Micrel, Inc.

MIC2298

October 2007

8

M9999-101707-B

Over-Voltage Protection

The MIC2298 offers over-voltage protection functionality.

If an LED is disconnected from the circuit or the

feedback pin is shorted to ground, the feedback pin will

fall to ground potential. This will cause the MIC2298 to

switch at full duty cycle in an attempt to maintain the

feedback voltage. As a result, the output voltage will

climb out of control. This may cause the switch node

voltage to exceed its maximum voltage rating, possibly

damaging the IC and the external components. To

ensure the highest level of protection, the MIC2298 OVP

pin will shut the switch off when an over-voltage

condition is detected, saving itself and the output

capacitor from damage. The OVP threshold can be

increased by adding a resistor divider between the

output and ground. Be careful not to exceed the 30V

rating of the switch.

Figure 4. Adjustable OVP Circuit

Brightness Control

Pin Brightness Functionality

BRT Pin

OPEN

GND

40mV

≥20kΩ to 100kΩ

SS/DIM Pin

OPEN

200mV

Voltage v

Table 1. BRT and SS/DIM Brightness Control Functionality

The

MIC2298

has

built

in

brightness/dimming

functionally for white LED applications. The BRT and

SS/DIM pins are available for brightness/dimming control

functionality. Table 1 illustrates the different modes of

dimming functionality afforded by the BRT and SS/DIM

pins. The resulting LED current is then calculated as:

Hence, a 200m

Ω sense resistor will achieve nominally

1A when both DIM and BRT pins are left open.

PWM control of brightness

A control signal can be driven into the enable pin to vary

average current through the LED for applications not

sensitive to low frequency (~100Hz) light modulation.

For such applications, the SS/DIM pin capacitance

should be minimized to achieve a fast turn on time. 0nF

33nF.

For other applications, where no analog control voltage

is available, the BRT pin can be driven through a low

pass filter (18kΩ and 470nF) at a PWM frequency of

>5kHz to set the FB voltage, and therefore, the LED

current from 20% to 100% of Nominal LED current

(Figure 5).

Figure 5. High Frequency PWM Programming

Via BRT Pin

Since the DIM pin is typically utilized for soft start, it is

recommended to use the enable and BRT pins for the

PWM method of adjusting the average LED current.

Figures 6 and 7 below show typical results for this

method.

Figure 6. LED Current vs. DC (on enable pin)