The LM1578A is a switching regulator which can easily be
set up for such DC-to-DC voltage conversion circuits as the
buck, boost, and inverting configurations. The LM1578A
features a unique comparator input stage which not only
has separate pins for both the inverting and non-inverting
inputs, but also provides an internal 1.0V reference to each
input, thereby simplifying circuit design and p.c. board layout. The output can switch up to 750 mA and has output
pins for its collector and emitter to promote design flexibility.
An external current limit terminal may be referenced to either the ground or the V
plication. In addition, the LM1578A has an on board oscillator, which sets the switching frequency with a single external capacitor from
terminal, depending upon the ap-
in
k
1 Hz to 100 kHz (typical).
The LM1578A is an improved version of the LM1578, offering higher maximum ratings for the total supply voltage and
output transistor emitter and collector voltages.
Functional Diagram
Features
Y
Inverting and non-inverting feedback inputs
Y
1.0V reference at inputs
Y
Operates from supply voltages of 2V to 40V
Y
Output current up to 750 mA, saturation less than 0.9V
Y
Current limit and thermal shut down
Y
Duty cycle up to 90%
Applications
Y
Switching regulators in buck, boost, inverting, and
single-ended transformer configurations
Y
Motor speed control
Y
Lamp flasher
TL/H/8711– 1
C
1995 National Semiconductor CorporationRRD-B30M115/Printed in U. S. A.
TL/H/8711
Page 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Total Supply Voltage50V
Collector Output to Ground
Emitter Output to Ground (Note 2)
Power Dissipation (Note 3)Internally limited
Output Current750 mA
Storage Temperature
Lead Temperature
(soldering, 10 seconds)260
b
0.3V toa50V
b
1V toa50V
b
65§Ctoa150§C
Maximum Junction Temperature150
C
§
ESD Tolerance (Note 4)2 kV
Operating Ratings
Ambient Temperature Range
LM1578A
LM2578A
b
55§CsT
b
40§CsT
LM3578A0
Junction Temperature Range
LM1578A
C
§
LM2578A
LM3578A0
b
55§CsT
b
40§CsT
CsT
§
CsT
§
s
a
125§C
A
s
a
85§C
A
s
a
70§C
A
s
a
150§C
J
s
a
125§C
J
s
a
125§C
J
Electrical Characteristics
These specifications apply for 2VsV
s
duty cycles75%, unless otherwise specified. Values in standard typeface are for T
for operation over the specified operating junction temperature range.
Sense VoltageReferred to VINor Ground110mV
Shutdown Level(Note 10)9580mV (min)
140160mV (max)
DVCL/DTSense Voltage0.3%/§C
Temperature Drift
I
CL
Sense Bias CurrentReferred to V
Referred to ground0.4mA
IN
4.0mA
DEVICE POWER CONSUMPTION
I
S
Supply CurrentOutput OFF, V
Output ON, I
e
V
0V
Note 1: 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 2: For T
Note 3: At elevated temperatures, devices must be derated based on package thermal resistance. The device in the TO-99 package must be derated at 150
junction to ambient, or 45
package must be derated at 150
Note 4: Human body model, 1.5 kX in series with 100 pF.
Note 5: Typical values are for T
Note 6: All limits guaranteed and 100% production tested at room temperature (standard type face) and at temperature extremes (bold type face). All limits are
used to calculate Average Outgoing Quality Level (AOQL).
Note 7: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). Room temperature limits are 100%
production tested. Limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to
calculate AOQL.
Note 8: Input terminals are protected from accidental shorts to ground but if external voltages higher than the reference voltage are applied, excessive current will
flow and should be limited to less than 5 mA.
Note 9: I
Note 10: Connection of a 10 kX resistor from pin 1 to pin 4 will drive the duty cycle to its maximum, typically 90%. Applying the minimum Current Limit Sense
Voltage to pin 7 will not reduce the duty cycle to less than 50%. Applying the maximum Current Limit Sense Voltage to pin 7 is certain to reduce the duty cycle
below 50%. Increasing this voltage by 15 mV may be required to reduce the duty cycle to 0%, when the Collector output swing is 40V or greater (see Ground-Referred Current Limit Sense Voltage typical curve).
Note 11: A military RETS specification is available on request. At the time of printing, the LM1578A RETS spec complied with the boldface limits in this column.
The LM1578AH may also be procured as a Standard Military Drawing.
t
100§C, the Emitter pin voltage should not be driven more than 0.6V below ground (see Application Information).
J
C/W, junction to case. The device in the 8-pin DIP must be derated at 95§C/W, junction to ambient. The device in the surface-mount
§
C/W, junction-to-ambient.
§
e
J
and I2are the external sink currents at the inputs (refer to Test Circuit).
1
E
25§C and represent the most likely parametric norm.
e
0V2.0mA
E
e
750 mA pulsed,14mA
C
3.0/3.33.5/4.0mA (max)
LM2578A/
LM3578A
Limit
(Note 7)
Units
§
C/W,
Connection Diagram and Ordering Information
Metal Can
Order Number LM3578AM, LM2578AN or LM3578AN
Order Number LM1578AH/883 or SMD
Top View
See NS Package Number H08C
TL/H/8711– 28
Ý
5962-8958602
3
Dual-In-Line Package
TL/H/8711– 29
See NS Package Number M08A or N08E
Page 4
Typical Performance Characteristics
Oscillator Frequency Change
with Temperature
Collector Saturation Voltage
(Sinking Current,
Emitter Grounded)
Current Limit Sense Voltage
Drift with Temperature
Oscillator Voltage SwingDrift with Temperature
Emitter Saturation Voltage
(Sourcing Current,
Collector at V
Current Limit Response Time
for Various Over Drives
Parameter tests can be made using the test circuit shown.
Select the desired V
adjustable power supplies. A digital volt meter with an input
resistance greater than 100 MX should be used to measure
the following:
Input Reference Voltage to Ground; S1 in either position.
Level Shift Accuracy (%)
e
e
I
I
1
2
Input Current (mA)
e
I
0 mA.
2
Oscillator parameters can be measured at T
quency counter or an oscilloscope.
, collector voltage and duty cycle with
in
e
(TP3(V)/1V)c100%; S1 at
1mA
e
(1VbTp3(V))/1 MX:S1atI
using a fre-
p4
1
e
The Current Limit Sense Voltage is measured by connecting
an adjustable 0-to-1V floating power supply in series with
the current limit terminal and referring it to either the ground
or the V
test point T
age until the LM1578A’s duty cycle just reaches 0%. This
terminal. Set the duty cycle to 90% and monitor
in
while adjusting the floating power supply volt-
P5
voltage is the Current Limit Sense Voltage.
The Supply Current should be measured with the duty cycle
at 0% and S1 in the I
e
e
I
1
0 mA position.
2
*LM1578A specifications are measured using automated
test equipment. This circuit is provided for the customer’s
convenience when checking parameters. Due to possible
variations in testing conditions, the measured values from
these testing procedures may not match those of the
factory.
Note 1: Op amp supplies areg15V
Note 2: DVM input resistance
Note 3: *LM1578 max duty cycle is 90%
l
100 MX
Definition of Terms
Input Reference Voltage: The voltage (referred to ground)
that must be applied to either the inverting or non-inverting
input to cause the regulator switch to change state (ON or
OFF).
Input Reference Current: The current that must be drawn
from either the inverting or non-inverting input to cause the
regulator switch to change state (ON or OFF).
Input Level Shift Accuracy: This specification determines
the output voltage tolerance of a regulator whose output
control depends on drawing equal currents from the inverting and non-inverting inputs (see the Inverting Regulator of
Figure 21
ure 23
, and the RS-232 Line Driver Power Supply of
Fig-
).
Level Shift Accuracy is tested by using two equal-value resistors to draw current from the inverting and non-inverting
input terminals, then measuring the percentage difference in
the voltages across the resistors that produces a controlled
duty cycle at the switch output.
TL/H/8711– 3
Collector Saturation Voltage: With the inverting input terminal grounded thru a 10 kX resistor and the output transistor’s emitter connected to ground, the Collector SaturationVoltage is the collector-to-emitter voltage for a given collector current.
Emitter Saturation Voltage: With the inverting input terminal grounded thru a 10 kX resistor and the output transistor’s collector connected to V
age is the collector-to-emitter voltage for a given emitter
, the Emitter Saturation Volt-
in
current.
Collector Emitter Sustaining Voltage: The collector-emitter breakdown voltage of the output transistor, measured at
a specified current.
Current Limit Sense Voltage: The voltage at the Current
Limit pin, referred to either the supply or the ground terminal, which (via logic circuitry) will cause the output transistor
to turn OFF and resets cycle-by-cycle at the oscillator frequency.
5
Page 6
Definition of Terms (Continued)
Current Limit Sense Current: The bias current for the Cur-
rent Limit terminal with the applied voltage equal to the Current Limit Sense Voltage.
Supply Current: The IC power supply current, excluding the
current drawn through the output transistor, with the oscillator operating.
Functional Description
The LM1578A is a pulse-width modulator designed for use
as a switching regulator controller. It may also be used in
other applications which require controlled pulse-width voltage drive.
A control signal, usually representing output voltage, fed
into the LM1578A’s comparator is compared with an internally-generated reference. The resulting error signal and the
oscillator’s output are fed to a logic network which determines when the output transistor will be turned ON or OFF.
The following is a brief description of the subsections of the
LM1578A.
COMPARATOR INPUT STAGE
The LM1578A’s comparator input stage is unique in that
both the inverting and non-inverting inputs are available to
the user, and both contain a 1.0V reference. This is accomplished as follows: A 1.0V reference is fed into a modified
voltage follower circuit (see FUNCTIONAL DIAGRAM).
When both input pins are open, no current flows through R1
and R2. Thus, both inputs to the comparator will have the
potential of the 1.0V reference, V
example the non-inverting input, is pulled DV away from V
a current of DV/R1 will flow through R1. This same current
flows through R2, and the comparator sees a total voltage
of 2DV between its inputs. The high gain of the system,
through feedback, will correct for this imbalance and return
both inputs to the 1.0V level.
This unusual comparator input stage increases circuit flexibility, while minimizing the total number of external components required for a voltage regulator system. The inverting
switching regulator configuration, for example, can be set up
without having to use an external op amp for feedback polarity reversal (see TYPICAL APPLICATIONS).
OSCILLATOR
The LM1578A provides an on-board oscillator which can be
adjusted up to 100 kHz. Its frequency is set by a single
external capacitor, C
equation
, as shown in
1
e
f
OSC
The oscillator provides a blanking pulse to limit maximum
duty cycle to 90%, and a reset pulse to the internal circuitry.
. When one input, for
A
Figure 1
b
5
8c10
/C
, and follows the
1
OUTPUT TRANSISTOR
The output transistor is capable of delivering up to 750 mA
with a saturation voltage of less than 0.9V. (see
Saturation Voltage
The emitter must not be pulled more than 1V below ground
(this limit is 0.6V for T
external transistor must be used to develop negative output
and
Emitter Saturation Voltage
t
100§C). Because of this limit, an
J
voltages (see the Inverting Regulator Typical Application).
Other configurations may need protection against violation
of this limit (see the Emitter Output section of the Applications Information).
CURRENT LIMIT
The LM1578A’s current limit may be referenced to either
the ground or the V
basis.
pins, and operates on a cycle-by-cycle
in
The current limit section consists of two comparators: one
with its non-inverting input referenced to a voltage 110 mV
below V
110 mV above ground (see FUNCTIONAL DIAGRAM). The
, the other with its inverting input referenced
in
current limit is activated whenever the current limit terminal
is pulled 110 mV away from either V
in
Applications Information
CURRENT LIMIT
As mentioned in the functional description, the current limit
terminal may be referenced to either the V
terminal. Resistor R3 converts the current to be sensed into
a voltage for current limit detection.
,
A
FIGURE 2. Current Limit, Ground Referred
or ground.
or the ground
in
Collector
curves).
TL/H/8711– 15
FIGURE 1. Value of Timing Capacitor vs
Oscillator Frequency
FIGURE 3. Current Limit, VinReferred
TL/H/8711– 16
TL/H/8711– 4
6
Page 7
Applications Information (Continued)
CURRENT LIMIT TRANSIENT SUPPRESSION
When noise spikes and switching transients interfere with
proper current limit operation, R1 and C1 act together as a
low pass filter to control the current limit circuitry’s response
time.
Because the sense current of the current limit terminal varies according to where it is referenced, R1 should be less
than 2 kX when referenced to ground, and less than 100X
when referenced to V
.
in
FIGURE 4. Current Limit Transient Suppressor,
TL/H/8711– 17
Ground Referred
FIGURE 5. Current Limit Transient Suppressor,
TL/H/8711– 18
V
Referred
in
C.L. SENSE VOLTAGE MULTIPLICATION
When a larger sense resistor value is desired, the voltage
divider network, consisting of R1 and R2, may be used. This
effectively multiplies the sense voltage by (1
Also, R1 can be replaced by a diode to increase current limit
sense voltage to about 800 mV (diode V
a
f
a
R1/R2).
110 mV).
FIGURE 7. Current Limit Sense Voltage Multiplication,
TL/H/8711– 20
V
Referred
in
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout is accomplished with few external
components. When V
breakdown voltage, the output transistor is turned off. This
becomes lower than the zener
in
occurs because diode D1 will then become forward biased,
allowing resistor R3 to sink a greater current from the noninverting input than is sunk by the parallel combination of R1
and R2 at the inverting terminal. R3 should be one-fifth of
the value of R1 and R2 in parallel.
FIGURE 8. Under-Voltage Lockout
TL/H/8711– 22
MAXIMUM DUTY CYCLE LIMITING
The maximum duty cycle can be externally limited by adjusting the charge to discharge ratio of the oscillator capacitor
with a single external resistor. Typical values are 50 mA for
the charge current, 450 mA for the discharge current, and a
voltage swing from 200 mV to 750 mV. Therefore, R1 is
selected for the desired charging and discharging slopes
and C1 is readjusted to set the oscillator frequency.
FIGURE 6. Current Limit Sense Voltage Multiplication,
TL/H/8711– 19
Ground Referred
7
Page 8
Applications Information (Continued)
FIGURE 9. Maximum Duty Cycle Limiting
TL/H/8711– 21
DUTY CYCLE ADJUSTMENT
When manual or mechanical selection of the output transistor’s duty cycle is needed, the cirucit shown below may be
used. The output will turn on with the beginning of each
oscillator cycle and turn off when the current sunk by R2
and R3 from the non-inverting terminal becomes greater
than the current sunk from the inverting terminal.
With the resistor values as shown, R3 can be used to adjust
the duty cycle from 0% to 90%.
When the sum of R2 and R3 is twice the value of R1, the
duty cycle will be about 50%. C1 may be a large electrolytic
capacitor to lower the oscillator frequency below 1 Hz.
TL/H/8711– 23
FIGURE 10. Duty Cycle Adjustment
REMOTE SHUTDOWN
The LM1578A may be remotely shutdown by sinking a
greater current from the non-inverting input than from the
inverting input. This may be accomplished by selecting resistor R3 to be approximately one-half the value of R1 and
R2 in parallel.
FIGURE 11. Shutdown Occurs when VLis High
TL/H/8711– 24
EMITTER OUTPUT
When the LM1578A output transistor is in the OFF state, if
the Emitter output swings below the ground pin voltage, the
output transistor will turn ON because its base is clamped
near ground. The
low Ground
Collector Current with Emitter Output Be-
curve shows the amount of Collector current
drawn in this mode, vs temperature and Emitter voltage.
When the Collector-Emitter voltage is high, this current will
cause high power dissipation in the output transistor and
should be avoided.
This situation can occur in the high-current high-voltage
buck application if the Emitter output is used and the catch
diode’s forward voltage drop is greater than 0.6V. A fast-recovery diode can be added in series with the Emitter output
to counter the forward voltage drop of the catch diode (see
Figure 2
). For better efficiency of a high output current buck
regulator, an external PNP transistor should be used as
shown in
Figure 16.
TL/H/8711– 30
FIGURE 12. D1 Prevents Output Transistor from
Improperly Turning ON due to D2’s Forward Voltage
8
Page 9
Applications Information (Continued)
SYNCHRONIZING DEVICES
When several devices are to be operated at once, their oscillators may be synchronized by the application of an external signal. This drive signal should be a pulse waveform with
a minimum pulse width of 2 ms. and an amplitude from 1.5V
to 2.0V. The signal source must be capable of 1.) driving
capacitive loads and 2.) delivering up to 500 mA for each
LM1578A.
Capacitors C1 thru CN are to be selected for a 20% slower
frequency than the synchronization frequency.
FIGURE 13. Synchronizing Devices
TL/H/8711– 25
Typical Applications
The LM1578A may be operated in either the continuous or
the discontinuous conduction mode. The following applications (except for the Buck-Boost Regulator) are designed
for continuous conduction operation. That is, the inductor
current is not allowed to fall to zero. This mode of operation
has higher efficiency and lower EMI characteristics than the
discontinuous mode.
BUCK REGULATOR
The buck configuration is used to step an input voltage
down to a lower level. Transistor Q1 in
input DC voltage into a squarewave. This squarewave is
then converted back into a DC voltage of lower magnitude
by the low pass filter consisting of L1 and C1. The duty
cycle, D, of the squarewave relates the output voltage to the
input voltage by the following equation:
DcV
e
V
in
in
e
V
out
FIGURE 14. Basic Buck Regulator
Figure 15
rent, I
20% of I
ciency of 75%, a load regulation of 30 mV (70 mA to
is a 15V to 5V buck regulator with an output cur-
, of 350 mA. The circuit becomes discontinuous at
o
, has 10 mV of output voltage ripple, an effi-
o(max)
350 mA) and a line regulation of 10 mV (12
c
(ton)/(t
Figure 14
a
on
s
chops the
t
).
off
TL/H/8711– 5
s
V
in
18V).
Component values are selected as follows:
e
R1
R3
R3e0.15X
b
(V
1)cR2 where R2e10 kX
o
e
V/I
sw(max)
where:
V is the current limit sense voltage, 0.11V
is the maximum allowable current thru the out-
I
sw(max)
put transistor.
L1 is the inductor and may be found from the inductance
calculation chart (
Given V
in
I
o(max)
Discontinuous at 20% of I
Figure 16
e
15VV
e
350 mA f
) as follows:
e
5V
o
e
50 kHz
OSC
o(max)
.
Note that since the circuit will become discontinuous at
20% of I
below 70 mA.
, the load current must not be allowed to fall
o(max)
Step 1: Calculate the maximum DC current through the inductor, I
the top of the chart and show that I
buck configuration. Thus, I
Step 2: Calculate the inductor Volts-sec product, E-T
cording to the equations given from the chart. For the Buck:
E-T
op
e
(15b5) (5/15) (1000/50)
e
66V-ms.
with the oscillator frequency, f
. The necessary equations are indicated at
e
L(max)
b
(V
Vo)(Vo/Vin) (1000/f
in
L(max)
e
350 mA.
L(max)
osc
, expressed in kHz.
osc
)
e
I
o(max)
V
in
V
o
V
ripple
I
o
f
osc
R1
R2
R3
C1
C2
C3
L1
D1
e
e
e
e
e
e
e
e
e
e
e
for the
op
15V
5V
e
10 mV
350 mA
e
50 kHz
40 kX
10 kX
0.15X
1820 pF
220 mF
20 pF
470 mH
1N5818
TL/H/8711– 6
, ac-
FIGURE 15. Buck or Step-Down Regulator
Step 3: Using the graph with axis labeled ‘‘Discontinuous At
%I
’’ and ‘‘I
OUT
maximum inductor current, I
L(max, DC)
discontinuity percentage.
’’ find the point where the desired
L(max, DC)
intercepts the desired
In this example, the point of interest is where the 0.35A line
intersects with the 20% line. This is nearly the midpoint of
the horizontal axis.
Step 4: This last step is merely the translation of the point
found in Step 3 to the graph directly below it. This is accomplished by moving straight down the page to the point which
intercepts the desired E-T
66V-ms and the desired inductor value is 470 mH. Since this
. For this example, E-Topis
op
example was for 20% discontinuity, the bottom chart could
have been used directly, as noted in step 3 of the chart
instructions.
9
Page 10
Typical Applications (Continued)
TL/H/8711– 31
FIGURE 16. DC/DC Inductance Calculator
10
Page 11
Typical Applications (Continued)
For a full line of standard inductor values, contact Pulse
Engineering (San Diego, Calif.) regarding their PE526XX series, or A. I. E. Magnetics (Nashville, Tenn.).
A more precise inductance value may be calculated for the
Buck, Boost and Inverting Regulators as follows:
BUCK
e
L
BOOST
e
L
INVERT
e
L
where DI
usually chosen based on the minimum load current expected of the circuit. For the buck regulator, since the inductor
current I
DI
L
where the Discontinuity Factor is the ratio of the minimum
load current to the maximum load current. For this example,
the Discontinuity Factor is 0.2.
b
Vo(V
Vin(V
V
e
Vo)/(DILVinf
in
b
Vin)/(DILf
o
V
/[DIL(V
l
l
in
o
is the current ripple through the inductor. DILis
L
equals the load current IO,
L
a
in
DI
L
oscVo
V
l
o
e
2#I
)
osc
)
]
)f
l
osc
O(min)
140 mA for this circuit. DILcan also be interpreted as
e
DI
2#(Discontinuity Factor)#I
L
L
The remainder of the components of
Figure 15
are chosen
as follows:
C1 is the timing capacitor found in
t
C2
where V
b
Vo(V
Vo)/(8f
in
is the peak-to-peak output voltage ripple.
ripple
osc
2
VinV
Figure 1
ripple
.
L1)
C3 is necessary for continuous operation and is generally in the 10 pF to 30 pF range.
D1 should be a Schottky type diode, such as the
1N5818 or 1N5819.
BUCK WITH BOOSTED OUTPUT CURRENT
For applications requiring a large output current, an external
transistor may be used as shown in
Figure 17
. This circuit
steps a 15V supply down to 5V with 1.5A of output current.
The output ripple is 50 mV, with an efficiency of 80%, a load
regulation of 40 mV (150 mA to 1.5A), and a line regulation
of 20 mV (12V
s
s
V
18V).
in
Component values are selected as outlined for the buck
regulator with a discontinuity factor of 10%, with the addition of R4 and R5:
e
R4
10V
BE1Bf/Ip
R5e(V
in
bVb
b
V
V
sat)Bf
/(I
L(max, DC)
BE1
a
IR4)
where:
V
is the VBEof transistor Q1.
BE1
V
is the saturation voltage of the LM1578A output
sat
transistor.
V is the current limit sense voltage.
B
is the forced current gain of transistor Q1 (B
f
for
Figure 17
e
I
R4
e
I
p
V
/R4
BE1
I
L(max, DC)
).
a
0.5DI
L
e
f
30
e
V
15VR4e200X
in
e
5VR5e330X
V
o
e
50 mVC1e1820 pF
V
ripple
e
1.5AC2e330 mF
I
o
e
50 kHzC3e20 pF
f
osc
e
40 kXL1e220 mH
R1
e
10 kXD1e1N5819
R2
e
0.05XQ1eD45
R3
TL/H/8711– 8
FIGURE 17. Buck Converter with Boosted Output Current
11
Page 12
Typical Applications (Continued)
BOOST REGULATOR
The boost regulator converts a low input voltage into a higher output voltage. The basic configuration is shown in
18
. Energy is stored in the inductor while the transistor is on
and then transferred with the input voltage to the output
capacitor for filtering when the transistor is off. Thus,
e
V
a
V
in
Vin(ton/t
o
).
off
Figure
t
Io(V
b
Vin)/(f
o
oscVoVripple
).
C2
D1 is a Schottky type diode such as a IN5818 or IN5819.
L1 is found as described in the buck converter section, using the inductance chart for
Figure 16
for the boost configu-
ration and 20% discontinuity.
INVERTING REGULATOR
Figure 20
shows the basic configuration for an inverting regulator. The input voltage is of a positive polarity, but the
output is negative. The output may be less than, equal to, or
greater in magnitude than the input. The relationship between the magnitude of the input voltage and the output
voltage is V
c
e
V
(ton/t
o
in
).
off
FIGURE 18. Basic Boost Regulator
TL/H/8711– 9
The circuit of
Figure 19
converts a 5V supply into a 15V
supply with 150 mA of output current, a load regulation of
14 mV (30 mA to 140 mA), and a line regulation of 35 mV
(4.5V
s
s
V
8.5V).
in
e
V
5V
in
e
15V
V
o
e
V
ripple
e
140 mA
I
o
e
50 kHz
f
osc
e
140 kX
R1
e
10 kX
R2
e
0.15X
R3
e
220 kX
R4
e
1820 pF
C1
e
470 mF
C2
e
20 pF
C3
e
0.0022 mF
C4
e
330 mH
L1
e
1N5818
D1
TL/H/8711– 11
10 mV
FIGURE 19. Boost or Step-Up Regulator
R1e(V
R3eV/(I
b
1) R2 where R2e10 kX.
o
L(max, DC)
a
0.5 DIL)
where:
e
DI
2(I
L
LOAD(min)
)(Vo/Vin)
DILis 200 mA in this example.
R4, C3 and C4 are necessary for continuous operation
and are typically 220 kX, 20 pF, and 0.0022 mF respectively.
C1 is the timing capacitor found in
Figure 1
.
FIGURE 21. Inverting Regulator
FIGURE 20. Basic Inverting Regulator
TL/H/8711– 10
Figure 21
shows an LM1578A configured as a 5V tob15V
polarity inverter with an output current of 300 mA, a load
regulation of 44 mV (60 mA to 300 mA) and a line regulation
of 50 mV (4.5V
R1
R4e10V
s
V
in
e
(lV
l
o
R3eV/(I
BE1Bf
s
8.5V).
a
1) R2 where R2e10 kX.
L(max, DC)
/(I
L (max, DC)
a
0.5 DIL).
a
0.5 DIL)
where:
V, V
with Boosted Output Current’’ section.
DI
L
, and Bfare defined in the ‘‘Buck Converter
BE1,Vsat
e
2(I
LOAD(min)
)(V
a
V
)/V
l
l
in
o
IN
R5 is defined in the ‘‘Buck with Boosted Output Current’’ section.
R6 serves the same purpose as R4 in the Boost Regulator circuit and is typically 220 kX.
C1, C3 and C4 are defined in the ‘‘Boost Regulator’’
section.
C2
t
I
V
/[f
l
l
o
o
osc
(lV
a
Vin)V
l
o
ripple
]
L1 is found as outlined in the section on buck converters, using the inductance chart of
Figure 16
for the in-
vert configuration and 20% discontinuity.
e
V
5V
in
eb
15V
V
o
e
5mV
V
ripple
TL/H/8711– 12
e
300 mA, I
I
o
e
f
osc
e
160 kX R2e10 kX
R1
e
0.01 X R4e190X
R3
e
82X R6e220 kX
R5
e
1820 pF
C1
e
1000 mF
C2
e
20 pF
C3
e
0.0022 mF
C4
e
150 mH
L1
e
1N5818
D1
50 kHz
e
60 mA
min
12
Page 13
Typical Applications (Continued)
BUCK-BOOST REGULATOR
The Buck-Boost Regulator, shown in
voltage up or down, depending upon whether or not the
desired output voltage is greater or less than the input voltage. In this case, the output voltage is 12V with an input
voltage from 9V to 15V. The circuit exhibits an efficiency of
75%, with a load regulation of 60 mV (10 mA to 100 mA)
and a line regulation of 52 mV.
e
R1
b
(V
1) R2 where R2e10 kX
o
R3eV/0. 75A
R4, C1, C3 and C4 are defined in the ‘‘Boost Regulator’’
section.
D1 and D2 are Schottky type diodes such as the 1N5818 or
1N5819.
(Io/V
C2
t
[
f
osc(Vin
ripple
a
V
o
where:
is the forward voltage drop of the diodes.
V
d
V
is the saturation voltage of the LM1578A output
sat
transistor.
V
is the saturation voltage of transistor Q1.
sat1
L1t(V
b
V
in
where:
(1/f
)(V
osc
e
t
on
a
(V
V
o
a
2Io(V
in
e
I
p
(V
o
a
b
2V
in
d
a
V
2V
o
b
V
in
sat
Figure 22
a
)(V
o
a
b
2V
d
b
V
sat
sat1
a
2Vd)
b
V
V
sat
b
V
d
sat
b
V
)
sat1
2Vd)
V
sat
)(ton/Ip)
sat1
b
V
b
)
sat1
V
)
, may step a
]
)
sat1
RS-232 LINE DRIVER POWER SUPPLY
The power supply, shown in
Figure 23
, operates from an
input voltage as low as 4.2V (5V nominal), and delivers an
output of
The circuit provides a load regulation of
10% to 100% of full load) and a line regulation of
g
12V atg40 mA with better than 70% efficiency.
g
150 mV (from
g
10 mV.
Other notable features include a cycle-by-cycle current limit
and an output voltage ripple of less than 40 mVp-p.
A unique feature of this circuit is its use of feedback from
both outputs. This dual feedback configuration results in a
sharing of the output voltage regulation by each output so
that neither side becomes unbalanced as in single feedback
systems. In addition, since both sides are regulated, it is not
necessary to use a linear regulator for output regulation.
The feedback resistors, R2 and R3, may be selected as
follows by assuming a value of 10 kX for R1;
e
R2
R3e(lV
b
(V
1V)/45.8 mAe240 kX
o
a
1V)/54.2 mAe240 kX
l
o
Actually, the currents used to program the values for the
feedback resistors may vary from 40 mAto60mA, as long
as their sum is equal to the 100 mA necessary to establish
the 1V threshold across R1. Ideally, these currents should
be equal (50 mA each) for optimal control. However, as was
done here, they may be mismatched in order to use standard resistor values. This results in a slight mismatch of
regulation between the two outputs.
The current limit resistor, R4, is selected by dividing the current limit threshold voltage by the maximum peak current
level in the output switch. For our purposes R4
110 mV/750 mAe0.15X. A value of 0.1X was used.
Capacitor C1 sets the oscillator frequency and is selected
from
Figure 1
.
Capacitor C2 serves as a compensation capacitor for synchronous operation and a value of 10 to 50 pF should be
sufficient for most applications.
A minimum value for an ideal output capacitor C3, could be
calculated as C
is the transistor on time (typically 0.4/f
peak-to-peak output voltage ripple. A larger output capacitor
than this theoretical value should be used since electrolytics
have poor high frequency performance. Experience has
shown that a value from 5 to 10 times the calculated value
should be used.
c
e
I
t/DV where Iois the load current, t
o
), and DVisthe
osc
For good efficiency, the diodes must have a low forward
voltage drop and be fast switching. 1N5819 Schottky diodes
work well.
Transformer selection should be picked for an output transistor ‘‘on’’ time of 0.4/f
enough to prevent the output transistor switch from ramping
higher than the transistor’s rating of 750 mA. Pulse Engineering (San Diego, Calif.) and Renco Electronics, Inc.
(Deer Park, N.Y.) can provide further assistance in selecting
the proper transformer for a specific application need. The
transformer used in
PE-64287.
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DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL
SEMICONDUCTOR CORPORATION. As used herein:
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systems which, (a) are intended for surgical implantsupport device or system whose failure to perform can
into the body, or (b) support or sustain life, and whosebe reasonably expected to cause the failure of the life
failure to perform, when properly used in accordancesupport device or system, or to affect its safety or
with instructions for use provided in the labeling, caneffectiveness.
be reasonably expected to result in a significant injury
to the user.
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