LM2524DMDC [NSC]

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LM2524DMDC
型号: LM2524DMDC
厂家: National Semiconductor    National Semiconductor
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March 2005  
LM2524D/LM3524D  
Regulating Pulse Width Modulator  
one pulse per period even in noisy environments. The  
LM3524D includes double pulse suppression logic that in-  
sures when a shut-down condition is removed the state of  
the T-flip-flop will change only after the first clock pulse has  
arrived. This feature prevents the same output from being  
pulsed twice in a row, thus reducing the possibility of core  
saturation in push-pull designs.  
General Description  
The LM3524D family is an improved version of the industry  
standard LM3524. It has improved specifications and addi-  
tional features yet is pin for pin compatible with existing 3524  
families. New features reduce the need for additional exter-  
nal circuitry often required in the original version.  
The LM3524D has a 1% precision 5V reference. The cur-  
rent carrying capability of the output drive transistors has  
been raised to 200 mA while reducing VCEsat and increasing  
VCE breakdown to 60V. The common mode voltage range of  
the error-amp has been raised to 5.5V to eliminate the need  
for a resistive divider from the 5V reference.  
Features  
n Fully interchangeable with standard LM3524 family  
n
1% precision 5V reference with thermal shut-down  
n Output current to 200 mA DC  
n 60V output capability  
In the LM3524D the circuit bias line has been isolated from  
the shut-down pin. This prevents the oscillator pulse ampli-  
tude and frequency from being disturbed by shut-down. Also  
at high frequencies (.300 kHz) the max. duty cycle per  
output has been improved to 44% compared to 35% max.  
duty cycle in other 3524s.  
n Wide common mode input range for error-amp  
n One pulse per period (noise suppression)  
n Improved max. duty cycle at high frequencies  
n Double pulse suppression  
n Synchronize through pin 3  
In addition, the LM3524D can now be synchronized exter-  
nally, through pin 3. Also a latch has been added to insure  
Connection Diagram  
00865002  
Top View  
Order Number LM2524DN or LM3524DN  
See NS Package Number N16E  
Order Number LM3524DM  
See NS Package Number M16A  
© 2005 National Semiconductor Corporation  
DS008650  
www.national.com  
Block Diagram  
00865001  
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2
Absolute Maximum Ratings (Note 5)  
Internal Power Dissipation  
Operating Junction Temperature  
Range (Note 2)  
1W  
If Military/Aerospace specified devices are required,  
please contact the National Semiconductor Sales Office/  
Distributors for availability and specifications.  
LM2524D  
−40˚C to +125˚C  
0˚C to +125˚C  
150˚  
Supply Voltage  
40V  
LM3524D  
Collector Supply Voltage  
(LM2524D)  
Maximum Junction Temperature  
Storage Temperature Range  
Lead Temperature (Soldering 4 sec.)  
M, N Pkg.  
55V  
40V  
−65˚C to +150˚C  
(LM3524D)  
Output Current DC (each)  
Oscillator Charging Current (Pin 7)  
200 mA  
5 mA  
260˚C  
Electrical Characteristics  
(Note 1)  
LM2524D  
Tested  
Limit  
LM3524D  
Symbol  
Parameter  
Conditions  
Design  
Limit  
Tested  
Limit  
Design  
Limit  
Units  
Typ  
Typ  
(Note 3) (Note 4)  
(Note 3) (Note 4)  
REFERENCE SECTION  
VREF  
Output Voltage  
5
4.85  
5.15  
15  
4.80  
5.20  
30  
5
4.75  
5.25  
VMin  
VMax  
mVMax  
mVMax  
dB  
VRLine  
VRLoad  
Line Regulation  
Load Regulation  
Ripple Rejection  
VIN = 8V to 40V  
10  
10  
66  
10  
10  
66  
25  
25  
50  
50  
IL = 0 mA to 20 mA  
f = 120 Hz  
15  
25  
IOS  
Short Circuit  
Current  
VREF = 0  
25  
25  
mA Min  
50  
50  
180  
200  
mA Max  
µVrms Max  
mV/kHr  
NO  
Output Noise  
Long Term  
Stability  
10 Hz f 10 kHz  
40  
20  
100  
500  
40  
20  
100  
TA = 125˚C  
OSCILLATOR SECTION  
fOSC  
Max. Freq.  
RT = 1k, CT = 0.001 µF  
(Note 7)  
550  
20  
350  
20  
kHzMin  
kHzMin  
fOSC  
Initial  
RT = 5.6k, CT = 0.01 µF  
(Note 7)  
17.5  
17.5  
Accuracy  
22.5  
34  
22.5  
30  
kHzMax  
kHzMin  
RT = 2.7k, CT = 0.01 µF  
(Note 7)  
38  
38  
42  
1
46  
kHzMax  
fOSC  
fOSC  
Freq. Change  
with VIN  
VIN = 8 to 40V  
0.5  
0.5  
1.0  
%
Max  
Freq. Change  
with Temp.  
TA = −55˚C to +125˚C  
at 20 kHz RT = 5.6k,  
CT = 0.01 µF  
5
5
3
%
VOSC  
tPW  
Output Amplitude  
(Pin 3) (Note 8)  
Output Pulse  
Width (Pin 3)  
Sawtooth Peak  
Voltage  
RT = 5.6k, CT = 0.01 µF  
3
2.4  
1.5  
3.6  
2.4  
1.5  
3.8  
VMin  
µsMax  
VMax  
RT = 5.6k, CT = 0.01 µF  
RT = 5.6k, CT = 0.01 µF  
0.5  
3.4  
0.5  
3.8  
3
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Electrical Characteristics (Continued)  
(Note 1)  
LM2524D  
Tested  
Limit  
LM3524D  
Tested  
Limit  
Symbol  
Parameter  
Conditions  
Design  
Limit  
Design  
Limit  
Units  
Typ  
Typ  
(Note 3) (Note 4)  
(Note 3) (Note 4)  
0.6  
Sawtooth Valley  
Voltage  
RT = 5.6k, CT = 0.01 µF  
1.1  
0.8  
0.6  
VMin  
ERROR-AMP SECTION  
VIO  
Input Offset  
Voltage  
VCM = 2.5V  
2
1
8
8
10  
10  
1
2
1
10  
10  
1
mVMax  
µAMax  
µAMax  
µAMin  
IIB  
Input Bias  
Current  
VCM = 2.5V  
IIO  
Input Offset  
Current  
VCM = 2.5V  
0.5  
1.0  
65  
0.5  
ICOSI  
Compensation  
Current (Sink)  
VIN(I) − VIN(NI) = 150 mV  
65  
95  
95  
125  
125  
µAMax  
µAMin  
ICOSO  
Compensation  
VIN(NI) − VIN(I) = 150 mV  
−125  
−125  
Current (Source)  
−95  
80  
−95  
80  
−65  
74  
−65  
µAMax  
dBMin  
VMin  
AVOL  
Open Loop Gain  
Common Mode  
Input Voltage  
Range  
RL  
=
, VCM = 2.5 V  
60  
1.4  
5.4  
70  
1.5  
5.5  
60  
VCMR  
1.5  
5.5  
VMax  
CMRR  
GBW  
Common Mode  
Rejection Ratio  
Unity Gain  
90  
3
80  
90  
2
80  
dBMin  
MHz  
AVOL = 0 dB, VCM = 2.5V  
Bandwidth  
VO  
Output Voltage  
Swing  
RL  
=
0.5  
5.5  
0.5  
5.5  
65  
VMin  
VMax  
dbMin  
PSRR  
Power Supply  
Rejection Ratio  
VIN = 8 to 40V  
80  
70  
80  
COMPARATOR SECTION  
Minimum Duty  
Pin 9 = 0.8V,  
0
0
0
0
%
Max  
Cycle  
[RT = 5.6k, CT = 0.01 µF]  
Pin 9 = 3.9V,  
Maximum Duty  
Cycle  
49  
44  
45  
35  
49  
44  
45  
35  
%
Min  
[RT = 5.6k, CT = 0.01 µF]  
Pin 9 = 3.9V,  
Maximum Duty  
Cycle  
%
Min  
[RT = 1k, CT = 0.001 µF]  
Zero Duty Cycle  
VCOMPZ  
VCOMPM  
IIB  
Input Threshold  
(Pin 9)  
1
1
V
V
Input Threshold  
(Pin 9)  
Maximum Duty Cycle  
3.5  
−1  
3.5  
−1  
Input Bias  
Current  
µA  
CURRENT LIMIT SECTION  
VSEN  
Sense Voltage  
V(Pin 2) − V(Pin 1)  
150 mV  
180  
220  
180  
220  
mVMin  
mVMax  
200  
4
200  
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Electrical Characteristics (Continued)  
(Note 1)  
LM2524D  
Tested  
Limit  
LM3524D  
Tested  
Limit  
Symbol  
Parameter  
Conditions  
Design  
Limit  
Design  
Limit  
Units  
Typ  
Typ  
(Note 3) (Note 4)  
(Note 3) (Note 4)  
TC-Vsense  
Sense Voltage T.C.  
Common Mode  
Voltage Range  
0.2  
−0.7  
1
0.2  
−0.7  
1
mV/˚C  
VMin  
V5 − V4 = 300 mV  
VMax  
SHUT DOWN SECTION  
VSD  
High Input  
Voltage  
V(Pin 2) − V(Pin 1)  
150 mV  
1
1
0.5  
1.5  
1
1
0.5  
1.5  
VMin  
VMax  
mA  
ISD  
High Input  
Current  
I(pin 10)  
OUTPUT SECTION (EACH OUTPUT)  
VCES  
Collector Emitter  
IC 100 µA  
55  
50  
40  
VMin  
Voltage Breakdown  
ICES  
Collector Leakage VCE = 60V  
Current  
VCE = 55V  
VCE = 40V  
IE = 20 mA  
IE = 200 mA  
IE = 50 mA  
0.1  
µAMax  
VMax  
VMin  
0.1  
0.2  
1.5  
18  
50  
0.7  
2.5  
17  
VCESAT  
VEO  
tR  
Saturation  
Voltage  
0.2  
1.5  
18  
0.5  
2.2  
17  
Emitter Output  
Voltage  
Rise Time  
VIN = 20V,  
IE = −250 µA  
RC = 2k  
200  
100  
200  
100  
ns  
ns  
tF  
Fall Time  
RC = 2k  
SUPPLY CHARACTERISTICS SECTION  
VIN  
T
Input Voltage  
Range  
After Turn-on  
8
8
VMin  
VMax  
˚C  
40  
40  
Thermal Shutdown (Note 2)  
Temp.  
160  
5
160  
5
IIN  
Stand By Current  
VIN = 40V (Note 6)  
10  
10  
mA  
Note 1: Unless otherwise stated, these specifications apply for T = T = 25˚C. Boldface numbers apply over the rated temperature range: LM2524D is −40˚ to 85˚C  
A
J
and LM3524D is 0˚C to 70˚C. V = 20V and f  
= 20 kHz.  
IN  
OSC  
Note 2: For operation at elevated temperatures, devices in the N package must be derated based on a thermal resistance of 86˚C/W, junction to ambient. Devices  
in the M package must be derated at 125˚C/W, junction to ambient.  
Note 3: Tested limits are guaranteed and 100% tested in production.  
Note 4: Design limits are guaranteed (but not 100% production tested) over the indicated temperature and supply voltage range. These limits are not used to  
calculate outgoing quality level.  
Note 5: Absolute maximum ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating  
the device beyond its rated operating conditions.  
Note 6: Pins 1, 4, 7, 8, 11, and 14 are grounded; Pin 2 = 2V. All other inputs and outputs open.  
Note 7: The value of a C capacitor can vary with frequency. Careful selection of this capacitor must be made for high frequency operation. Polystyrene was used  
t
in this test. NPO ceramic or polypropylene can also be used.  
Note 8: OSC amplitude is measured open circuit. Available current is limited to 1 mA so care must be exercised to limit capacitive loading of fast pulses.  
5
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Typical Performance Characteristics  
Switching Transistor  
Peak Output Current  
vs Temperature  
Maximum Average Power  
Dissipation (N, M Packages)  
00865029  
00865028  
Maximum & Minimum  
Duty Cycle Threshold  
Voltage  
Output Transistor  
Saturation Voltage  
00865030  
00865031  
Output Transistor Emitter  
Voltage  
Reference Transistor  
Peak Output Current  
00865032  
00865033  
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6
Typical Performance Characteristics (Continued)  
Standby Current  
vs Voltage  
Standby Current  
vs Temperature  
00865034  
00865035  
Current Limit Sense Voltage  
00865036  
Test Circuit  
00865004  
7
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Functional Description  
INTERNAL VOLTAGE REGULATOR  
The LM3524D has an on-chip 5V, 50 mA, short circuit pro-  
tected voltage regulator. This voltage regulator provides a  
supply for all internal circuitry of the device and can be used  
as an external reference.  
For input voltages of less than 8V the 5V output should be  
shorted to pin 15, VIN, which disables the 5V regulator. With  
these pins shorted the input voltage must be limited to a  
maximum of 6V. If input voltages of 6V–8V are to be used, a  
pre-regulator, as shown in Figure 1, must be added.  
00865005  
FIGURE 2.  
00865010  
*Minimum C of 10 µF required for stability.  
O
FIGURE 1.  
OSCILLATOR  
The LM3524D provides a stable on-board oscillator. Its fre-  
quency is set by an external resistor, RT and capacitor, CT. A  
graph of RT, CT vs oscillator frequency is shown is Figure 2.  
The oscillator’s output provides the signals for triggering an  
internal flip-flop, which directs the PWM information to the  
outputs, and a blanking pulse to turn off both outputs during  
transitions to ensure that cross conduction does not occur.  
The width of the blanking pulse, or dead time, is controlled  
by the value of CT, as shown in Figure 3. The recommended  
values of RT are 1.8 kto 100 k, and for CT, 0.001 µF to  
0.1 µF.  
00865006  
FIGURE 3.  
ERROR AMPLIFIER  
If two or more LM3524D’s must be synchronized together,  
the easiest method is to interconnect all pin 3 terminals, tie  
all pin 7’s (together) to a single CT, and leave all pin 6’s open  
except one which is connected to a single RT. This method  
works well unless the LM3524D’s are more than 6" apart.  
The error amplifier is a differential input, transconductance  
amplifier. Its gain, nominally 86 dB, is set by either feedback  
or output loading. This output loading can be done with  
either purely resistive or a combination of resistive and re-  
active components. A graph of the amplifier’s gain vs output  
load resistance is shown in Figure 4.  
A second synchronization method is appropriate for any  
circuit layout. One LM3524D, designated as master, must  
have its RTCT set for the correct period. The other slave  
LM3524D(s) should each have an RTCT set for a 10% longer  
period. All pin 3’s must then be interconnected to allow the  
master to properly reset the slave units.  
The oscillator may be synchronized to an external clock  
source by setting the internal free-running oscillator fre-  
quency 10% slower than the external clock and driving pin 3  
with a pulse train (approx. 3V) from the clock. Pulse width  
should be greater than 50 ns to insure full synchronization.  
00865007  
FIGURE 4.  
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8
CURRENT LIMITING  
Functional Description (Continued)  
The function of the current limit amplifier is to override the  
error amplifier’s output and take control of the pulse width.  
The output duty cycle drops to about 25% when a current  
limit sense voltage of 200 mV is applied between the +CL  
and −CLsense terminals. Increasing the sense voltage ap-  
proximately 5% results in a 0% output duty cycle. Care  
should be taken to ensure the −0.7V to +1.0V input common-  
mode range is not exceeded.  
The output of the amplifier, or input to the pulse width modu-  
lator, can be overridden easily as its output impedance is  
very high (ZO . 5 M). For this reason a DC voltage can be  
applied to pin 9 which will override the error amplifier and  
force a particular duty cycle to the outputs. An example of  
this could be a non-regulating motor speed control where a  
variable voltage was applied to pin 9 to control motor speed.  
A graph of the output duty cycle vs the voltage on pin 9 is  
shown in Figure 5.  
In most applications, the current limit sense voltage is pro-  
duced by a current through a sense resistor. The accuracy of  
this measurement is limited by the accuracy of the sense  
resistor, and by a small offset current, typically 100 µA,  
flowing from +CL to −CL.  
The duty cycle is calculated as the percentage ratio of each  
output’s ON-time to the oscillator period. Paralleling the out-  
puts doubles the observed duty cycle.  
OUTPUT STAGES  
The outputs of the LM3524D are NPN transistors, capable of  
a maximum current of 200 mA. These transistors are driven  
180˚ out of phase and have non-committed open collectors  
and emitters as shown in Figure 6.  
00865009  
00865008  
FIGURE 6.  
FIGURE 5.  
The amplifier’s inputs have a common-mode input range of  
1.5V–5.5V. The on board regulator is useful for biasing the  
inputs to within this range.  
9
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Typical Applications  
00865011  
FIGURE 7. Positive Regulator, Step-Up Basic Configuration (IIN(MAX) = 80 mA)  
00865012  
FIGURE 8. Positive Regulator, Step-Up Boosted Current Configuration  
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10  
Typical Applications (Continued)  
00865013  
FIGURE 9. Positive Regulator, Step-Down Basic Configuration (IIN(MAX) = 80 mA)  
11  
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Typical Applications (Continued)  
00865014  
FIGURE 10. Positive Regulator, Step-Down Boosted Current Configuration  
00865015  
FIGURE 11. Boosted Current Polarity Inverter  
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12  
Typical Applications (Continued)  
BASIC SWITCHING REGULATOR THEORY  
AND APPLICATIONS  
The circuit works as follows: Q1 is used as a switch, which  
has ON and OFF times controlled by the pulse width modu-  
lator. When Q1 is ON, power is drawn from VIN and supplied  
to the load through L1; VA is at approximately VIN, D1 is  
reverse biased, and Co is charging. When Q1 turns OFF the  
inductor L1 will force VA negative to keep the current flowing  
in it, D1 will start conducting and the load current will flow  
through D1 and L1. The voltage at VAis smoothed by the L1,  
Co filter giving a clean DC output. The current flowing  
through L1 is equal to the nominal DC load current plus  
some IL which is due to the changing voltage across it. A  
good rule of thumb is to set ILP-P . 40% x Io.  
The basic circuit of a step-down switching regulator circuit is  
shown in Figure 12, along with a practical circuit design  
using the LM3524D in Figure 15.  
00865016  
FIGURE 12. Basic Step-Down Switching Regulator  
00865017  
FIGURE 13. Relation of Switch Timing to Inductor Current in Step-Down Regulator  
+
Neglecting VSAT, VD, and settling IL = IL  
;
ηMAX will be further decreased due to switching losses in  
Q1. For this reason Q1 should be selected to have the  
maximum possible fT, which implies very fast rise and fall  
times.  
where T = Total Period  
CALCULATING INDUCTOR L1  
The above shows the relation between VIN, Vo and duty  
cycle.  
as Q1 only conducts during tON  
.
Since IL+ = IL = 0.4Io  
Solving the above for L1  
The efficiency, η, of the circuit is:  
13  
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Typical Applications (Continued)  
where: L1 is in Henrys  
f is switching frequency in Hz  
Also, see LM1578 data sheet for graphical methods of in-  
ductor selection.  
CALCULATING OUTPUT FILTER CAPACITOR Co:  
00865019  
Figure 13 shows L1’s current with respect to Q1’s tON and  
tOFF times (VA is at the collector of Q1). This curent must  
flow to the load and Co. Co’s current will then be the differ-  
ence between IL, and Io.  
FIGURE 14. Inductor Current Slope in Step-Down  
Regulator  
Ico = IL − Io  
A complete step-down switching regulator schematic, using  
the LM3524D, is illustrated in Figure 15. Transistors Q1 and  
Q2 have been added to boost the output to 1A. The 5V  
regulator of the LM3524D has been divided in half to bias the  
error amplifier’s non-inverting input to within its common-  
mode range. Since each output transistor is on for half the  
period, actually 45%, they have been paralleled to allow  
longer possible duty cycle, up to 90%. This makes a lower  
possible input voltage. The output voltage is set by:  
From Figure 13 it can be seen that current will be flowing into  
Co for the second half of tON through the first half of tOFF, or  
a time, tON/2 + tOFF/2. The current flowing for this time is  
IL/4. The resulting Vc or Vo is described by:  
where VNI is the voltage at the error amplifier’s non-inverting  
input.  
Resistor R3 sets the current limit to:  
Figures 16, 17 and show a PC board layout and stuffing  
diagram for the 5V, 1A regulator of Figure 15. The regulator’s  
performance is listed in Table 1.  
For best regulation, the inductor’s current cannot be allowed  
to fall to zero. Some minimum load current Io, and thus  
inductor current, is required as shown below:  
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14  
Typical Applications (Continued)  
00865020  
*Mounted to Staver Heatsink No. V5-1.  
Q1 = BD344  
Q2 = 2N5023  
>
L1 = 40 turns No. 22 wire on Ferroxcube No. K300502 Torroid core.  
FIGURE 15. 5V, 1 Amp Step-Down Switching Regulator  
15  
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Typical Applications (Continued)  
TABLE 1.  
Parameter  
Conditions  
Typical  
Characteristics  
Output Voltage  
Switching Frequency  
Short Circuit  
VIN = 10V, Io = 1A  
VIN = 10V, Io = 1A  
VIN = 10V  
5V  
20 kHz  
1.3A  
Current Limit  
Load Regulation  
VIN = 10V  
3 mV  
6 mV  
Io = 0.2 − 1A  
VIN = 10 − 20V,  
Io = 1A  
Line Regulation  
Efficiency  
VIN = 10V, Io = 1A  
VIN = 10V, Io = 1A  
80%  
Output Ripple  
10 mVp-p  
00865021  
FIGURE 16. 5V, 1 Amp Switching Regulator, Foil Side  
00865022  
FIGURE 17. Stuffing Diagram, Component Side  
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16  
Typical Applications (Continued)  
THE STEP-UP SWITCHING REGULATOR  
Figure 18 shows the basic circuit for a step-up switching  
regulator. In this circuit Q1 is used as a switch to alternately  
apply VIN across inductor L1. During the time, tON, Q1 is ON  
and energy is drawn from VIN and stored in L1; D1 is reverse  
biased and Io is supplied from the charge stored in Co. When  
Q1 opens, tOFF, voltage V1 will rise positively to the point  
where D1 turns ON. The output current is now supplied  
through L1, D1 to the load and any charge lost from Co  
during tON is replenished. Here also, as in the step-down  
regulator, the current through L1 has a DC component plus  
some IL. IL is again selected to be approximately 40% of  
IL. Figure 19 shows the inductor’s current in relation to Q1’s  
ON and OFF times.  
00865023  
FIGURE 18. Basic Step-Up Switching Regulator  
00865024  
FIGURE 19. Relation of Switch Timing to Inductor Current in Step-Up Regulator  
In calculating input current IIN(DC), which equals the induc-  
tor’s DC current, assume first 100% efficiency:  
Since IL+ = IL−, VIN ON  
t
= VotOFF − VINtOFF,  
and neglecting VSAT and VD1  
for η = 100%, POUT = PIN  
The above equation shows the relationship between VIN, Vo  
and duty cycle.  
17  
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Typical Applications (Continued)  
This equation shows that the input, or inductor, current is  
larger than the output current by the factor (1 + tON/tOFF).  
Since this factor is the same as the relation between Vo and  
VIN, IIN(DC) can also be expressed as:  
So far it is assumed η = 100%, where the actual efficiency or  
ηMAX will be somewhat less due to the saturation voltage of  
Q1 and forward on voltage of D1. The internal power loss  
This equation assumes only DC losses, however ηMAX is  
further decreased because of the switching time of Q1 and  
D1.  
due to these voltages is the average IL current flowing, or IIN  
,
through either VSAT or VD1. For VSAT = VD1 = 1V this power  
loss becomes IIN(DC) (1V). ηMAX is then:  
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18  
The network D1, C1 forms a slow start circuit.  
Typical Applications (Continued)  
This holds the output of the error amplifier initially low thus  
reducing the duty-cycle to a minimum. Without the slow start  
circuit the inductor may saturate at turn-on because it has to  
supply high peak currents to charge the output capacitor  
from 0V. It should also be noted that this circuit has no  
supply rejection. By adding a reference voltage at the non-  
inverting input to the error amplifier, see Figure 21, the input  
voltage variations are rejected.  
In calculating the output capacitor Co it can be seen that Co  
supplies Io during tON. The voltage change on Co during this  
time will be some Vc = Vo or the output ripple of the  
regulator. Calculation of Co is:  
The LM3524D can also be used in inductorless switching  
regulators. Figure 22 shows a polarity inverter which if con-  
nected to Figure 20 provides a −15V unregulated output.  
where: Co is in farads, f is the switching frequency,  
Vo is the p-p output ripple  
Calculation of inductor L1 is as follows:  
VIN is applied across L1  
where: L1 is in henrys, f is the switching frequency in Hz  
To apply the above theory, a complete step-up switching  
regulator is shown in Figure 20. Since VIN is 5V, VREF is tied  
to VIN. The input voltage is divided by 2 to bias the error  
amplifier’s inverting input. The output voltage is:  
19  
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Typical Applications (Continued)  
00865025  
>
L1 = 25 turns No. 24 wire on Ferroxcube No. K300502 Toroid core.  
FIGURE 20. 15V, 0.5A Step-Up Switching Regulator  
00865026  
FIGURE 21. Replacing R3/R4 Divider in Figure 20 with Reference Circuit Improves Line Regulation  
00865027  
FIGURE 22. Polarity Inverter Provides Auxiliary −15V Unregulated Output from Circuit of Figure 20  
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20  
Physical Dimensions inches (millimeters)  
unless otherwise noted  
Molded Surface-Mount Package (M)  
Order Number LM3524DM  
NS Package Number M16A  
21  
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)  
Molded Dual-In-Line Package (N)  
Order Number LM2524DN or LM3524DN  
NS Package Number N16E  
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.  
For the most current product information visit us at www.national.com.  
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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.  
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