February 5, 2008
LM5008
High Voltage (100V) Step Down Switching Regulator
LM5008 High Voltage (100V) Step Down Switching Regulator
General Description
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 compensation. An intelligent current limit is implemented with forced
OFF time, which is inversely proportional to Vout. This
scheme ensures short circuit protection while providing minimum foldback. Other protection features include: Thermal
Shutdown, VCC under-voltage lockout, Gate drive under-voltage lockout, and Max Duty Cycle limiter
Features
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
■
Highly efficient operation
■
Precision internal reference
■
Low bias current
■
Intelligent current limit protection
■
Thermal shutdown
■
Typical Applications
Non-Isolated Telecommunication Buck Regulator
■
Secondary High Voltage Post Regulator
■
+42V Automotive Systems
■
Package
MSOP - 8
■
LLP - 8 (4mm x 4mm)
■
Connection Diagram
8-Lead MSOP, LLP
20097902
Ordering Information
Order Number Package Type NSC Package Drawing Supplied As
LM5008MM
LM5008MMX 3500 Units on Tape and Reel
LM5008SD
LM5008SDX 4500 Units on Tape and Reel
LM5008SDC
LM5008SDCX 4500 Units on Tape and Reel
MSOP-8 MUA08A
SDC08A
LLP-8
SDC08B
1000 Units on Tape and Reel
1000 Units on Tape and Reel
1000 Units on Tape and Reel
© 2008 National Semiconductor Corporation 200979 www.national.com
Typical Application Circuit and Block Diagram
LM5008
20097901
FIGURE 1.
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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 BST 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.
3 R
4 RTN Ground pin Ground for the entire circuit.
5 FB Feedback input from Regulated Output This pin is connected to the inverting input of the
6 RON/SD On time set pin
7 V
CC
8 V
EP Exposed Pad The exposed pad has no electrical contact. Connect
Current Limit OFF time set pin
CL
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.
internal regulation comparator. The regulation
threshold is 2.5V.
A resistor between this pin and VIN sets the switch on
Ton = 1.25 x 10
-10
RON / V
IN
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.
Output from the internal high voltage series pass
regulator. Regulated at 7.0V.
If an auxiliary voltage is available to raise the voltage
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.
Input voltage Recommended operating range: 9.5V to 95V.
IN
to system ground plane for reduced thermal
resistance.
LM5008
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
LM5008
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN to GND -0.3V to 100V
BST to GND -0.3V to 114V
SW to GND (Steady State) -1V
ESD Rating (Note 5)
Human Body Model 2kV
BST to V
CC
100V
BST to SW 14V
VCC to GND 14V
All Other Inputs to GND -0.3 to 7V
Lead Temperature (Soldering 4 sec) 200°C
Storage Temperature Range -55°C to +150°C
Operating Ratings (Note 1)
V
IN
Operating Junction Temperature −40°C to + 125°C
Electrical Characteristics
Specifications with standard typeface are for TJ = 25°C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 48V, unless otherwise stated (Note 3).
Symbol Parameter Conditions Min Typ Max Units
VCC Supply
VCC Reg VCC Regulator Output 6.6 7 7.4 V
VCC Current Limit (Note 4) 9.5 mA
VCC undervoltage Lockout
Voltage (VCC increasing)
VCC Undervoltage Hysteresis 200
VCC UVLO Delay (filter) 100mV overdrive 10
I
I
Operating Current Non-Switching, FB = 3V 485 675 µA
IN
Shutdown Current RON/SD = 0V 76 150 µA
IN
Switch Characteristics
Buck Switch Rds(on) I
Gate Drive UVLO V
Gate Drive UVLO Hysteresis 430 mV
Current Limit
Current Limit Threshold 0.41 0.51 0.61 A
Current Limit Response Time I
OFF time generator (test 1) FB=0V, RCL = 100K 35 µs
OFF time generator (test 2) FB=2.3V, RCL = 100K 2.56 µs
On Time Generator
TON - 1 Vin = 10V
TON - 2 Vin = 95V
Remote Shutdown Threshold Rising 0.40 0.70 1.05 V
Remote Shutdown Hysteresis 35 mV
6.3 V
= 200mA, (Note 6) 1.15 2.47
TEST
− VSW Rising 3.4 4.5 5.5 V
BST
Overdrive = 0.1A Time
switch
400 ns
to Switch Off
2.15 2.77 3.5 µs
Ron = 200K
200 300 420 ns
Ron = 200K
9.5V to 95V
mV
µs
Ω
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Symbol Parameter Conditions Min Typ Max Units
Minimum Off Time
Minimum Off Timer FB = 0V 300 ns
Regulation and OV Comparators
FB Reference Threshold Internal reference
2.445 2.5 2.550 V
Trip point for switch ON
FB Over-Voltage Threshold Trip point for switch OFF 2.875 V
FB Bias Current 100 nA
Thermal Shutdown
Tsd Thermal Shutdown Temp. 165 °C
Thermal Shutdown Hysteresis 25 °C
Thermal Resistance
θ
JA
Junction to Ambient MUA Package
200
°C/W
SDC Package 40 °C/W
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.
LM5008
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Typical Performance Characteristics
LM5008
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FIGURE 2. ICC Current vs Applied VCC Voltage
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FIGURE 3. ON-Time vs Input Voltage and R
ON
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FIGURE 5. Current Limit Off-Time vs VFB and R
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FIGURE 6. Efficiency vs V
(Circuit of Figure 13)
IN
CL
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FIGURE 4. Maximum Frequency vs V
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OUT
and V
IN
LM5008
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FIGURE 7. Efficiency vs Load Current vs V
(Circuit of Figure 13)
IN
Functional Description
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 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. The
hysteretic control requires no loop compensation. Current
limit is implemented with forced off-time, which is inversely
proportional to V
tection while providing minimum foldback. The Functional
. This scheme ensures short circuit pro-
OUT
Block Diagram of the LM5008 is shown in Figure 1.
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 power bus ranges. Protection features include: Thermal Shutdown, 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
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 oneshot, 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 period. This will continue until regulation is achieved.
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
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FIGURE 8. Output Voltage vs Load Current
(Circuit of Figure 13)
voltage at FB falls below the internal reference - until then the
inductor current remains zero. In this mode the operating frequency is lower than in continuous conduction mode, and
varies with load current. Therefore at light loads the conversion efficiency is maintained, since the switching losses reduce with the reduction in load and frequency. The discontinuous operating frequency can be calculated as follows:
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 discontinuous mode frequency and remains relatively constant with load
and line variations. The approximate continuous mode operating frequency can be calculated as follows:
(1)
The output voltage (V
nal resistors as shown in Figure 1. The regulation point can
) can be programmed by two exter-
OUT
be calculated as follows:
V
= 2.5 x (R1 + R2) / R2
OUT
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).
For applications where lower output voltage ripple is required
the output can be taken directly from a low ESR output capacitor, as shown in Figure 9. However, R3 slightly degrades
the load regulation.
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LM5008
20097905
FIGURE 9. Low Ripple Output Configuration
High Voltage Start-Up Regulator
The LM5008 contains an internal high voltage startup regulator. 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 reaches the under-voltage lockout threshold of 6.3V, the buck
switch is enabled.
In applications involving a high value for VIN, where power
dissipation in the VCC regulator is a concern, an auxiliary voltage 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.
FIGURE 10. Self Biased Configuration
Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5V
reference. In normal operation (the output voltage is regulated), 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 offtime of 300 ns. Bias current at the FB pin is nominally 100 nA.
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20097906
Over-Voltage Comparator
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.
LM5008
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), resulting in a nearly constant frequency as Vin is varied over its
range. The on-time equation for the LM5008 is:
TON = 1.25 x 10
See Figure 3. RON should be selected for a minimum on-time
(at maximum VIN) greater than 400 ns, for proper current limit
-10
x RON / V
IN
FIGURE 11. Shutdown Implementation
(2)
Current Limit
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:
T
= 10-5 / (0.285 + (VFB / 6.35 x 10-6 x RCL)) (3)
OFF
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.
N - Channel Buck Switch and Driver
The LM5008 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capacitor 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.
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 minimum time each cycle to recharge the bootstrap capacitor.
An external re-circulating diode (D1) carries the inductor current 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.
operation. This requirement limits the maximum frequency for
each application, depending on VIN and V
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.
20097907
. See Figure 4.
OUT
Thermal Protection
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 temperature 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 illustrated with a design example. Refer to Figure 1. The following
steps will configure the LM5008 for:
•
Input voltage range (Vin): 12V to 95V
•
Output voltage (V
•
Load current (for continuous conduction mode): 100 mA
to 300 mA
•
Maximum ripple at V
voltage
R1 and R2: From Figure 1, V
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 chosen. 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.
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 generally a compromise since it affects the size of L1 and C2, and
): 10V
OUT1
: 100 mVp-p at maximum input
OUT2
= VFB x (R1 + R2) / R2, and
OUT1
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the switching losses. The maximum allowed frequency,
based on a minimum on-time of 400 ns, is calculated from:
LM5008
F
MAX
= V
OUT
/ (V
INMAX
x 400ns)
For this exercise, Fmax = 263kHz. From equation 1, RON calculates to 304 kΩ. A standard value 357 kΩ resistor will be
used to allow for tolerances in equation 1, resulting in a frequency of 224kHz.
L1: The main parameter affected by the inductor is the output
current ripple amplitude. The choice of inductor value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum ripple current occurs
at maximum Vin.
a) Minimum load current: To maintain continuous conduction 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 I
calculates to 181 mA p-p at Vin = 95V, and 34 mA p-p at Vin
= 12V.
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 saturating, since the current limit is reached during startup.
OR
The DC resistance of the inductor should be as low as possible. 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.
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 transitions. For this reason, C3 should be no smaller than 0.1 µF.
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 operation. For the LM5008 the minimum ripple required at pin 5 is
25 mV p-p, requiring a minimum ripple at V
Since the minimum ripple current (at minimum Vin) is 34 mA
p-p, the minimum ESR required at V
2.94Ω. Since quality capacitors for SMPS applications have
OUT1
of 100 mV.
OUT1
is 100mV/34mA =
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.
b) Nature of the Load: The load can be connected to
V
or V
OUT1
ripple voltage which ranges from 100 mV (@ Vin = 12V) to
500mV (@Vin = 95V). Alternatively, V
ple, but lower regulation due to R3.
OUT2
. V
provides good regulation, but with a
OUT1
provides low rip-
OUT2
For a maximum allowed ripple voltage of 100 mVp-p at
V
(@ Vin = 95V), assume an ESR of 0.4Ω for C2. At
OUT2
maximum Vin, the ripple current is 181 mAp-p, creating a ripple 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.
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 increase to the peak (391 mA), and then decreases to 300 mA
half way through the off-time. The average value of this portion of the waveform is 45.5mA, and will cause half of the
voltage ripple, or 14 mV. The interval is one half of the frequency cycle time, or 2.23 µs. Using the capacitor’s basic
equation:
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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20097926
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 capacitor 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 optimum values for R3 and C2.
LM5008
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 offtime 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 increased to allow for:
The response time of the current limit detection loop
(400ns),
The off-time determined by equation 3 has a ±25% tolerance,
t
OFFCL(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.
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, although that affects efficiency. A good choice is an ultrafast
power diode, such as the MURA110T3 from ON Semiconductor. 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 maximum current limit threshold (610 mA).
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 current, 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 exercise), C1 calculates to:
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 appropriate in the majority of applications. A high quality ceramic
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 startup, 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.
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.
FINAL CIRCUIT
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 bootstrap 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 optimum 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 direct as possible.
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 dissipate 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 temperatures.
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LM5008
FIGURE 13. LM5008 Example Circuit
Bill of Materials (Circuit of Figure 13)
Item Description Part Number Value
C1 Ceramic Capacitor TDK C4532X7R2A105M 1µF, 100V
C2 Ceramic Capacitor TDK C4532X7R1E156M 15µF, 25V
C3 Ceramic Capacitor Kemet C1206C104K5RAC 0.1µF, 50V
C4 Ceramic Capacitor Kemet C1206C103K5RAC 0.01µF, 50V
C5 Ceramic Capacitor TDK C3216X7R2A104M 0.1µF, 100V
D1 UltraFast Power Diode ON Semi MURA110T3 100V, 1A
L1 Power Inductor Coilcraft DO3316-224 or 220 µH
TDK SLF10145T-221MR65
R1 Resistor Vishay CRCW12063011F
R2 Resistor Vishay CRCW12061001F
R3 Resistor Vishay CRCW12062R00F
R
ON
R
CL
U1 Switching Regulator National Semiconductor
Resistor Vishay CRCW12063573F
Resistor Vishay CRCW12062673F
LM5008
20097922
3.01 kΩ
1.0 kΩ
2.0 Ω
357 kΩ
267 kΩ
www.national.com 12
Physical Dimensions inches (millimeters) unless otherwise noted
LM5008
8-Lead MSOP Package
NS Package Number MUA08A
8-Lead LLP Package
NS Package Number SDC08A
13 www.national.com
LM5008
8-Lead LLP Package
NS Package Number SDC08B
www.national.com 14
Notes
LM5008
15 www.national.com
Notes
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