. Current
limit is implemented with forced off-time, which is inversely
proportional to VOUT. This scheme ensures short circuit pro-
tection while providing minimum foldback. The Functional
Block Diagram of the LM5008 is shown in Figure 1.
where RL = the load resistance
In continuous conduction mode, current flows continuously
through the inductor and never ramps down to zero. In this
mode the operating frequency is greater than the discontinu-
ous mode frequency and remains relatively constant with load
and line variations. The approximate continuous mode oper-
ating frequency can be calculated as follows:
The LM5008 can be applied in numerous applications to ef-
ficiently regulate down higher voltages. This regulator is well
suited for 48 Volt Telecom and the new 42V Automotive pow-
er bus ranges. Protection features include: Thermal Shut-
down, VCC under-voltage lockout, Gate drive under-voltage
lockout, Max Duty Cycle limit timer and the intelligent current
limit off timer.
Hysteretic Control Circuit Overview
(1)
The LM5008 is a Buck DC-DC regulator that uses a control
scheme in which the on-time varies inversely with line voltage
(VIN). Control is based on a comparator and the on-time one-
shot, with the output voltage feedback (FB) compared to an
internal reference (2.5V). If the FB level is below the reference
the buck switch is turned on for a fixed time determined by the
line voltage and a programming resistor (RON). Following the
ON period the switch will remain off for at least the minimum
off-timer period of 300ns. If FB is still below the reference at
that time the switch will turn on again for another on-time pe-
riod. This will continue until regulation is achieved.
The output voltage (VOUT) can be programmed by two exter-
nal resistors as shown in Figure 1. The regulation point can
be calculated as follows:
VOUT = 2.5 x (R1 + R2) / R2
All hysteretic regulators regulate the output voltage based on
ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor C2. A minimum of
25mV to 50mV of ripple voltage at the feedback pin (FB) is
required for the LM5008. In cases where the capacitor ESR
is too small, additional series resistance may be required (R3
in Figure 1).
The LM5008 operates in discontinuous conduction mode at
light load currents, and continuous conduction mode at heavy
load current. In discontinuous conduction mode, current
through the output inductor starts at zero and ramps up to a
peak during the on-time, then ramps back to zero before the
end of the off-time. The next on-time period starts when the
For applications where lower output voltage ripple is required
the output can be taken directly from a low ESR output ca-
pacitor, as shown in Figure 9. However, R3 slightly degrades
the load regulation.
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20097905
FIGURE 9. Low Ripple Output Configuration
In applications involving a high value for VIN, where power
dissipation in the VCC regulator is a concern, an auxiliary volt-
age can be diode connected to the VCC pin. Setting the
auxiliary voltage to 8.0 -14V will shut off the internal regulator,
reducing internal power dissipation. See Figure 10. The cur-
rent required into the VCC pin is shown in Figure 2.
High Voltage Start-Up Regulator
The LM5008 contains an internal high voltage startup regu-
lator. The input pin (VIN) can be connected directly to the line
voltages up to 95 Volts, with transient capability to 100 volts.
The regulator is internally current limited to 9.5mA at VCC
.
Upon power up, the regulator sources current into the external
capacitor at VCC (C3). When the voltage on the VCC pin reach-
es the under-voltage lockout threshold of 6.3V, the buck
switch is enabled.
20097906
FIGURE 10. Self Biased Configuration
Regulation Comparator
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal 2.5V
reference. In normal operation (the output voltage is regulat-
ed), an on-time period is initiated when the voltage at FB falls
below 2.5V. The buck switch will stay on for the on-time,
causing the FB voltage to rise above 2.5V. After the on-time
period, the buck switch will stay off until the FB voltage again
falls below 2.5V. During start-up, the FB voltage will be below
2.5V at the end of each on-time, resulting in the minimum off-
time of 300 ns. Bias current at the FB pin is nominally 100 nA.
The feedback voltage at FB is compared to an internal 2.875V
reference. If the voltage at FB rises above 2.875V the on-time
pulse is immediately terminated. This condition can occur if
the input voltage, or the output load, change suddenly. The
buck switch will not turn on again until the voltage at FB falls
below 2.5V.
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operation. This requirement limits the maximum frequency for
each application, depending on VIN and VOUT. See Figure 4.
On-Time Generator and Shutdown
The on-time for the LM5008 is determined by the RON resistor,
and is inversely proportional to the input voltage (Vin), result-
ing in a nearly constant frequency as Vin is varied over its
range. The on-time equation for the LM5008 is:
The LM5008 can be remotely disabled by taking the RON/SD
pin to ground. See Figure 11. The voltage at the RON/SD pin
is between 1.5 and 3.0 volts, depending on Vin and the value
of the RON resistor.
TON = 1.25 x 10-10 x RON / VIN
(2)
See Figure 3. RON should be selected for a minimum on-time
(at maximum VIN) greater than 400 ns, for proper current limit
20097907
FIGURE 11. Shutdown Implementation
Current Limit
Thermal Protection
The LM5008 contains an intelligent current limit OFF timer. If
the current in the Buck switch exceeds 0.5A the present cycle
is immediately terminated, and a non-resetable OFF timer is
initiated. The length of off-time is controlled by an external
resistor (RCL) and the FB voltage (see Figure 5). When FB =
0V, a maximum off-time is required, and the time is preset to
35µs. This condition occurs when the output is shorted, and
during the initial part of start-up. This amount of time ensures
safe short circuit operation up to the maximum input voltage
of 95V. In cases of overload where the FB voltage is above
zero volts (not a short circuit) the current limit off-time will be
less than 35µs. Reducing the off-time during less severe
overloads reduces the amount of foldback, recovery time, and
the start-up time. The off-time is calculated from the following
equation:
The LM5008 should be operated so the junction temperature
does not exceed 125°C during normal operation. An internal
Thermal Shutdown circuit is provided to protect the LM5008
in the event of a higher than normal junction temperature.
When activated, typically at 165°C, the controller is forced into
a low power reset state, disabling the buck switch and the
VCC regulator. This feature prevents catastrophic failures from
accidental device overheating. When the junction tempera-
ture reduces below 140°C (typical hysteresis = 25°C), the Vcc
regulator is enabled, and normal operation is resumed.
Applications Information
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values will be illus-
trated with a design example. Refer to Figure 1. The following
steps will configure the LM5008 for:
TOFF = 10-5 / (0.285 + (VFB / 6.35 x 10-6 x RCL))
(3)
The current limit sensing circuit is blanked for the first 50-70ns
of each on-time so it is not falsely tripped by the current surge
which occurs at turn-on. The current surge is required by the
re-circulating diode (D1) for its turn-off recovery.
•
•
•
Input voltage range (Vin): 12V to 95V
Output voltage (VOUT1): 10V
Load current (for continuous conduction mode): 100 mA
to 300 mA
•
Maximum ripple at VOUT2: 100 mVp-p at maximum input
voltage
N - Channel Buck Switch and Driver
The LM5008 integrates an N-Channel Buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor and an internal high voltage diode. A 0.01µF ceramic
capacitor (C4) connected between the BST pin and SW pin
provides the voltage to the driver during the on-time.
R1 and R2: From Figure 1, VOUT1 = VFB x (R1 + R2) / R2, and
since VFB = 2.5V, the ratio of R1 to R2 calculates as 3:1.
Standard values of 3.01 kΩ (R1) and 1.00 kΩ (R2) are cho-
sen. Other values could be used as long as the 3:1 ratio is
maintained. The selected values, however, provide a small
amount of output loading (2.5 mA) in the event the main load
is disconnected. This allows the circuit to maintain regulation
until the main load is reconnected.
During each off-time, the SW pin is at approximately 0V, and
the bootstrap capacitor charges from Vcc through the internal
diode. The minimum OFF timer, set to 300ns, ensures a min-
imum time each cycle to recharge the bootstrap capacitor.
Fs and RON: The recommended operating frequency range
for the LM5008 is 50kHz to 600 kHz. Unless the application
requires a specific frequency, the choice of frequency is gen-
erally a compromise since it affects the size of L1 and C2, and
An external re-circulating diode (D1) carries the inductor cur-
rent after the internal Buck switch turns off. This diode must
be of the Ultra-fast or Schottky type to minimize turn-on losses
and current over-shoot.
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the switching losses. The maximum allowed frequency,
based on a minimum on-time of 400 ns, is calculated from:
The DC resistance of the inductor should be as low as pos-
sible. For example, if the inductor’s DCR is one ohm, the
power dissipated at maximum load current is 0.09W. While
small, it is not insignificant compared to the load power of 3W.
FMAX = VOUT / (VINMAX x 400ns)
For this exercise, Fmax = 263kHz. From equation 1, RON cal-
culates to 304 kΩ. A standard value 357 kΩ resistor will be
used to allow for tolerances in equation 1, resulting in a fre-
quency of 224kHz.
C3: The capacitor on the VCC output provides not only noise
filtering and stability, but its primary purpose is to prevent false
triggering of the VCC UVLO at the buck switch on/off transi-
tions. For this reason, C3 should be no smaller than 0.1 µF.
L1: The main parameter affected by the inductor is the output
current ripple amplitude. The choice of inductor value there-
fore depends on both the minimum and maximum load cur-
rents, keeping in mind that the maximum ripple current occurs
at maximum Vin.
C2, and R3: When selecting the output filter capacitor C2, the
items to consider are ripple voltage due to its ESR, ripple
voltage due to its capacitance, and the nature of the load.
a) ESR and R3: A low ESR for C2 is generally desirable so
as to minimize power losses and heating within the capacitor.
However, a hysteretic regulator requires a minimum amount
of ripple voltage at the feedback input for proper loop opera-
tion. For the LM5008 the minimum ripple required at pin 5 is
25 mV p-p, requiring a minimum ripple at VOUT1 of 100 mV.
Since the minimum ripple current (at minimum Vin) is 34 mA
p-p, the minimum ESR required at VOUT1 is 100mV/34mA =
2.94Ω. Since quality capacitors for SMPS applications have
an ESR considerably less than this, R3 is inserted as shown
in Figure 1. R3’s value, along with C2’s ESR, must result in
at least 25 mV p-p ripple at pin 5. Generally, R3 will be 0.5 to
3.0Ω.
a) Minimum load current: To maintain continuous conduc-
tion at minimum Io (100 mA), the ripple amplitude (IOR) must
be less than 200 mA p-p so the lower peak of the waveform
does not reach zero. L1 is calculated using the following
equation:
At Vin = 95V, L1(min) calculates to 200 µH. The next larger
standard value (220 µH) is chosen and with this value IOR
calculates to 181 mA p-p at Vin = 95V, and 34 mA p-p at Vin
= 12V.
b) Nature of the Load: The load can be connected to
VOUT1 or VOUT2. VOUT1 provides good regulation, but with a
ripple voltage which ranges from 100 mV (@ Vin = 12V) to
500mV (@Vin = 95V). Alternatively, VOUT2 provides low rip-
ple, but lower regulation due to R3.
b) Maximum load current: At a load current of 300 mA, the
peak of the ripple waveform must not reach the minimum
guaranteed value of the LM5008’s current limit threshold (410
mA). Therefore the ripple amplitude must be less than 220
mA p-p, which is already satisfied in the above calculation.
With L1 = 220 µH, at maximum Vin and Io, the peak of the
ripple will be 391 mA. While L1 must carry this peak current
without saturating or exceeding its temperature rating, it also
must be capable of carrying the maximum guaranteed value
of the LM5008’s current limit threshold (610 mA) without sat-
urating, since the current limit is reached during startup.
For a maximum allowed ripple voltage of 100 mVp-p at
VOUT2 (@ Vin = 95V), assume an ESR of 0.4Ω for C2. At
maximum Vin, the ripple current is 181 mAp-p, creating a rip-
ple voltage of 72 mVp-p. This leaves 28 mVp-p of ripple due
to the capacitance. The average current into C2 due to the
ripple current is calculated using the waveform in Figure 12.
20097926
FIGURE 12. Inductor Current Waveform
Starting when the current reaches Io (300 mA in Figure 12)
half way through the on-time, the current continues to in-
crease to the peak (391 mA), and then decreases to 300 mA
half way through the off-time. The average value of this por-
tion of the waveform is 45.5mA, and will cause half of the
voltage ripple, or 14 mV. The interval is one half of the fre-
quency cycle time, or 2.23 µs. Using the capacitor’s basic
equation:
two numbers do not add directly. However, this calculation
provides a practical minimum value for C2 based on its ESR,
and the target spec. To allow for the capacitor’s tolerance,
temperature effects, and voltage effects, a 15 µF, X7R ca-
pacitor will be used.
c) In summary: The above calculations provide a minimum
value for C2, and a calculation for R3. The ESR is just as
important as the capacitance. The calculated values are
guidelines, and should be treated as starting points. For each
application, experimentation is needed to determine the op-
timum values for R3 and C2.
C = I x Δt / ΔV
the minimum value for C2 is 7.2 µF. The ripple due to C2’s
capacitance is 90° out of phase from the ESR ripple, and the
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RCL: When a current limit condition is detected, the minimum
off-time set by this resistor must be greater than the maximum
normal off-time which occurs at maximum Vin. Using equation
2, the minimum on-time is 0.470 µs, yielding a maximum off-
time of 3.99 µs. This is increased by 117 ns (to 4.11 µs) due
to a ±25% tolerance of the on-time. This value is then in-
creased to allow for:
capacitor, with low ESR is recommended as C4 supplies the
surge current to charge the buck switch gate at turn-on. A low
ESR also ensures a quick recharge during each off-time. At
minimum Vin, when the on-time is at maximum, it is possible
during start-up that C4 will not fully recharge during each 300
ns off-time. The circuit will not be able to complete the start-
up, and achieve output regulation. This can occur when the
frequency is intended to be low (e.g., RON = 500K). In this
case C4 should be increased so it can maintain sufficient
voltage across the buck switch driver during each on-time.
The response time of the current limit detection loop
(400ns),
The off-time determined by equation 3 has a ±25% toler-
ance,
C5: This capacitor helps avoid supply voltage transients and
ringing due to long lead inductance at VIN. A low ESR, 0.1µF
ceramic chip capacitor is recommended, located close to the
LM5008.
tOFFCL(MIN) = (4.11 µs + 0.40µs) x 1.25 = 5.64 µs
Using equation 3, RCL calculates to 264kΩ (at VFB = 2.5V).
The closest standard value is 267 kΩ.
FINAL CIRCUIT
D1: The important parameters are reverse recovery time and
forward voltage. The reverse recovery time determines how
long the reverse current surge lasts each time the buck switch
is turned on. The forward voltage drop is significant in the
event the output is short-circuited as it is only this diode’s
voltage which forces the inductor current to reduce during the
forced off-time. For this reason, a higher voltage is better, al-
though that affects efficiency. A good choice is an ultrafast
power diode, such as the MURA110T3 from ON Semicon-
ductor. Its reverse recovery time is 30ns, and its forward
voltage drop is approximately 0.72V at 300 mA at 25°C. Other
types of diodes may have a lower forward voltage drop, but
may have longer recovery times, or greater reverse leakage.
D1’s reverse voltage rating must be at least as great as the
maximum Vin, and its current rating be greater than the max-
imum current limit threshold (610 mA).
The final circuit is shown in Figure 13. The circuit was tested,
and the resulting performance is shown in Figure 6 through
Figure 8.
MINIMUM LOAD CURRENT
A minimum load current of 1 mA is required to maintain proper
operation. If the load current falls below that level, the boot-
strap capacitor may discharge during the long off-time, and
the circuit will either shutdown, or cycle on and off at a low
frequency. If the load current is expected to drop below 1 mA
in the application, the feedback resistors should be chosen
low enough in value so they provide the minimum required
current at nominal Vout.
PC BOARD LAYOUT
The LM5008 regulation and over-voltage comparators are
very fast, and as such will respond to short duration noise
pulses. Layout considerations are therefore critical for opti-
mum performance. The components at pins 1, 2, 3, 5, and 6
should be as physically close as possible to the IC, thereby
minimizing noise pickup in the PC tracks. The current loop
formed by D1, L1, and C2 should be as small as possible. The
ground connection from C2 to C1 should be as short and di-
rect as possible.
C1: This capacitor’s purpose is to supply most of the switch
current during the on-time, and limit the voltage ripple at Vin,
on the assumption that the voltage source feeding Vin has an
output impedance greater than zero. At maximum load cur-
rent, when the buck switch turns on, the current into pin 8 will
suddenly increase to the lower peak of the output current
waveform, ramp up to the peak value, then drop to zero at
turn-off. The average input current during this on-time is the
load current (300 mA). For a worst case calculation, C1 must
supply this average load current during the maximum on-time.
To keep the input voltage ripple to less than 2V (for this ex-
ercise), C1 calculates to:
If the internal dissipation of the LM5008 produces excessive
junction temperatures during normal operation, good use of
the pc board’s ground plane can help considerably to dissi-
pate heat. The exposed pad on the bottom of the LLP-8
package can be soldered to a ground plane on the PC board,
and that plane should extend out from beneath the IC to help
dissipate the heat. Additionally, the use of wide PC board
traces, where possible, can also help conduct heat away from
the IC. Judicious positioning of the PC board within the end
product, along with use of any available air flow (forced or
natural convection) can help reduce the junction tempera-
tures.
Quality ceramic capacitors in this value have a low ESR which
adds only a few millivolts to the ripple. It is the capacitance
which is dominant in this case. To allow for the capacitor’s
tolerance, temperature effects, and voltage effects, a 1.0 µF,
100V, X7R capacitor will be used.
C4: The recommended value is 0.01µF for C4, as this is ap-
propriate in the majority of applications. A high quality ceramic
11
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20097922
FIGURE 13. LM5008 Example Circuit
Bill of Materials (Circuit of Figure 13)
Item
C1
C2
C3
C4
C5
D1
L1
Description
Part Number
Value
Ceramic Capacitor
Ceramic Capacitor
Ceramic Capacitor
Ceramic Capacitor
Ceramic Capacitor
UltraFast Power Diode
Power Inductor
TDK C4532X7R2A105M
TDK C4532X7R1E156M
Kemet C1206C104K5RAC
Kemet C1206C103K5RAC
TDK C3216X7R2A104M
ON Semi MURA110T3
Coilcraft DO3316-224 or
TDK SLF10145T-221MR65
Vishay CRCW12063011F
Vishay CRCW12061001F
Vishay CRCW12062R00F
Vishay CRCW12063573F
Vishay CRCW12062673F
1µF, 100V
15µF, 25V
0.1µF, 50V
0.01µF, 50V
0.1µF, 100V
100V, 1A
220 µH
R1
Resistor
3.01 kΩ
1.0 kΩ
2.0 Ω
R2
Resistor
R3
Resistor
RON
RCL
U1
Resistor
357 kΩ
267 kΩ
Resistor
Switching Regulator
National Semiconductor
LM5008
www.national.com
12
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead MSOP Package
NS Package Number MUA08A
8-Lead LLP Package
NS Package Number SDC08A
13
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8-Lead LLP Package
NS Package Number SDC08B
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14
Notes
15
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Notes
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