MIC2287 Datasheet, PDF (7/10 Pages) Micrel Semiconductor – 1.2MHz PWM White LED Driver with OVP in 2mm × 2mm MLF™ and Thin SOT-23

MIC2287
Dimming Control
There are two techniques for dimming control. One is PWM
dimming, and the other is continuous dimming.
1. PWM dimming control is implemented by applying
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.
2. Continuous dimming control is implemented by
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_IN
Micrel
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.
V_IN
VIN SW
EN
FB
GND OVP
Figure 3. MLF™ Package OVP Circuit
V_IN
PWM
VIN SW
EN
FB
GND
VIN SW
EN
FB
GND
5.11k
Figure 1. PWM Dimming Method
V_IN
VIN SW
EN
FB
GND
5.11k
49.9k
DC
Equivalent
Figure 2. 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 × 2mm MLF™ with
overvoltage protection (OVP). The extra pins of the 8-lead
2mm × 2mm MLF™ package allow a dedicated OVP monitor
Figure 4. Thin SOT-23 Package OVP Circuit
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_SS, provides over-
drive to the FB pin at start-up, resulting in gradual increase of
switch duty cycle and limited inrush current.
V_IN
C_SS
2200pF
VIN SW
EN
FB
R
GND
10k
Figure 5. Soft-Start Circuit
August 2004
7
M0510-081104