Fairchild RC4391 service manual

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PRODUCT SPECIFICATION RC4391

2

Pin Assignments

LBR

LBD

C

X

GND

V

FB

V

REF

+V

S

L

X

8

65-3471-02

7

6

5

1

2

3

4

Absolute Maximum Ratings

Note:

1. The maximum allowable supply voltage (+V

S

) in inverting applications will be reduced by the value of the negative output

voltage, unless an external power transistor is used in place of Q1.

Thermal Characteristics

Parameter Conditions Min Typ Max Unit

Internal Power Dissipation 500 mW
Supply Voltage

1

(Pin 6 to Pin 4 or Pin 6 to Pin 5) +30 V

Operating Temperature RC4391 0 70 °C

RV4391 -25 85 °C

RM4391 -55 125 °C
Storage Temperature -65 150 °C
Junction Temperature PDIP, SOIC 125 °C

CerDIP 175 °C
Switch Current (I

MAX

) Peak 375 mA

PD TA <50˚C PDIP 468 mW

CerDIP 833 mW

SOIC 300 mW
Lead Soldering Temperature (10 seconds) 300 °C

8-Lead Plastic DIP 8-Lead Ceramic DIP Small Outline SO-8

Therm. Res q

JC

—45°C/W —

Therm. Res. q

JA

160°C/W 150°C/W 240°C/W

For TA >50˚C Derate at 6.25 mW/°C 8.33 mW/°C 4.17 mW/°C

Pin Descriptions

Pin
Number Pin Function Description

1 Low Battery Resistor (LBR)
2 Low Battery Detector (LBD)
3 Timing Capacitor (CX)
4 Ground
5 External Inductor (LX)
6 +Supply Voltage (+VS)
7 +1.25V Reference Voltage (V

REF

)

8 Feedback Voltage (V

FB

)

RC4391 PRODUCT SPECIFICATION

3

Electrical Characteristics

(VS = +6.0V, over the full operating temperature range unless otherwise noted)

Note:

1. The maximum allowable supply voltage (+V

S

) in inverting applications will be reduced by the value of the negative output

voltage, unless an external power transistor is used.

Symbol Parameters Condition Min Typ Max Units

+V

S

Supply Voltage (Note 1) 4.0 30 V

I

SY

Supply Current VS = +25V 300 500 mA

V

REF

Reference Voltage 1.13 1.25 1.36 V

V

OUT

Output Voltage V

OUT nom

= -5.0V -5.5 -5.0 -4.5 V

V

OUT nom

= -15V -16.5 -15.0 -13.5

LI

1

Line Regulation V

OUT nom

= -5.0V, %V

OUT

CX = 150pF
VS = +5.8V to +15V 2.0 4.0
V

OUT nom

= -15V,
CX = 150pF
VS = +5.8V to +15 1.5 3.0

L0

1

Load Regulation V

OUT nom

= -5.0V, %V

OUT

CX = 350pF, VS = +4.5V,
P

LOAD

= 0mW to 75mW 0.2 0.5

V

OUT nom

= -15V,
CX = 350pF, VS = +4.5V,
P

LOAD

= 0mW to 75mW 0.2 0.3

I

CO

Switch Leakage Current Pin 5 = -20V 0.1 30 mA

PRODUCT SPECIFICATION RC4391

4

Electrical Characteristics

(VS = +6.0V, TA = +25°C unless otherwise noted)

Symbol Parameters Condition Min Typ Max Units

I

SY

Supply Voltage VS = +4.0V, 170 250 mA

No External Loads
VS = +25V 300 500
No External Loads

V

OUT

Output Voltage V

OUT nom

= -5.0V -5.35 -5.0 -4.65 V

V

OUT nom

= -15V -15.85 -15.0 -14.15

LI

1

Line Regulation V

OUT nom

= -5.0V %V

OUT

CX = 150pF, 1.5 3.0
VS = +5.8V to +15V
V

OUT nom

= -15V,
CX = 150pF 1.0 2.0
VS = +5.8V to +15V

L0

1

Load Regulation V

OUT nom

= -5.0V, %V

OUT

CX = 350pF, VS = +4.5V, 0.2 0.4
P

LOAD

= 0mW to 75mW

V

OUT nom

= -15V,
CX = 350pF, VS = +4.5V, 0.07 0.14
P

LOAD

= 0mW to 75mW

V

REF

Reference Voltage 1.18 1.25 1.32 V

I

SW

Switch Current Pin 5 = 5.5V 75 100 mA

I

CO

Switch Leakage Current Pin 5 = -24V 0.01 5.0 mA

I

CX

Cap. Charging Current Pin 3 = 0V 6.0 10 14 mA

I

LBDL

LBD Leakage Current Pin 1 = 1.5V, Pin 2 = 6.0V 0.01 5.0 mA

I

LBD0

LBD On Current Pin 1 = 1.1V, Pin 2 = 0.4V 210 600 mA

I

LBRB

LBR Bias Current Pin 1 = 1.5V 0.7 mA

RC4391 PRODUCT SPECIFICATION

5

Typical Performance Characteristics

Figure 1. Oscillator Frequency vs. Supply Voltage Figure 2. Oscillator Frequency vs. Temperature

Figure 3. Reference Voltage vs. Temperature Figure 4. Reference Voltage vs. Supply Voltage

Figure 5. Collector Current vs. Q1 Saturation Voltage Figure 6. Minimum Supply Voltage vs. Temperature

F

O

(kHz)

+VS (V)

65-3268

0

510152025

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

F (kHz)

TA (¡C)

65-3272

-55 0 25 70 125

8
7
6
5
4
3
2
1
0

O

V

REF

(V)

TA (¡C)

65-3269

-55 0 25 70 125

1.260

1.255

1.250

1.245

1.240

V

REF

(V)

+VS (V)

65-3273

4 6 10 20 30

1.260

1.255

1.250

1.245

1.240

I

C

(mA)

V

CE

(SAT) (V)

65-3270

600
500
400
300
200
100

0

123 5 7

8

20
10

46

TA (¡C)

65-3271

-55 0 25 70 125

4

3

2

1

0

+V

S

(V)

PRODUCT SPECIFICATION RC4391

6

Principles of Operation

The basic switching inverter circuit is the building block on
which the complete inverting application is based.

A simplified diagram of the voltage inverter circuit with
ideal components and no feedback circuitry is shown in
Figure 7. When the switch S is closed, charging current from
the battery flows through the inductor L, which builds up a
magnetic field, increasing as the switch is held closed. When
the switch is opened, the magnetic field collapses, and the
energy stored in the magnetic field is conv erted into a current
which flows through the inductor in the same direction as the
changing current. Because there is no path for this current to
flow through the switch, the current must flow through the
diode to charge the capacitor C. The key to the inversion is
the ability of the inductor to become a source when the
charging current is removed.

The equation V = L (di/dt) gives the maximum possible
voltage across the inductor; in the actual application, feed­back circuitry and the output capacitor will decrease the
output voltage to a regulated fixed value.

A complete schematic for the standard inverting application
is shown in Figure 8. The ideal switch in the simplified
diagram is replaced by the PNP transistor switch between
pins 5 and 6. C

F

functions as the output filter capacitor, and

D1 and LX replace D and L.

When power is first applied, the ground sensing comparator
(pin 8) compares the output voltage to the +1.25V voltage

reference. Because C

F

is initially discharged a positive
voltage is applied to the comparator, and the output of the
comparator gates the squarewave oscillator. This gated
squarewave signal turns on, then off, the PNP output transis­tor. This turning on and off of the output transistor performs
the same function as opening and closing the ideal switch in
the simplified diagram; i.e., it stores energy in the inductor
during the on time and releases it into the capacitor during
the off time.

The comparator will continue to allow the oscillator to turn
the switch transistor on and off until enough energy has been
stored in the output capacitor to make the comparator input
voltage decrease to less than 0V. The voltage applied to the
comparator is set by the output voltage, the reference volt­age, and the ratio of R1 to R2.

Figure 7. Simple Inverting Regulator

D

(+)

(–)

V

OUT

C

S

L

+V

S

65-1601

*Caution: Use current limiting protection circuit for high values of CF (Figure 13)

Figure 8. Inverting Regulator – Standard Circuit

C2

C1

OSC

+1.25V

REF/Bias

RC4391

65-1602

REF

V

Q1

Q2

C

x

A

To

+V

s

LBD

Output

R3

260K

R4

590K

R6

100K

B

C *

33µF

F

R1

R2

+V

s

V

OUT

1N914

F

E

L

x

C1

0.1µF
D1

-

Parts

List

-5.0V

Output

-15V

Output
R1 =
R2 =
C =
L =

300 k
75 k
150 pF

1.0 mH Dale TE3 Q4 TA

900 k
75 k
150 pF

= Optional

-V = (1.25V) ( )

x
x

W

W

W

W

OUT

R1
R2

D

LBR

GND

C

X

LBD

L

X

+V

S

V

REF

V

FB

RC4391 PRODUCT SPECIFICATION

7

Figure 9. Inverting Regulator Waveforms

B

A

C

D

E

F

G

1.78V

0.62V

(Internal)

I

0 mA

+V
(Internal)
+V - 0.7V
Max

0 mA
I

0 mA

+V - V

-V - V

Ground

C

O

I

V

I

I

V

X

SC

LOAD

BEQ1

LX

D

LX

L

S

S

S

MAX

SW

OUT D

65-2472

BAT

OUT

X

X

V

L

V

L

This feedback system will vary the duration of the on time in
response to changes in load current or battery voltage (see
Figure 9). If the load current increases (waveform C), then
the transistor will remain on (waveform D) for a longer por­tion of the oscillator cycle, (waveform B) to build up to a
higher peak value. The duty cycle of the switch transistor
varies in response to changes in load and line.

Step-Down Regulator

The step-down circuit function is similar to inv ersion; it uses
the same components (switch, inductor, diode, filter capaci­tor), and charges and discharges the inductor by closing and
opening the switch. The great difference is that the inductor
is in series with the load; therefore, both the charging current
and the discharge current flow into the load. In the inverting
circuit only the discharge current flows into the load. Refer
to Figure 10.

When the switch S is closed, current flows from the battery,
through the inductor, and through the load resistor to ground.
After the switch is opened, stored energy in the inductor
causes current to keep flowing through the load, the circuit
being completed by the catch diode D. Since current flows to
the load during charge and discharge, the average load cur-

rent will be greater than in an inverting circuit. The signifi­cance of that is that for equal load currents the step-down
circuit will require less peak inductor current than an invert­ing circuit. Therefore, the inductor will not require as large
of a core, and the switch transistor will not be stressed as
heavily for equal load currents.

Figure 11 depicts a complete schematic for a step-down cir­cuit using the RC4391. Observe that the ground lead of the
4391 is not connected to circuit ground; instead, it is tied to
the output voltage. It is by this rearrangement that the feed­back system, which senses voltages more negative than the
ground lead, can be used to regulate a non-negative output
voltage.

Figure 10. Simple Step-Down Regulator

D

(+)

(-)

V

OUT

S

C

S

L

+V

65-2473

R

L