MIC3223 Datasheet, PDF (11/24 Pages) Micrel Semiconductor – High Power Boost LED Driver with Integrated FET

Micrel, Inc.
Application Information
Constant Output Current Converter
The MIC3223 is a peak current mode boost converter
designed to drive high power LEDs with a constant
current output. The MIC3223 operates with an input
voltage range from 4.5V to 20V. In the boost
configuration, the output can be set from V_IN up to 37V.
The peak current mode control architecture of the
MIC3223 provides the advantages of superior line
transient response as well as an easier to design
compensation.
The MIC3223 LED driver 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 below a safe lower limit
Setting the LED Current
The current through the LED string is set via the value
chosen for the current sense resistor R_ADJ which is R5 in
the schematic of the Typical Application. This value can
be calculated using Equation 1:
Eq. (1)
ILED = ^0.2V
R _ADJ
Another important parameter to be aware of in the boost
converter 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. The
equation is shown in Equation 2.
Eq. (2)
ΔI_LED
≈
V_OUTRIPPLE
(R_LED + R_ADJ )
Where
V_OUTRIPPLE
= ^{ILED × D}
C_OUT × F_SW
Reference Voltage
The voltage feedback loop the MIC3223 uses an internal
voltage of 200mV with an accuracy of ±5%. The
feedback voltage is the voltage drop across the current
sense resistor as shown in the Typical Application.
When in regulation the voltage at V_FB will equal 200mV.
MIC3223
Output Over Voltage Protection (OVP)
The MIC3223 provides an OVP circuitry in order to
protect the system from an overvoltage fault condition.
This OVP threshold can be programmed through the
use of external resistors (R3 and R4 in the Typical
Application). 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. Normally use 100k for
R3.
Eq. (3)
R4 =
R3
(V_OVP/1.245) − 1
VDD
An internal linear regulator is used to provide the
necessary internal bias voltages. When V_IN is 6V or
below connect the V_DD pin to V_IN. Use a 10µF ceramic
bypass capacitor.
DRVVDD
An internal linear regulator is used to provide the
necessary internal bias voltages to the gate driver that
drives the external FET. When V_IN is above 6V connect
DRVVDD to VDD.
When V_IN is 6V or below connect the DRVVDD pin to
VIN. Use a bypass capacitor, 10µF ceramic capacitor.
UVLO
Internal under voltage lock out (UVLO) prevents the part
from being used below a safe V_IN voltage. The UVLO is
3.7V. Operation below 4.5V is not recommended.
Soft Start
Soft start is employed to lessen the inrush currents
during turn on. At turn on the following occurs;
1. After about 1.5ms C_SS will start to rise in a
exponential manner according to;
V_SS
=
0.2_{⎜⎜⎜⎝⎛}1
−
e
−t
(37kΩ×C _SS
)
⎟⎞
⎟⎟⎠
2. According to the block diagram, V_SS is the ref
node of the error amp. PWM switching start
when V_SS begins to rise.
3. When the C_SS is fully charged, 0.2V will be at the
error amp reference and steady state operation
begins.
4. Design for soft-start time using the above
equation.
January 2010
11
M9999-011510-A