Philips TDA8385 Datasheet

INTEGRATED CIRCUITS

DATA SH EET

TDA8385

Control circuit for a Self-Oscillating Power Supply (SOPS)

Preliminary speciﬁcation Supersedes data of September 1991 File under Integrated Circuits, IC02

Philips Semiconductors

March 1994

Philips Semiconductors Preliminary speciﬁcation

Control circuit for a Self-Oscillating Power Supply (SOPS)

FEATURES

• Bandgap reference generator

• Slow-start circuitry

• Low-loss peak current sensing

• Over-voltage protection

• Hysteresis controlled stand-by

function

• Error amplifier with gain setting

• Programmable transfer character

generator

• Protection against open- and short-circuited feedback loop

ORDERING INFORMATION

EXTENDED TYPE

NUMBER

TDA8385 16 DIL plastic SOT38WBE

PINS PIN POSITION MATERIAL CODE

• Over-load current fold back characteristic

• LED driver

• Demagnetization protection

• Programmable determination of

switch-on moment of switching transistor for low-switching losses

• Feed-forward input

• Regulation-indicator output

• Programmable minimum on-time of

switching transistor

• Accurate peak-current setting.

TDA8385

GENERAL DESCRIPTION

The TDA8385 is intended to be used in combination with the opto-coupler (CNR50) as a control unit for a self-oscillating power supply.

PACKAGE

March 1994 2

Philips Semiconductors Preliminary speciﬁcation

Control circuit for a Self-Oscillating Power Supply (SOPS)

BLOCK DIAGRAMS

handbook, full pagewidth

current reference

setting

feed forward

input

regulation indicator

output

differential amplifier

output

ref

1 RIO

11Vdiff

REFERENCE

BLOCK

ref

REGULATION

INDICATOR

DIFFERENTIAL AMPLIFIER

TDA8385

P GND

16 14

latch

diff

P (min)

DETECTOR

reset

2812

(28, 27, 23)

STABILIZED

SUPPLY

ref

stab

SUPPLY REFERENCES

ref

(2.5 V)

feedback voltage

input

transistor-on setting input

slow start voltage

input

on(min)

7Vss

50 µA

CLAMP

TCG

2.5 V 4

on (min)

reset (28)

quick

discharge

Fig.1 Block diagram; part A (continued in Fig.2; part B).

TCG

CONTROL PART

SLOW START

MINIMUM

VOLTAGE

CLAMP

50 µA

charge

III

VII

MCD417

March 1994 3

Philips Semiconductors Preliminary speciﬁcation

Control circuit for a Self-Oscillating Power Supply (SOPS)

handbook, full pagewidth

stand-by voltage input

latch

2.5 V

sim

(28)18

2.5 V

STAND-BY

2 V

PWM

comparator

latch

OUTPUT

STAGE

LED DRIVER

TDA8385

LED

Q (23)

TDA8385

LED driver output

demagnetization

100 mV

100 µA

slow

discharge

OVER-VOLTAGE PROTECTION

(17) (28)

DELAY

SAWTOOTH GENERATOR

demagnetization

LED CONTROL

sim

0.2 I

peak

DEM

115 mV

over voltage

2.5 V

VIII

current simulation

input

peak-current

setting input

demagnetization

input

delay setting

over-voltage

protection

Fig.2 Block diagram; part B (continued from Fig.1; part A).

March 1994 4

MCD418

Philips Semiconductors Preliminary speciﬁcation

Control circuit for a Self-Oscillating Power Supply (SOPS)

PINNING

SYMBOL PIN DESCRIPTION

RIO 1 regulation indicator output LED 2 LED driver output I

ref

on(min)

peak

DELAY 6 delay setting V

OVP 8 over-voltage protection V

diff

sim

GND 14 ground (0 V) DEM 15 demagnetization input V

3 current reference setting 4 transistor-on setting input 5 peak current setting input

7 slow start voltage input

9 feedback voltage input 10 stand-by voltage input 11 differential ampliﬁer output 12 current simulation input 13 feed forward input

16 positive supply voltage

handbook, 2 columns

on(min)

DELAY

peak

OVP

RIO

LED

ref

1 2 3 4 5 6 7

Fig.3 Pinning diagram.

TDA8385

MCD402

DEM

GND

sim

diff

FUNCTIONAL DESCRIPTION

The TDA8385 can be divided into 10 functional blocks as shown in Fig.1 and Fig.2.

Block for Figs 1 and 2

BLOCK

NO.

DESCRIPTION

I supply references II sawtooth generator III control part IV pulse width modulator

(PWM) V LED control VI LED driver VII slow-start circuitry VIII over-voltage protection IX stand-by circuit X regulation-indicator

output

These 10 functional blocks of Fig.1 and Fig.2 contain sub-sections numbered 1 to 28 which are

cross-referenced in the following description.

Supply references (Block I)

The TDA8385 is intended to be used on the secondary side of the self-oscillating power supply. It can be supplied either by an auxiliary winding of the transformer or an external supply e.g. 50 Hz transformer. Charging of the capacitor C

(see

Fig.16) takes place during transistor on-time (Ton; see Fig.17). During stand-by the IC is supplied by the stand-by voltage Vsb (pin 10). The operating voltage range is from 7.5 to 20 V. The supply current, inclusive drive current for the LED, is less than 20 mA. A bandgap based reference (2.5 V) generates a stabilized voltage V

of 3.9 V to supply all internal

stab

circuits of the IC except the LED driver. The LED driver is directly supplied by VP. The reference block generates all the reference voltages in the circuit. By means of a resistor connected to pin 3, a reference current (I

) is defined.

ref

This current is reflected several times and is used to obtain IC-independent settings e.g. T

setting, delay

on(min)

setting, charging and discharging of slow-start capacitor Css on pin 7 (see Fig.16).

The power supply is released by the opto-coupler IC at an input voltage level, which is high enough to guarantee correct operation of the TDA8385 e.g. VP = 10 V by sensing the mains voltage VI. As soon as the SOPS switching transistor (T1, see Fig.16) is conductive the capacitor C is charged. As long as the IC supply voltage is below 7.5 V the LED driver is blocked (see latch output; sub-section 28) in order to guarantee start-up of SOPS.

During the initialization phase the quick-discharge-switch (sub-section 27), set input of flip-flop (13) and reset input of flip-flop (23) are also activated. As soon as the voltage of 7.5 V is reached the control functions of the IC are operative. Hysteresis on the initialization level is 2.3 V.

March 1994 5

Philips Semiconductors Preliminary speciﬁcation

Control circuit for a Self-Oscillating Power Supply (SOPS)

Sawtooth generator (Block II)

URRENT SIMULATION

C (SEE FIGS 5 AND 16)

The current of the power supply switching transistor is detected on the secondary side by an indirect method of current sensing.

Information of the collector current (Ic) is obtained by integrating the voltage of an auxiliary winding of the transformer during transistor on-time (Ton). An external capacitor C on pin 5 is charged during Ton by the current source I reflection of the current which flows into pin 12. This current is obtained by connecting an external resistor R12 to the auxiliary transformer winding. During transistor on-time this current is related to the input voltage VI. During transistor off time (T capacitor C is discharged by switch sw1. This switch is active during the total T voltage Vc is formed across C. This sawtooth is a measure for the collector current of the switching transistor T1.

For the voltage Vc yields:

. The current I

sim

time. In this way a sawtooth

off

simTon

------------------------ C

sim

is the

) the

off

(1)

TDA8385

latch

initialization

operation

5.2 7.5 20

Fig.4 Latch initialization as a function of supply voltage VP.

R12

VP(V)

sw1

MCD403

sim

---------- -

----- n

R12

Where: p = reflection factor;

sim

--------I

0.2==

(2)

×=

(2) → (1) gives:

----- -

--- C

× T

---------- R12

×=

(3)

March 1994 6

off

MCD404

Fig.5 Determination of the peak current Ic.

Philips Semiconductors Preliminary speciﬁcation

Control circuit for a Self-Oscillating Power Supply (SOPS)

For ‘Ton’ yields:

VcC× np× R12×

---------------------------------------------- -

× V

For the primary current Ic yields:

×=

-----

Substitution (4) into (5):

--- -

× R12× V

-- -

----- -

×=

Equation (6) shows that by limiting the voltage V

the collector peak current

can be limited. The peak current is limited by means of the clamping circuit in the transfer character generator (TCG); see Fig.1 sub-section 4.

ELAY SETTING (PIN 6)

The output of sub-section 11 is

(4)

extended by the delay circuit of sub-section 12. The starting (reference) point of the delay circuit is the falling edge of the output of demagnetizing comparator (11) The

(5)

delay can be determined externally by capacitor (C

The switch-on moment of the switching transistor can be

(6)

determined by capacitor C A minimum delay time is required to

prevent transistor T1 from switching during demagnetization of the transformer because of oscillations caused by the leakage inductance.

delay

) on pin 6.

delay

TDA8385

Control part (Block III)

The differential amplifier, sub-section 3, compares the feedback voltage (Vfb) with the reference voltage V the differential amplifier is available on pin 11 to allow gain setting. The differential amplifier is internally compensated for 0 dB feedback stability.

The feedback input (pin 9) is also

used as the input for the TCG (see Fig.6) with which a current foldback characteristic can be obtained as shown in Fig.7.

. The output of

ref

The clamping level can be externally influenced by means of a resistor on pin 7.

The collector peak current can be influenced in several ways:

• Resistor R12 on pin 12

• Capacitor C on pin 5

• Capacitor on pin 7

• Transfer ratio nh/n

• Inductance L Before comparing the sawtooth

voltage Vc with the control voltage V in the pulse width modulator, a voltage of 100 mV is added to Vc. In this way it will be possible for Vr to become smaller than V

, which is

sim

important for a stabilized no-load operation (see Fig.6 area 3).

EMAGNETIZATION INPUT (PIN 15)

This input prevents the switching transistor from conducting during demagnetization of the transformer in order to prevent the transformer from going into saturation. The output of comparator (11) is HIGH as soon as the voltage of the transformer winding exceeds 115 mV.

(3)

(4)

(2)

(1)

(5)

(1), (2), (3) = V (4), (5) = V

TCG

diff

clamp

Ton(min)

MCD405

Fig.6 Reference voltage (Vmv) as a function of feedback voltage (Vfb).

March 1994 7

Philips Semiconductors Preliminary speciﬁcation

Control circuit for a Self-Oscillating Power Supply (SOPS)

(5)

(1)

(1), (2), (3) = V (4), (5) = V

TCG

diff

Fig.7 Current foldback characteristic; stabilized output voltage (VO)

as function of load current (IO).

(4)

(2)

(3)

MCD406

TDA8385

The voltage V minimum on-time of the switching transistor. This voltage can be determined externally with a resistor on pin 4. With this resistor the current foldback characteristic can be influenced (see dotted line in Figs 6 and 7).

The minimum on-time is of importance for the following.

• Stand-by operation

• Starting-up of power supply

• Overload and short-circuit

conditions.

The output of the differential amplifier (V

), the output of the TCG (V

diff

and the voltage Vss + V compared in a minimum voltage clamping circuit (see Fig.1 sub-section 6). The output voltage is equal to the lowest input voltage.

Some relevant characteristics of the control part are depicted in Fig.8.

Ton(min)

determines the

TCG

are

Ton(min)

)

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The voltage Vmv determines the collector peak current Ic of transistor T1. The right-hand curve is passed through at start-up. When the feedback voltage slowly increases from zero, the peak current starts at I the regulation slope is reached, which is approximately V

The plateau of the top between the points x and y has to be kept as small as possible. The voltage V Due to the characteristic of the TCG open- and short-circuit feedback loop will result in low peak current. An additional signal on pin 13 can be supplied which is subtracted from the signal Vmv. This input can be used for feed forward information. If no feed forward information is used, pin 13 should be connected to ground.

c (max)

c (min)

Ton(min)

decreases with the decreasing load. For good no-load operation the peak current has to be made zero with V

diff

and rises along the straight line until I

c(min)

ref

TCG

diff

external peak-current setting (pin 7)

ref

MCD407

is reached. At a slightly higher feedback voltage

c(max)

diff

Fig.8 Characteristics of the control part.

March 1994 8

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