MIC3263 Datasheet, PDF (19/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
MIC3263
Slope Compensation
The boost stage uses peak current mode and requires
slope compensation. Slope compensation is required to
maintain internal stability of the boost stage across all duty
cycles and to prevent any unstable oscillations. The
MIC3263 uses a combination of internal slope
compensation and a additional slope compensation that is
set by an external resistor, RSLP. The ability to set the
proper slope compensation through the use of a single
external component results in design flexibility. This slope
compensation resistor, RSLP, can be calculated as
follows:
Figure 11. Boost Stage
The operating duty cycle can be calculated using the
equation provided below:
D = ^{(VOUT - eff × VIN ) and D′ = 1 − D}
V_OUT
Find L using the following equation:
L = ^{VIN ×D}
I_{L_PP} ×Fsw
I_{L_PP} is the inductor peak-to-peak ripple current.
Use a I_{L_PP} of 20% to 40% of the total load current. F_SW is
the boost switching frequency.
Output Capacitor
In a boost converter, to find the C_OUT for a given V_OUT
ripple use the following calculation:
RSLP
=
V_OUT(MAX)
8.64 ×10-6
- L ×F_sw
× V_IN(MIN)
where V_IN(MAX) and V_OUT(MAX) can be selected to system
specifications. The lowest value of RSLP should be 15kΩ.
Calculate RSLP using the lowest V_IN and maximum V_OUT
the system will operate.
Example: For these operating conditions:
V_IN(MIN) = 12V, V_OUT(MAX) = 32V, L = 22μH,
F_SW = 1MHz
RSLP
=
32V - 22μH×1Mhz
8.64 ×10-6 ×12V
=
96.5kΩ
Use the next highest standard value.
Table 3 compiles and lists RSLP values for one set of
operating conditions. Select RSLP for V_{IN_MIN} and V_{O_MAX}.
C_OUT
=
I_LEDtotal × D
V_RIPPLE ×Fsw
V_RIPPLE can usually be kept below 50mV:
I_{LED_TOTAL} = 6 × 30mA = 180mA
In the MIC3263, the LED current in each channel is
individually regulated by that channels current amplifier
(linear current regulator). These current regulators are fast
enough to follow the boost output voltage ripple and to
keep the LED ripple currents much lower than C_OUT can
filter the output ripple voltage.
January 2010
19
M9999-012110