30V 1.5A 1MHz Sync Step-Down Dimmable LED Driver

Peak Current Limit Mode

The HM2802 has both a peak current limit mode and a valley current limit mode to protect the chip from overcurrent

damage. The device implements current-mode control, which uses the internal COMP voltage to control the turn-

off of the high-side MOSFET and the turn-on of the low-side MOSFET on a cycle-by-cycle basis. During each

cycle, the switch current and the current reference generated by the internal COMP voltage are compared. When the

peak switch current intersects the current reference, the high-side switch turns off. During overcurrent conditions,

such as when the sensing resistor is shorted, or an open circuit occurs in the feedback-filter RC network that drives

FB low, the error amplifier responds by driving the COMP pin high, increasing the switch current. The error

amplifier output is clamped internally. This clamp functions as a switch-current limit. This current limit is fixed at

3.5A (typical).

Valley Current Limit

While the low-side MOSFET is turned on, the conduction current is monitored by the internal circuitry. During

normal operation, the low-side MOSFET sources current to the load. At the end of every clock cycle, the low-side

MOSFET sourcing current is compared to the internally set low-side sourcing current-limit. If the low-side sourcing-

current limit is exceeded, the high-side MOSFET does not turn on and the low-side MOSFET stays on for the next

cycle. The high-side MOSFET turns on again when the low-side current is below the low-side sourcing current-

limit at the start of a cycle.

LED-Open

When the LED load is open, the FB voltage is low, and the internal COMP voltage is driven high and clamped, then

output rises close to the input. At this time, the chip does not have load capacity.

LED Short Protection

When LED+ and LED- are shorted, the output current will be limited to near

starts to drop until FB is below the Under-Voltage (UV) threshold. Once a UV is triggered, the HM2802 enters

hiccup mode to periodically restart the part. At this time, the output voltage is close to VFB.

LED+ Short to GND Protection

low and clamped, and the high-side MOSFET is commanded on for a minimum on-time each cycle. The device

enters fold-back mode, meanwhile, chip reaches internal current limit.

Efficiency and Feedback Voltage

The feedback voltage has a direct effect on the converter efficiency. Because the voltage drop across the feedback

resistor does not contribute to the output power (LED brightness), the lower the feedback voltage, the higher the

efficiency. Especially when powering only three or less LEDs, the feedback voltage impacts the efficiency around

2% depending on the sum of the forward voltage of the LEDs.

Thermal Shutdown

Thermal shutdown prevents the chip from operating at exceedingly high temperatures. When the silicon die

temperature exceeds 160°C, it shuts down the whole chip. When the temperature falls below its lower threshold

(Typ. 140°C) the chip is enabled again.