MIC2287-15BML MICREL [Micrel Semiconductor], MIC2287-15BML Datasheet - Page 7

• Manufacturer Part Number: MIC2287-15BML
• Manufacturer: MICREL [Micrel Semiconductor]

Dimming Control
There are two techniques for dimming control. One is PWM
dimming, and the other is continuous dimming.
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 MIC2287 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 MIC2287 has 3
product options in the 8-lead 2mm
overvoltage protection (OVP). The extra pins of the 8-lead
2mm
August 2004
MIC2287
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
MIC2287 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
PWM frequency should be between 100Hz and
10kHz.
applying a DC control voltage to the FB pin of the
MIC2287 through a series resistor as shown in
Figure 2. The LED intensity (current) can be dy-
namically 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 a high frequency PWM signal (>10kHz)
that can be used to control LED intensity.
V
2mm MLF™ package allow a dedicated OVP monitor
IN
V
IN
Figure 1. PWM Dimming Method
Figure 2. Continuous Dimming
PWM
VIN
EN
GND
VIN
EN
SW
FB
GND
Equivalent
SW
FB
DC
49.9k
5.11k
2mm MLF™ with
7
with options 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, as well as smallest
size and lowest cost. The OVP will clamp the output voltage
to within the specified limits. For the Thin SOT-23-5 package,
an OVP pin is not available. An external zener diode can be
connected from the output of the converter to FB pin as shown
in Figure 4. to implement similar protection.
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 5 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.
Figure 4. Thin SOT-23 Package OVP Circuit
V
V
IN
IN
V
Figure 3. MLF™ Package OVP Circuit
IN
Figure 5. Soft-Start Circuit
VIN
EN
VIN
EN
GND
GND
VIN
EN
GND
SW
FB
SW
FB
OVP
SW
FB
2200pF
C
10k
SS
R
5.11k
SS
, provides over-
M0510-081104
Micrel