MP4050A – NON-ISOLATED, HIGH-BRIGHTNESS LED DRIVER, WITH ENHANCED THERMAL FEATURE

MP4050A Rev. 1.01

www.MonolithicPower.com

13

1/14/2016

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Minimum Operating Frequency Limit

The

MP4050A

incorporates

a

minimum

operating frequency (22kHz) to eliminate

audible noise.

When the operating frequency is less than

22kHz, the internal peak-current regulator

decreases the peak-current value to keep the

operating frequency constant at about 22kHz.

If the inductance is too large to make the

operating frequency drop to the minimum

operating frequency, the converter enters CCM

operation. Generally, the converter works in

DCM when the operating frequency is larger

than 22kHz for normal operation.

Minimum Off-Time Limit

A minimum off-time limit is implemented. During

normal operation, the minimum off-time limit is

4.7μs. During the start-up period, the minimum

off-time limit is shortened gradually from

16.45μs to 4.7μs (see Fig. 4). Each minimum

off-time limit maintains 32 switching cycles. This

soft-start function enables a safe start-up.

FIGURE 4. Minimum Off-Time Limit at

Start-up

Thermal Shutdown (TSD)

To prevent thermal damage to the system and

IC, the chip reduces the reference to decrease

the output power if the junction temperature

exceeds

145

C.

This

limits

the

rising

temperature speed of the IC. Typically, the

reference voltage drops to around 20% when

the junction temperature rises to 160

C. If the

temperature exceeds 160

C, the MP4050A

stops switching, and the IC is latched off. Once

the junction temperature drops below 110

C,

the chip resumes operation.

Open LED Protection (OLP)

the MOSFET turns off, the MP4050A stops

working, and a re-start cycle begins. Open LED

protection operates in hiccup mode. The

MP4050A

monitors

the

PRO

voltage

continuously, and the VCC cap is discharged

and

charged

repeatedly.

The

MP4050A

resumes work 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 can reduce

automatically the switching frequency and

achieve close loop control. Then the output

power at this condition is limited within a safe

range. The MP4050A resumes work in normal

operation once the device recovers from the

short circuit.

Leading Edge Blanking (LEB)

Internal leading edge blanking (LEB) is

employed to prevent a switching pulse from

terminating

prematurely

due

to

parasitic

capacitance discharging when the MOSFET

turns on. During the blanking time, the path

from SOURCE to the current comparator input

is blocked (see Fig. 5)

FIGURE 5. Leading Edge Blanking (LEB)