MIC2289 Micrel Semiconductor, MIC2289 Datasheet - Page 7

• Manufacturer Part Number: MIC2289
• Description: 2mm 2mm White LED Driver with Internal Schottky Diode and OVP
• Manufacturer: Micrel Semiconductor

Dimming Control
There are two techniques for dimming control. One is PWM
dimming, and the other is continuous dimming.
August, 2004
MIC2289
1. PWM dimming control is implemented by applying
2. Continuous dimming control is implemented by
a PWM signal on EN pin as shown in Figure 1. The
MIC2289 is turned on and off by the PWM signal.
With this method, the LEDs operate with either
zero or full current. The average LED current is
increased proportionally to the duty-cycle of the
PWM signal. This technique has high-efficiency
because the IC and the LEDs consume no current
during the off cycle of the PWM signal. Typical
frequency should be between 100Hz and 10kHz.
applying a DC control voltage to the FB pin of the
MIC2289 through a series resistor as shown in
Figure 2. The LED current is decreased propor-
tionally with the amplitude of the control voltage.
The LED intensity (current) can be dynamically
varied applying a DC voltage to the FB pin. The DC
voltage can come from a DAC signal, or a filtered
PWM signal . The advantage of this approach is
that a high frequency PWM signal (>10kHz) can be
used to control LED intensity.
V
IN
V
IN
Figure 1. PWM Dimming Method
Figure 2. Continuous Dimming
PWM
VIN
EN
VIN
EN
GND
OUT
GND
SW
FB
Equivalent
OUT
SW
FB
DC
49.9k
5.11k
7
Open-Circuit Protection
If the LEDs are disconnected from the circuit, or in case an
LED fails open, the sense resistor will pull the FB pin to
ground. This will cause the MIC2289 to switch with a high
duty-cycle, resulting in output overvoltage. This may cause
the SW pin voltage to exceed its maximum voltage rating,
possibly damaging the IC and the external components. To
ensure the highest level of protection, the MIC2289 has 3
product options in the 2mm 2mm MLF™-8 with overvoltage
protection, OVP. The extra pins of the 2mm
MLF™-8 package allow a dedicated OVP monitor with op-
tions for 15V, 24V, or 34V (see Figure 3). The reason for the
three OVP levels is to let users choose the suitable level of
OVP for their application. For example, a 3-LED application
would typically see an output voltage of no more than 12V, so
a 15V OVP option would offer a suitable level of protection.
This allows the user to select the output diode and capacitor
with the lowest voltage ratings, therefore smallest size and
lowest cost. The OVP will clamp the output voltage to within
the specified limits.
Start-Up and Inrush Current
During start-up, inrush current of approximately double the
nominal current flows to set up the inductor current and the
voltage on the output capacitor. If the inrush current needs to
be limited, a soft-start circuit similar to Figure 4 could be
implemented. The soft-start capacitor, C
drive to the FB pin at start-up, resulting in gradual increase of
switch duty cycle and limited inrush current.
V
IN
Figure 3. MLF™ Package OVP Circuit
V
Figure 4. One of Soft-Start Circuit
IN
VIN
EN
GND
OUT
SW
VIN
EN
FB
GND
OUT
SW
FB
C
SS
R
2200pF
10k
SS
, provides over-
M9999-081104
Micrel
2mm