MC34262 데이터 시트보기 (PDF) - Motorola => Freescale

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MC34262

Power Factor Controllers

Motorola => Freescale 

MC34262 MC33262

Figure 18. Inductor Current and MOSFET

Gate Voltage Waveforms

Peak

Inductor Current

Average

Timer

A watchdog timer function was added to the IC to eliminate

the need for an external oscillator when used in stand–alone

applications. The Timer provides a means to automatically

start or restart the preconverter if the Drive Output has been

off for more than 620 µs after the inductor current reaches

zero. The restart time delay versus temperature is shown in

Figure 8.

0

On

MOSFET

Q1 Off

Current Sense Comparator and RS Latch

The Current Sense Comparator RS Latch configuration

used ensures that only a single pulse appears at the Drive

Output during a given cycle. The inductor current is

converted to a voltage by inserting a ground–referenced

sense resistor R7 in series with the source of output switch

Q1. This voltage is monitored by the Current Sense Input and

compared to a level derived from the Multiplier output. The

peak inductor current under normal operating conditions is

controlled by the threshold voltage of Pin 4 where:

IL(pk ) =

Pin 4 Threshold

R7

Abnormal operating conditions occur during preconverter

startup at extremely high line or if output voltage sensing is

lost. Under these conditions, the Multiplier output and Current

Sense threshold will be internally clamped to 1.5 V.

Therefore, the maximum peak switch current is limited to:

Ipk(max) =

1.5 V

R7

An internal RC filter has been included to attenuate any

high frequency noise that may be present on the current

waveform. This filter helps reduce the ac line current

distortion especially near the zero crossings. With the

component values shown in Figure 20, the Current Sense

Comparator threshold, at the peak of the haversine varies

from 1.1 V at 90 Vac to 100 mV at 268 Vac. The Current

Sense Input to Drive Output turn–off propagation delay is

typically less than 200 ns.

Undervoltage Lockout and Quickstart

An Undervoltage Lockout comparator has been

incorporated to guarantee that the IC is fully functional before

enabling the output stage. The positive power supply terminal

(VCC) is monitored by the UVLO comparator with the upper

threshold set at 13 V and the lower threshold at 8.0 V. In the

stand–by mode, with VCC at 7.0 V, the required supply current

is less than 0.4 mA. This large hysteresis and low startup

current allow the implementation of efficient bootstrap startup

techniques, making these devices ideally suited for wide

input range off–line preconverter applications. An internal

36 V clamp has been added from VCC to ground to protect

the IC and capacitor C4 from an overvoltage condition. This

feature is desirable if external circuitry is used to delay the

startup of the preconverter. The supply current, startup, and

operating voltage characteristics are shown in Figures 13

and 14.

A Quickstart circuit has been incorporated to optimize

converter startup. During initial startup, compensation

capacitor C1 will be discharged, holding the error amp output

below the Multiplier threshold. This will prevent Drive Output

switching and delay bootstrapping of capacitor C4 by diode

D6. If Pin 2 does not reach the multiplier threshold before C4

discharges below the lower UVLO threshold, the converter

will “hiccup” and experience a significant startup delay. The

Quickstart circuit is designed to precharge C1 to 1.7 V, Figure

7. This level is slightly below the Pin 2 Multiplier threshold,

allowing immediate Drive Output switching and bootstrap

operation when C4 crosses the upper UVLO threshold.

Drive Output

The MC34262/MC33262 contain a single totem–pole

output stage specifically designed for direct drive of power

MOSFETs. The Drive Output is capable of up to ± 500 mA

peak current with a typical rise and fall time of 50 ns with a

1.0 nF load. Additional internal circuitry has been added to

keep the Drive Output in a sinking mode whenever the

Undervoltage Lockout is active. This characteristic eliminates

the need for an external gate pull–down resistor. The

totem–pole output has been optimized to minimize

cross–conduction current during high speed operation. The

addition of two 10 Ω resistors, one in series with the source

output transistor and one in series with the sink output

transistor, helps to reduce the cross–conduction current and

radiated noise by limiting the output rise and fall time. A 16 V

clamp has been incorporated into the output stage to limit the

high state VOH. This prevents rupture of the MOSFET gate

when VCC exceeds 20 V.

8

MOTOROLA ANALOG IC DEVICE DATA

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