C1888CT datasheet(2/6 Pages) TI

2 / 6 page

UCC1888

UCC2888

UCC3888

OPERATION (cont.)

At the end of the on time, Q1 is turned off and the current

through RON is again diverted to VCC. Thus the current

through RON, which charges CT during the on time, con-

tributes to supplying power to the chip during the off time.

The power switch off time is controlled by the discharge

of CT which, in turn, is programmed by the regulated out-

put voltage. The relationship between CT discharge cur-

rent, IDCHG, and output voltage is illustrated as follows:

Region 1. When VOUT = 0, the off time is infinite. This

feature provides inherent short circuit protec-

tion. However, to ensure output voltage

startup when the output is not a short, a high

value resistor, RS, is placed in parallel with CT

to establish a minimum switching frequency.

Region 2. As VOUT rises above approximately 0.7V to its

regulated value, IDCHG is defined by ROFF,

and is equal to:

IDCHG = (VOUT - 0.7V) / ROFF

As VOUT increases, IDCHG increases reducing off time.

The operating frequency increases and VOUT rises

quickly to its regulated value.

Region 3. In this region, a transconductance amplifier re-

duces IDCHG in order to maintain a regulated

VOUT.

Region 4. If VOUT should rise above its regulation range,

IDCHG falls to zero and the circuit returns to

the minimum frequency established by RS and

CT.

The range of switching frequencies is established by

RON, ROFF, RS, and CT as follows:

Frequency = 1/(TON + TOFF)

TON = RON

• CT • 4.6 V/(VIN - 4.5V)

TOFF (max) = 1.4

• RS • CT

Regions 1 and 4

TOFF = ROFF

• CT • 3.7V /(VOUT - 0.7V)

Region 2, excluding the effects of RS

which have a minimal impact on TOFF.

The above equations assume that VCC equals 9V. The

voltage at TON increases from approximately 2.5V to

6.5V while CT is charging. To take this into account, VIN

is adjusted by 4.5V in the calculation of TON. The voltage

at TOFF is approximately 0.7V.

DESIGN EXAMPLE

The UCC3888 regulates a 5 volt, 1 Watt nonisolated DC output from AC inputs between 80 and 265 volts. In this ex-

ample, the IC is programmed to deliver a maximum on time gate drive pulse width of 2.2 microseconds which occurs

at 80 VAC. The corresponding switching frequency is approximately 100kHz at low line, and overall efficiency is ap-

proximately 50%. Additional design information is available in Unitrode Application Note U-149.

UDG-96014

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