MIC2298_11 Datasheet, PDF (8/13 Pages) Micrel Semiconductor – 3.5A Minimum, 1MHz Boost High Brightness White LED Driver

Micrel, Inc.
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.
MIC2298
Hence, a 200mΩ 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
will achieve approximately 1.5ms with a C_COMP value of
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 4. Adjustable OVP Circuit
Brightness Control
Pin Brightness Functionality
BRT Pin
OPEN
GND
V_FB (V)
200mV or V_SS/DIM
40mV
≥20kΩ to 100kΩ
[R_BRT] to GND
(10µA × R_BRT)/5
SS/DIM Pin
OPEN
Voltage v
V_FB (V)
200mV
V_FB = 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:
I_LED = V_FB/R_SENSE
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)
October 2007
8
M9999-101707-B