MP4068 – HIGH PF, NON-ISOLATED, TRIAC DIMMABLE LED DRIVER

MP4068 Rev. 1.01

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14

7/30/2015

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turned on. The on period is determined by the

peak current limit. After the on period elapses,

the integrated MOSFET is turned off (see Fig.

4).

By monitoring the internal sampling capacitor

voltage, the output current can be regulated.

The output current is determined by the

following equation:

=

O

0.195V

I

R1

The peak current can be calculated as follows:

=

Peak

0.46V

I

R1

Where R1 is the sense resistor.

Minimum Operating Frequency Limit

The MP4068 incorporates a minimum operating

frequency (22kHz) to eliminate audible noise

when the frequency is less than 20kHz.

When the operating frequency is less than

22kHz, the internal peak current regulator will

decrease the peak current value to keep the

operating frequency constant (about 22kHz).

Minimum Off-Time Limit

A minimum off-time limit is implemented. During

normal operation, the minimum off-time limit is

7μs. During the start-up period, the minimum

off-time limit is shortened gradually from 24μs

to 12μs to 7μs (see Fig. 5). Each minimum off

time maintains a 32 switching cycle. This soft-

start function enables a safe start-up.

Thermal Protection (TSD)

To prevent the IC and system from thermal

damage, the MP4068 reduces the reference to

decrease the output current. This limits the

temperature rising speed of the IC when the

junction temperature exceeds 145

°C. Typically,

the output current drops to around 20% when

the IC temperature rises to 160

°C. Once the

junction temperature exceeds 160

°C, the

MP4068 shuts down the switching cycle. As

soon as the junction temperature drops below

110

°C, the power supply resumes operation.

During the thermal shutdown condition, the

charged by the internal high-voltage regulator.

Over-Voltage Protection (OVP)

start cycle begins. When OVP occurs, the chip

works in hiccup mode; the MP4068 monitors

the OVF voltage continuously, and the VCC

voltage discharges and re-charges repeatedly.

The MP4068 resumes operation once the fault

disappears.

Short-Circuit Protection (SCP)

When an LED short circuit occurs, the switching

off time is extended. Due to the minimum

operating frequency limit, the IC reduces

automatically the switching frequency and

achieves close loop control. Then the output

power at this condition is limited at a safe range.

The MP4068 resumes working in normal

operation once the short circuit is released.