LM5000SDX-6EP [NSC]

High Voltage Switch Mode Regulator; 高压开关模式稳压器
LM5000SDX-6EP
型号: LM5000SDX-6EP
厂家: National Semiconductor    National Semiconductor
描述:

High Voltage Switch Mode Regulator
高压开关模式稳压器

稳压器 开关式稳压器或控制器 电源电路 开关式控制器 光电二极管 高压
文件: 总18页 (文件大小:434K)
中文:  中文翻译
下载:  下载PDF数据表文档文件
May 2004  
LM5000EP  
Enhanced Plastic High Voltage Switch Mode Regulator  
General Description  
Features  
n 80V internal switch  
The LM5000EP is a monolithic integrated circuit specifically  
designed and optimized for flyback, boost or forward power  
converter applications. The internal power switch is rated for  
a maximum of 80V, with a current limit set to 2A. Protecting  
the power switch are current limit and thermal shutdown  
circuits. The current mode control scheme provides excellent  
rejection of line transients and cycle-by-cycle current limiting.  
An external compensation pin and the built-in slope compen-  
sation allow the user to optimize the frequency compensa-  
tion. Other distinctive features include softstart to reduce  
stresses during start-up and an external shutdown pin for  
remote ON/OFF control. There are two operating frequency  
ranges available. The LM5000-3EP is pin selectable for  
either 300kHz (FS Grounded) or 700kHz (FS Open). The  
LM5000-6EP is pin selectable for either 600kHz (FS  
Grounded) or 1.3MHz (FS Open). The device is available in  
a low profile 16-lead TSSOP package or a thermally en-  
hanced 16-lead LLP package.  
n Operating input voltage range of 3.1V to 40V  
n Pin selectable operating frequency  
300kHz/700kHz (-3)  
600kHz/1.3MHz (-6)  
n Adjustable output voltage  
n External compensation  
n Input undervoltage lockout  
n Softstart  
n Current limit  
n Over temperature protection  
n External shutdown  
n Small 16-Lead TSSOP or 16-Lead LLP package  
Applications  
n Flyback Regulator  
n Forward Regulator  
n Boost Regulator  
n Distributed Power Converters  
n Selected Military Applications  
n Selected Avionics Applications  
ENHANCED PLASTIC  
Extended Temperature Performance of −40˚C to +125˚C  
Baseline Control - Single Fab & Assembly Site  
Process Change Notification (PCN)  
Qualification & Reliability Data  
Solder (PbSn) Lead Finish is standard  
Enhanced Diminishing Manufacturing Sources (DMS)  
Support  
Ordering Information  
PART NUMBER  
LM5000-3MTCEP  
(Notes 1, 2)  
VID PART NUMBER  
V62/04632-01  
TBD  
NS PACKAGE NUMBER (Note 3)  
MTC16  
TBD  
Note 1: For the following (Enhanced Plastic) version, check for availability: LM5000-3MTCXEP, LM5000SD-3EP, LM5000SD-6EP, LM5000SDX-3EP,  
LM5000SDX-6EP. Parts listed with an "X" are provided in Tape & Reel and parts without an "X" are in Rails.  
Note 2: FOR ADDITIONAL ORDERING AND PRODUCT INFORMATION, PLEASE VISIT THE ENHANCED PLASTIC WEB SITE AT: www.national.com/  
mil  
Note 3: Refer to package details under Physical Dimensions  
© 2004 National Semiconductor Corporation  
DS200983  
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Typical Application Circuit  
20098301  
LM5000EP Flyback Converter  
Connection Diagram  
Top View  
20098304  
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2
Pin Description  
Pin  
Name  
Function  
Compensation network connection. Connected to the output of the voltage error amplifier.  
The RC compenstion network should be connected from this pin to AGND. An additional  
100pF high frequency capacitor to AGND is recommended.  
Output voltage feedback input.  
1
COMP  
2
3
FB  
SHDN  
AGND  
PGND  
PGND  
PGND  
PGND  
SW  
Shutdown control input, Open = enable, Ground = disable.  
Analog ground, connect directly to PGND.  
4
5
Power ground.  
6
Power ground.  
7
Power ground.  
8
Power ground.  
9
Power switch input. Switch connected between SW pins and PGND pins  
Power switch input. Switch connected between SW pins and PGND pins  
Power switch input. Switch connected between SW pins and PGND pins  
Bypass-Decouple Capacitor Connection, 0.1µF ceramic capacitor recommended.  
Analog power input. A small RC filter is recommended, to suppress line glitches. Typical  
values of 10and 0.1µF are recommended.  
10  
11  
12  
13  
SW  
SW  
BYP  
VIN  
14  
15  
16  
SS  
FS  
Softstart Input. External capacitor and internal current source sets the softstart time.  
Switching frequency select input. Open = Fhigh. Ground = Flow  
Factory test pin, connect to ground.  
TEST  
3
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Absolute Maximum Ratings (Note 4)  
If Military/Aerospace specified devices are required,  
please contact the National Semiconductor Sales Office/  
Distributors for availability and specifications.  
Infrared (15 sec.)  
ESD Susceptibility (Note 6)  
Human Body Model  
Machine Model  
235˚C  
2kV  
200V  
VIN  
-0.3V to 40V  
-0.3V to 80V  
-0.3V to 5V  
-0.3V to 3V  
-0.3V to 7V  
150˚C  
Storage Temperature  
−65˚C to +150˚C  
SW Voltage  
FB Voltage  
Operating Conditions  
Operating Junction  
Temperature Range  
(Note 10)  
COMP Voltage  
All Other Pins  
Maximum Junction Temperature  
Power Dissipation(Note 5)  
Lead Temperature  
−40˚C to +125˚C  
3.1V to 40V  
Internally Limited  
216˚C  
Supply Voltage (Note 10)  
Electrical Characteristics  
Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Tempera-  
ture Range (TJ = −40˚C to +125˚C) Unless otherwise specified. VIN = 12V and IL = 0A, unless otherwise specified.  
Min  
(Note 7)  
Typ  
(Note 8)  
Max  
(Note 7)  
Symbol  
Parameter  
Conditions  
Units  
mA  
IQ  
Quiescent Current  
FB = 2V (Not Switching)  
FS = 0V  
2.0  
2.1  
2.5  
2.5  
FB = 2V (Not Switching)  
FS = Open  
mA  
VSHDN = 0V  
18  
30  
1.2840  
2.7  
µA  
V
VFB  
ICL  
Feedback Voltage  
1.2330  
1.35  
1.259  
2.0  
Switch Current Limit  
A
%VFB/VIN Feedback Voltage Line  
3.1V VIN 40V  
0.001  
0.04  
%/V  
Regulation  
IB  
FB Pin Bias Current (Note 9)  
Output Switch Breakdown  
Voltage  
55  
200  
nA  
V
BV  
TJ = 25˚C, ISW = 0.1µA  
TJ = -40˚C to + 125˚C, ISW  
0.5µA  
80  
=
76  
VIN  
gm  
Input Voltage Range  
3.1  
40  
V
µmho  
V/V  
%
Error Amp Transconductance I = 5µA  
150  
410  
280  
90  
750  
AV  
Error Amp Voltage Gain  
DMAX  
Maximum Duty Cycle  
LM5000-3EP  
FS = 0V  
FS = 0V  
85  
85  
Maximum Duty Cycle  
LM5000-6EP  
90  
%
TMIN  
fS  
Minimum On Time  
Switching Frequency  
LM5000-3EP  
165  
300  
700  
600  
1.3  
ns  
FS = 0V  
240  
550  
360  
840  
715  
1.545  
-2  
kHz  
FS = Open  
FS = 0V  
Switching Frequency  
LM5000-6EP  
485  
FS = Open  
VSHDN = 0V  
VSW = 80V  
ISW = 1A  
1.055  
MHz  
µA  
µA  
mΩ  
V
ISHDN  
IL  
Shutdown Pin Current  
Switch Leakage Current  
Switch RDSON  
−1  
0.008  
160  
0.6  
5
RDSON  
ThSHDN  
445  
SHDN Threshold  
Output High  
Output Low  
0.9  
0.6  
0.3  
V
UVLO  
On Threshold  
Off Threshold  
VCOMP Trip  
2.74  
2.60  
2.92  
2. 77  
0.67  
11  
3.10  
2.96  
V
V
OVP  
ISS  
V
Softstart Current  
8
14  
µA  
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4
Electrical Characteristics (Continued)  
Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Tempera-  
ture Range (TJ = −40˚C to +125˚C) Unless otherwise specified. VIN = 12V and IL = 0A, unless otherwise specified.  
Min  
(Note 7)  
Typ  
(Note 8)  
150  
Max  
(Note 7)  
Symbol  
θJA  
Parameter  
Conditions  
Units  
Thermal Resistance  
TSSOP, Package only  
LLP, Package only  
˚C/W  
45  
Note 4: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to  
be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.  
Note 5: The maximum allowable power dissipation is a function of the maximum junction temperature, T (MAX), the junction-to-ambient thermal resistance, θ  
,
JA  
J
and the ambient temperature, T . See the Electrical Characteristics table for the thermal resistance of various layouts. The maximum allowable power dissipation  
A
at any ambient temperature is calculated using: P (MAX) = (T  
− T )/θ . Exceeding the maximum allowable power dissipation will cause excessive die  
D
J(MAX)  
A JA  
temperature, and the regulator will go into thermal shutdown.  
Note 6: The human body model is a 100 pF capacitor discharged through a 1.5kresistor into each pin. The machine model is a 200pF capacitor discharged  
directly into each pin.  
Note 7: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100%  
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to  
calculate Average Outgoing Quality Level (AOQL).  
Note 8: Typical numbers are at 25˚C and represent the most likely norm.  
Note 9: Bias current flows into FB pin.  
Note 10: Supply voltage, bias current product will result in aditional device power dissipation. This power may be significant. The thermal dissipation design should  
take this into account.  
5
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Typical Performance Characteristics  
@
@
Iq (non-switching) vs VIN fSW = 700kHz  
Iq (non-switching) vs VIN fSW = 300kHz  
20098320  
20098321  
@
@
Iq (switching) vs VIN fSW = 700kHz  
Iq (switching) vs VIN fSW = 300kHz  
20098322  
20098323  
@
RDS(ON) vs VIN ISW =1A  
Vfb vs Temperature  
20098324  
20098325  
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6
Typical Performance Characteristics (Continued)  
Current Limit vs Temperature  
Current Limit vs VIN  
20098326  
20098327  
@
@
fSW vs. VIN FS = OPEN (-3)  
fSW vs. VIN FS = Low (-3)  
20098328  
20098329  
@
@
fSW vs. Temperature FS = OPEN (-3)  
fSW vs. Temperature FS = Low (-3)  
20098331  
20098330  
7
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Typical Performance Characteristics (Continued)  
@
@
fSW vs. Temperature FS = OPEN (-6)  
fSW vs. Temperature FS = Low (-6)  
20098374  
20098375  
Error Amp. Transconductance vs Temp.  
BYP Pin Voltage vs VIN  
20098332  
20098333  
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8
20098353  
FIGURE 1. 300 kHz operation, 48V output  
20098354  
FIGURE 2. 700 kHz operation, 48V output  
9
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Block Diagram  
20098303  
The LM5000EP has dedicated protection circuitry running  
during the normal operation to protect the IC. The Thermal  
Shutdown circuitry turns off the NMOS power device when  
the die temperature reaches excessive levels. The UVP  
comparator protects the NMOS power device during supply  
power startup and shutdown to prevent operation at voltages  
less than the minimum input voltage. The OVP comparator is  
used to prevent the output voltage from rising at no loads  
allowing full PWM operation over all load conditions. The  
LM5000EP also features a shutdown mode. An external  
capacitor sets the softstart time by limiting the error amp  
output range, as the capacitor charges up via an internal  
10µA current source.  
Boost Regulator Operation  
The LM5000EP utilizes a PWM control scheme to regulate  
the output voltage over all load conditions. The operation  
can best be understood referring to the block diagram and  
Figure 3. At the start of each cycle, the oscillator sets the  
driver logic and turns on the NMOS power device conducting  
current through the inductor, cycle 1 of Figure 3 (a). During  
this cycle, the voltage at the COMP pin controls the peak  
inductor current. The COMP voltage will increase with larger  
loads and decrease with smaller. This voltage is compared  
with the summation of the SW volatge and the ramp com-  
pensation.The ramp compensation is used in PWM architec-  
tures to eliminate the sub-harmonic oscillations that occur  
during duty cycles greater than 50%. Once the summation of  
the ramp compensation and switch voltage equals the  
COMP voltage, the PWM comparator resets the driver logic  
turning off the NMOS power device. The inductor current  
then flows through the output diode to the load and output  
capacitor, cycle 2 of Figure 3 (b). The NMOS power device is  
then set by the oscillator at the end of the period and current  
flows through the inductor once again.  
The LM5000EP is available in two operating frequency  
ranges. The LM5000-3EP is pin selectable for either 300kHz  
(FS Grounded) or 700kHz (FS Open). The LM5000-6EP is  
pin selectable for either 600kHz (FS Grounded) or 1.3MHz  
(FS Open)  
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10  
Operation  
20098302  
FIGURE 3. Simplified Boost Converter Diagram  
(a) First Cycle of Operation (b) Second Cycle Of Operation  
CONTINUOUS CONDUCTION MODE  
The LM5000EP is a current-mode, PWM regulator. When  
used as a boost regulator the input voltage is stepped up to  
a higher output voltage. In continuous conduction mode  
(when the inductor current never reaches zero at steady  
state), the boost regulator operates in two cycles.  
INTRODUCTION TO COMPENSATION  
In the first cycle of operation, shown in Figure 3 (a), the  
transistor is closed and the diode is reverse biased. Energy  
is collected in the inductor and the load current is supplied by  
COUT  
.
The second cycle is shown in Figure 3 (b). During this cycle,  
the transistor is open and the diode is forward biased. The  
energy stored in the inductor is transferred to the load and  
output capacitor.  
The ratio of these two cycles determines the output voltage.  
The output voltage is defined approximately as:  
where D is the duty cycle of the switch, D and D' will be  
required for design calculations.  
SETTING THE OUTPUT VOLTAGE  
The output voltage is set using the feedback pin and a  
resistor divider connected to the output as shown in Figure 1.  
The feedback pin is always at 1.259V, so the ratio of the  
feedback resistors sets the output voltage.  
20098305  
FIGURE 4. (a) Inductor current. (b) Diode current.  
11  
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average inductor current (input current) plus iL. Care must  
be taken to make sure that the switch will not reach its  
current limit during normal operation. The inductor must also  
be sized accordingly. It should have a saturation current  
rating higher than the peak inductor current expected. The  
output voltage ripple is also affected by the total ripple cur-  
rent.  
Operation (Continued)  
The LM5000EP is a current mode PWM regulator. The signal  
flow of this control scheme has two feedback loops, one that  
senses switch current and one that senses output voltage.  
To keep a current programmed control converter stable  
above duty cycles of 50%, the inductor must meet certain  
criteria. The inductor, along with input and output voltage,  
will determine the slope of the current through the inductor  
(see Figure 4 (a)). If the slope of the inductor current is too  
great, the circuit will be unstable above duty cycles of 50%.  
DC GAIN AND OPEN-LOOP GAIN  
Since the control stage of the converter forms a complete  
feedback loop with the power components, it forms a closed-  
loop system that must be stabilized to avoid positive feed-  
back and instability. A value for open-loop DC gain will be  
required, from which you can calculate, or place, poles and  
zeros to determine the crossover frequency and the phase  
margin. A high phase margin (greater than 45˚) is desired for  
the best stability and transient response. For the purpose of  
stabilizing the LM5000EP, choosing a crossover point well  
below where the right half plane zero is located will ensure  
sufficient phase margin. A discussion of the right half plane  
zero and checking the crossover using the DC gain will  
follow.  
The LM5000EP provides a compensation pin (COMP) to  
customize the voltage loop feedback. It is recommended that  
a series combination of RC and CC be used for the compen-  
sation network, as shown in Figure 1. The series combina-  
tion of RC and CC introduces pole-zero pair according to the  
following equations:  
OUTPUT CAPACITOR SELECTION  
The choice of output capacitors is somewhat more arbitrary.  
It is recommended that low ESR (Equivalent Series Resis-  
tance, denoted RESR) capacitors be used such as ceramic,  
polymer electrolytic, or low ESR tantalum. Higher ESR ca-  
pacitors may be used but will require more compensation  
which will be explained later on in the section. The ESR is  
also important because it determines the output voltage  
ripple according to the approximate equation:  
where RO is the output impedance of the error amplifier,  
850k. For most applications, performance can be opti-  
mized by choosing values within the range 5kΩ ≤ RC 20kΩ  
and 680pF CC 4.7nF.  
COMPENSATION  
This section will present a general design procedure to help  
insure a stable and operational circuit. The designs in this  
datasheet are optimized for particular requirements. If differ-  
ent conversions are required, some of the components may  
need to be changed to ensure stability. Below is a set of  
general guidelines in designing a stable circuit for continu-  
ous conduction operation (loads greater than 100mA), in  
most all cases this will provide for stability during discontinu-  
ous operation as well. The power components and their  
effects will be determined first, then the compensation com-  
ponents will be chosen to produce stability.  
VOUT ) 2iLRESR (in Volts)  
After choosing the output capacitor you can determine a  
pole-zero pair introduced into the control loop by the follow-  
ing equations:  
INDUCTOR SELECTION  
To ensure stability at duty cycles above 50%, the inductor  
must have some minimum value determined by the mini-  
mum input voltage and the maximum output voltage. This  
equation is:  
Where RL is the minimum load resistance corresponding to  
the maximum load current. The zero created by the ESR of  
the output capacitor is generally very high frequency if the  
ESR is small. If low ESR capacitors are used it can be  
neglected. If higher ESR capacitors are used see the High  
Output Capacitor ESR Compensation section.  
RIGHT HALF PLANE ZERO  
A current mode control boost regulator has an inherent right  
half plane zero (RHP zero). This zero has the effect of a zero  
in the gain plot, causing an imposed +20dB/decade on the  
rolloff, but has the effect of a pole in the phase, subtracting  
another 90˚ in the phase plot. This can cause undesirable  
effects if the control loop is influenced by this zero. To ensure  
where fs is the switching frequency, D is the duty cycle, and  
RDSON is the ON resistance of the internal switch. This  
equation is only good for duty cycles greater than 50%  
>
(D 0.5).  
the RHP zero does not cause instability issues, the control  
1
loop should be designed to have a bandwidth of  
2 the  
frequency of the RHP zero or less. This zero occurs at a  
frequency of:  
The inductor ripple current is important for a few reasons.  
One reason is because the peak switch current will be the  
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12  
To ensure this equation is valid, and that CC2 can be used  
without negatively impacting the effects of RC and CC, fPC2  
Operation (Continued)  
must be greater than 10fPC  
.
CHECKING THE DESIGN  
The final step is to check the design. This is to ensure a  
1
where ILOAD is the maximum load current.  
bandwidth of  
2 or less of the frequency of the RHP zero.  
This is done by calculating the open-loop DC gain, ADC. After  
this value is known, you can calculate the crossover visually  
by placing a −20dB/decade slope at each pole, and a +20dB/  
decade slope for each zero. The point at which the gain plot  
SELECTING THE COMPENSATION COMPONENTS  
The first step in selecting the compensation components RC  
and CC is to set a dominant low frequency pole in the control  
loop. Simply choose values for RC and CC within the ranges  
given in the Introduction to Compensation section to set this  
pole in the area of 10Hz to 100Hz. The frequency of the pole  
created is determined by the equation:  
crosses unity gain, or 0dB, is the crossover frequency. If the  
1
crossover frequency is at less than  
2 the RHP zero, the  
phase margin should be high enough for stability. The phase  
margin can also be improved some by adding CC2 as dis-  
cussed earlier in the section. The equation for ADC is given  
below with additional equations required for the calculation:  
where RO is the output impedance of the error amplifier,  
850k. Since RC is generally much less than RO, it does not  
have much effect on the above equation and can be ne-  
glected until a value is chosen to set the zero fZC. fZC is  
created to cancel out the pole created by the output capaci-  
tor, fP1. The output capacitor pole will shift with different load  
currents as shown by the equation, so setting the zero is not  
exact. Determine the range of fP1 over the expected loads  
and then set the zero fZC to a point approximately in the  
middle. The frequency of this zero is determined by:  
mc ) 0.072fs (in A/s)  
Now RC can be chosen with the selected value for CC.  
Check to make sure that the pole fPC is still in the 10Hz to  
100Hz range, change each value slightly if needed to ensure  
both component values are in the recommended range. After  
checking the design at the end of this section, these values  
can be changed a little more to optimize performance if  
desired. This is best done in the lab on a bench, checking the  
load step response with different values until the ringing and  
overshoot on the output voltage at the edge of the load steps  
is minimal. This should produce a stable, high performance  
circuit. For improved transient response, higher values of RC  
(within the range of values) should be chosen. This will  
improve the overall bandwidth which makes the regulator  
respond more quickly to transients. If more detail is required,  
or the most optimal performance is desired, refer to a more  
in depth discussion of compensating current mode DC/DC  
switching regulators.  
where RL is the minimum load resistance, VIN is the maxi-  
mum input voltage, and RDSON is the value chosen from the  
graph "RDSON vs. VIN " in the Typical Performance Charac-  
teristics section.  
SWITCH VOLTAGE LIMITS  
In a flyback regulator, the maximum steady-state voltage  
appearing at the switch, when it is off, is set by the trans-  
former turns ratio, N, the output voltage, VOUT, and the  
maximum input voltage, VIN (Max):  
VSW(OFF) = VIN (Max) + (VOUT +VF)/N  
where VF is the forward biased voltage of the output diode,  
and is typically 0.5V for Schottky diodes and 0.8V for ultra-  
fast recovery diodes. In certain circuits, there exists a volt-  
age spike, VLL, superimposed on top of the steady-state  
voltage . Usually, this voltage spike is caused by the trans-  
former leakage inductance and/or the output rectifier recov-  
ery time. To clamp” the voltage at the switch from exceeding  
its maximum value, a transient suppressor in series with a  
diode is inserted across the transformer primary.  
HIGH OUTPUT CAPACITOR ESR COMPENSATION  
When using an output capacitor with a high ESR value, or  
just to improve the overall phase margin of the control loop,  
another pole may be introduced to cancel the zero created  
by the ESR. This is accomplished by adding another capaci-  
tor, CC2, directly from the compensation pin VC to ground, in  
parallel with the series combination of RC and CC. The pole  
should be placed at the same frequency as fZ1, the ESR  
zero. The equation for this pole follows:  
If poor circuit layout techniques are used, negative voltage  
transients may appear on the Switch pin. Applying a nega-  
tive voltage (with respect to the IC’s ground) to any mono-  
lithic IC pin causes erratic and unpredictable operation of  
that IC. This holds true for the LM5000EP IC as well. When  
used in a flyback regulator, the voltage at the Switch pin can  
go negative when the switch turns on. The “ringing” voltage  
13  
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tances and inductances in the LM5000EP switch, the output  
diode(s), and the transformersuch as reverse recovery  
time of the output diode (mentioned above).  
Operation (Continued)  
at the switch pin is caused by the output diode capacitance  
and the transformer leakage inductance forming a resonant  
circuit at the secondary(ies). The resonant circuit generates  
the “ringing” voltage, which gets reflected back through the  
transformer to the switch pin. There are two common meth-  
ods to avoid this problem. One is to add an RC snubber  
around the output rectifier(s). The values of the resistor and  
the capacitor must be chosen so that the voltage at the  
Switch pin does not drop below −0.4V. The resistor may  
range in value between 10and 1 k, and the capacitor will  
vary from 0.001 µF to 0.1 µF. Adding a snubber will (slightly)  
reduce the efficiency of the overall circuit.  
INPUT LINE CONDITIONING  
A small, low-pass RC filter should be used at the input pin of  
the LM5000EP if the input voltage has an unusually large  
amount of transient noise. Additionally, the RC filter can  
reduce the dissipation within the device when the input  
voltage is high.  
Flyback Regulator Operation  
The LM5000EP is ideally suited for use in the flyback regu-  
lator topology. The flyback regulator can produce a single  
output voltage, or multiple output voltages.  
The other method to reduce or eliminate the “ringing” is to  
insert a Schottky diode clamp between the SW pin and the  
PGND pin. The reverse voltage rating of the diode must be  
greater than the switch off voltage.  
The operation of a flyback regulator is as follows: When the  
switch is on, current flows through the primary winding of the  
transformer, T1, storing energy in the magnetic field of the  
transformer. Note that the primary and secondary windings  
are out of phase, so no current flows through the secondary  
when current flows through the primary. When the switch  
turns off, the magnetic field collapses, reversing the voltage  
polarity of the primary and secondary windings. Now rectifier  
D5 is forward biased and current flows through it, releasing  
the energy stored in the transformer. This produces voltage  
at the output.  
OUTPUT VOLTAGE LIMITATIONS  
The maximum output voltage of a boost regulator is the  
maximum switch voltage minus a diode drop. In a flyback  
regulator, the maximum output voltage is determined by the  
turns ratio, N, and the duty cycle, D, by the equation:  
VOUT N x VIN x D/(1 − D)  
The duty cycle of a flyback regulator is determined by the  
following equation:  
The output voltage is controlled by modulating the peak  
switch current. This is done by feeding back a portion of the  
output voltage to the error amp, which amplifies the differ-  
ence between the feedback voltage and a 1.259V reference.  
The error amp output voltage is compared to a ramp voltage  
proportional to the switch current (i.e., inductor current dur-  
ing the switch on time). The comparator terminates the  
switch on time when the two voltages are equal, thereby  
controlling the peak switch current to maintain a constant  
output voltage.  
Theoretically, the maximum output voltage can be as large  
as desiredjust keep increasing the turns ratio of the trans-  
former. However, there exists some physical limitations that  
prevent the turns ratio, and thus the output voltage, from  
increasing to infinity. The physical limitations are capaci-  
www.national.com  
14  
Flyback Regulator Operation (Continued)  
20098372  
FIGURE 5. LM5000EP Flyback Converter  
ITEM  
PART NUMBER  
C4532X7R2A105MT  
DESCRIPTION  
Capacitor, CER, TDK  
VALUE  
1µ, 100V  
C
C
C
C
C
C
C
C
C
C
C
1
2
C4532X7R2A105MT  
C1206C224K5RAC  
C1206C104K5RAC  
C1206C104K5RAC  
C1206C101K1GAC  
C1206C104K5RAC  
C4532X7S0G686M  
C4532X7S0G686M  
C1206C221K1GAC  
C1206C102K5RAC  
Capacitor, CER, TDK  
1µ, 100V  
3
Capacitor, CER, KEMET  
Capacitor, CER, KEMET  
Capacitor, CER, KEMET  
Capacitor, CER, KEMET  
Capacitor, CER, KEMET  
Capacitor, CER, TDK  
0.22µ, 50V  
0.1µ, 50V  
0.1µ, 50V  
100p, 100V  
0.1µ, 50V  
68µ, 4V  
4
5
6
7
8
9
Capacitor, CER, TDK  
68µ, 4V  
10  
11  
Capacitor, CER, KEMET  
Capacitor, CER, KEMET  
220p, 100V  
1000p, 500V  
D
D
D
D
D
1
2
3
4
5
BZX84C10-NSA  
CMZ5930B-NSA  
CMPD914-NSA  
CMPD914-NSA  
CMSH3-40L-NSA  
Central, 10V Zener, SOT-23  
Central, 16V Zener, SMA  
Central, Switching, SOT-23  
Central, Switching, SOT-23  
Central, Schottky, SMC  
15  
www.national.com  
ITEM  
PART NUMBER  
A0009-A  
DESCRIPTION  
Coilcraft, Transformer  
VALUE  
T
1
R
R
R
R
R
R
R
1
2
3
4
5
6
7
CRCW12064992F  
CRCW12061001F  
CRCW12061002F  
CRCW12066191F  
CRCW120610R0F  
CRCW12062003F  
CRCW12061002F  
Resistor  
Resistor  
Resistor  
Resistor  
Resistor  
Resistor  
Resistor  
49.9K  
1K  
10K  
6.19K  
10  
200K  
10K  
Q
U
1
1
CXT5551-NSA  
LM5000-3EP  
Central, NPN, 180V  
Regulator, National  
www.national.com  
16  
Physical Dimensions inches (millimeters)  
unless otherwise noted  
TSSOP-16 Pin Package (MTC)  
NS Package Number MTC16  
LLP-16 Pin Package (SDA)  
NS Package Number SDA16A  
17  
www.national.com  
Notes  
LIFE SUPPORT POLICY  
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT  
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL  
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:  
1. Life support devices or systems are devices or  
systems which, (a) are intended for surgical implant  
into the body, or (b) support or sustain life, and  
whose failure to perform when properly used in  
accordance with instructions for use provided in the  
labeling, can be reasonably expected to result in a  
significant injury to the user.  
2. A critical component is any component of a life  
support device or system whose failure to perform  
can be reasonably expected to cause the failure of  
the life support device or system, or to affect its  
safety or effectiveness.  
BANNED SUBSTANCE COMPLIANCE  
National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products  
Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification  
(CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2.  
National Semiconductor  
Americas Customer  
Support Center  
National Semiconductor  
Europe Customer Support Center  
Fax: +49 (0) 180-530 85 86  
National Semiconductor  
Asia Pacific Customer  
Support Center  
National Semiconductor  
Japan Customer Support Center  
Fax: 81-3-5639-7507  
Email: new.feedback@nsc.com  
Tel: 1-800-272-9959  
Email: europe.support@nsc.com  
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www.national.com  
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.  

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