MIC3230 Datasheet, PDF (9/20 Pages) Micrel Semiconductor – Constant Current Boost Controller for Driving High Power LEDs

Micrel, Inc.
MIC3230/1/2
Power Topology
Constant Output Current Controller
The MIC323x family are peak current mode boost
controllers designed to drive high power LEDs. Unlike a
standard constant output voltage controller, the MIC323x
family has been designed to provide a constant output
current. The MIC323x family is designed for a wide input
voltage range, from 6V to 45V. In the boost configuration,
the output can be set from V_IN up to 100V.
As a peak current mode controller, the MIC323x family
provides the benefits of superior line transient response as
well as an easier to design compensation.
This family of LED drivers features a built-in soft-start
circuitry in order to prevent start-up surges. Other
protection features include:
• Current Limit (I_LIMIT) - Current sensing for over current
and overload protection
• Over Voltage Protection (OVP) - Output over voltage
protection to prevent operation above a safe upper
limit
• Under Voltage Lockout (UVLO) – UVLO designed to
prevent operation at very low input voltages
Setting the LED Current
The current through the LED string is set via the value
chosen for the current sense resistor, R_ADJ. This value can
be calculated using Equation 1:
Eq. (1)
I_LED
=
0.25V
R_ADJ
Another important parameter to be aware of in the boost
controller design, is the ripple current. The amount of
ripple current through the LED string is equal to the output
ripple voltage divided by the LED AC resistance (R_LED –
provided by the LED manufacturer) plus the current sense
resistor (R_ADJ). The amount of allowable ripple through the
LED string is dependent upon the application and is left to
the designer’s discretion. This equation is shown in
Equation 2:
Eq. (2)
ΔI_LED
≈
V_OUTRIPPLE
(R_LED + R_ADJ )
Where
V_OUTRIPPLE
=
I _LED × D × T
C_OUT
Reference Voltage
The voltage feedback loop of the MIC323x uses an
internal reference voltage of 0.25V with an accuracy of
±3%. The feedback voltage is the voltage drop across the
current setting resistor (R_ADJ) as shown in Figure 1. When
in regulation the voltage at I_ADJ will equal 0.25V.
Output Over Voltage Protection (OVP)
The MIC323x provides an OVP circuitry in order to help
protect the system from an overvoltage fault condition.
This OVP point can be programmed through the use of
external resistors (R8 and R9 in Figure 1). A reference
value of 1.245V is used for the OVP. Equation 3 can be
used to calculate the resistor value for R9 to set the OVP
point.
Eq. (3)
R9 =
R8
(V_OVP /1.245) − 1
LED Dimming
The MIC323x family of LED drivers can control the
brightness of the LED string via the use of pulse width
modulated (PWM) dimming. A PWM input signal of up to
500Hz can be applied to the PWM DIM pin (see Figure 1)
to pulse the LED string ON and OFF. It is recommended
to use PWM dimming signals above 120Hz to avoid any
recognizable flicker by the human eye. PWM dimming is
the preferred way to dim a LED in order to prevent
color/wavelength shifting, as occurs with analog dimming.
The output current level remains constant during each
PWMD pulse.
Oscillator and Switching Frequency Selection
The MIC323x family features an internal oscillator that
synchronizes all of the switching circuits internal to the IC.
This frequency is adjustable on the MIC3230 and MIC3231
and fixed at 400kHz in the MIC3232.
In the MIC3230/1, the switching frequency can be set by
choosing the appropriate value for the resistor, R₁
according to Equation 4:
Eq. (4)
R_FS
(kΩ)
=
⎜⎜⎝⎛
7526
F_SW (kHz)
⎟⎟⎠⎞^1.035
SYNC (MIC3230 Only)
Multiple MIC3230 ICs can be synchronized by connecting
their SYNC pins together. When synchronized, the
MIC3230 with the highest frequency (master) will override
the other MIC3230s (slaves). The internal oscillator of the
master IC will override the oscillator of the slave part(s)
and all MIC3230 will be synchronized to the same master
switching frequency.
The SYNC pin is designed to be used only by other
MIC3230s and is available on the MIC3230 only. If the
SYNC pin is being unused, it is to be left floating (open).
In the MIC3231, the SYNC pin is to be left floating (open).
January 2009
9
M9999-011409-A