TK11830M データシートの表示（PDF） - Toko America Inc

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TK11830M

POSITIVE-TO-NEGATIVE DC-DC CONVERTER

Toko America Inc  

TK11830

APPLICATION INFORMATION (CONT.)

INTERMITTENT OSCILLATION

When the ripple voltage applied to the feedback pin is large

and CFB is small, the power transistor switch drive is large

and the output voltage exceeds the desired value. This

causes the oscillator to stop for a period of ti. When the

ripple voltage is large and the power transistor is driven at

maximum capacity, a current up to ILPK(MAX) goes through

the inductor.

IL

tON

tOFF

IC

∆Q1-

∆Q+

ti

∆Q2-

t

ILPK(MAX)

ILPK(MAX)

-ILOAD

ILOAD

mounting. The package power dissipation curve on a

printed circuit board is estimated as follows:

When Pin 4 is connected to GND (Power transistor switch

is at maximum conductance), all input power is dissipated

by the IC at TA = room temperature. In this state Tj goes up

to 150 °C and thermal protection operates. Input power is

defined as PIN = VIN x <IIN>, where <IIN> is the average of

input current. From Tj = Oja x P + TA and Tj = 150 °C.

P = PIN, TA = Room temp., Oja can be found. The power

dissipation curve shows the effect of mounting on thermal

characteristics.

PLOSS, must be within this curve. The efficiency, E (%), is

the ratio between input and output power when the dc-dc

converter is operating.

PLOSS = PIN - POUT

= POUT x [(100 / E) - 1]

= |VOUT| x ILOAD x [(100 / E) - 1]

Note: tON/tOFF = (|VOUT| + VF) / (VIN - VSAT)

tON = [ILPK(MAX) / (VIN - VSAT)] x L

tOFF = [ILPK(MAX) / (|VOUT| + VF)] x L

Since the charge of the capacitor is equivalent to the

discharge (∆Q+ = ∆Q1- + ∆Q2-):

VIN

+

IIN

+

VIN

Vref

VOSC

VFB

ILPK(MAX) = 2 x ILOAD x [(tON / tOFF) + 1] + 2 x ILOAD x (ti / tOFF)

ti = ([ILPK(MAX) / (2 x ILOAD)] x tOFF) - (tON + tOFF)

f = 1 / (tON + tOFF + ti)

When load current increases, ti becomes shorter.

As in the case above, if the load current is too small, the

power transistor becomes overdriven and intermittent

oscillation will occur.

PACKAGE POWER DISSIPATION

The internal thermal protection circuit will operate when Tj

is approximately 150 °C. When thermal protection operates,

the power transistor switch will cycle between on and off to

keep Tj ≤ 150 °C. Thermal resistance Oja is determined by

January 1999 TOKO, Inc.

TA = 25 °C

MOUNTED ON PCB

IIN

<IIN>

t

IIN WAVEFORM WHEN THERMAL PROTECTION IS

OPERATING

Page 11

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