MIC3223 Datasheet, PDF (16/24 Pages) Micrel Semiconductor – High Power Boost LED Driver with Integrated FET

Micrel, Inc.
MIC3223
Eq. (9)
P_INDUCTOR = I_{IN_RMS(max)2} × DCR
A Coilcraft # MSS1260-223ML is used in this example. Its
DCR is 52mΩ, I_SAT =2.7A
P_INDUCTOR = 1.54² × 52 mΩ = 0.123W
Output Capacitor
In this LED driver application, the I_LED ripple current is a
more important factor when compared to that of the output
ripple voltage (although the two are directly related). To
find the C_OUT for a required I_LED ripple use the following
calculation:
For an output ripple I_LED(ripple) = 20ma
Eq. (10)
C _OUT
=
I_LED(nom) × D_nom
I_LED(ripple) × (R _ADJ + R_{LED_total} ) × F_SW
Find the equivalent ac resistance R_{LED_ac} from the
datasheet of the LED. This is the inverse slope of the I_LED
vs. V_f curve i.e.:
Eq. (11)
R_{LED_ac}
=
ΔV_f
ΔLED
In this example use R_{LED_ac} = 0.6Ω for each LED.
If the LEDs are connected in series, multiply R_{LED_ac} = 0.6Ω
by the total number of LEDs. In this example of six LEDs,
we obtain the following:
Eq. (12)
R_{LED_total} ≡ R_dynamic = 6 × 0.6Ω = 3.6Ω
C _OUT
=
I_LED(nom) × D_nom
I_LED(ripple) × (R _ADJ + R_{LED_total} ) × F_SW
= 1.9μF
Use 2.2µF or higher.
There is a trade off between the output ripple and the
rising edge of the DIM_IN pulse. This is because between
PWM dimming pulses, the converter stops pulsing and
C_OUT will start to discharge. The amount that C_OUT will
discharge depends on the time between PWM Dimming
pluses. At the next DIM_IN pulse, C_OUT has to be charged
up to the full output voltage V_OUT before the desired LED
current flows.
Input Capacitor
The input capacitor is shown in the Typical Application.
For superior performance, ceramic capacitors should be
used because of their low equivalent series resistance
(ESR). The input capacitor C_IN ripple current is equal to the
ripple in the inductor. The ripple voltage across the input
capacitor, C_IN is the ESR of C_IN times the inductor ripple.
The input capacitor will also bypass the EMI generated by
the converter as well as any voltage spikes generated by
the inductance of the input line. For a required V_IN(ripple):
Eq. (13)
C_IN =
I_{IN_PP}
V_IN(ripple) × F_SW
=
(0.3A)
= 0.75μF
8 × 50mV × 1MHz
This is the minimum value that should be used. To protect
the IC from inductive spikes or any overshoot, a larger
value of input capacitance may be required.
Use 2.2µF or higher as a good safe min.
Rectifier Diode Selection
A schottky diode is best used here because of the lower
forward voltage and the low reverse recovery time. The
voltage stress on the diode is the max V_OUT and therefore
a diode with a higher rating than max V_OUT should be used.
An 80% de-rating is recommended here as well.
Eq. (14)
I_DIODE(max) = I_OUT(max) = 0.36A
Since I_{IN_AVE(max)} occurs when D is at a maximum.
Eq. (15)
P_DIODE(max) ≈ V_DIODE × I_{DIODE_(max)}
A SK35B is used in this example, it’s V_DIODE is 0.5V
P_DIODE(max) ≈ 0.5V × 0.36A = 0.18W
MIC3223 Power Losses
To find the power losses in the MIC3223:
There is about 6mA input from V_IN into the V_DD pin.
The internal power switch has an RDS_ON of about 170mΩ
at.
P_MIC3223 = V_IN × 6mA + PwrFET
Eq. (16)
PwrFET = I_{FET_RMS(max)2} × R_{ds_on_@100°}
+ V_OUT(max) × I_{IN_AVE(max)} × tsw × Fsw
R_{ds_on_@100°} ≈ 160mΩ
tsw ≈ 30ns is the internal Power FET ON an OFF
transition time.
I = _SWRMS(max)
D_{⎜⎜⎛}I_{IN_AVE(max)} ²
⎝
+
I_{L_PP} ²
12
⎟⎞
⎟
⎠
= 1.3A
PwrFET = 1.3A² × 160mΩ + 28V × 1.54A × 30ns
× 1MHz = 1.6W
P_MIC3223 = 8 × 6mA + 1.77W = 1.66W
Snubber
A snubber is a damping resistor in series with a DC
blocking capacitor in parallel with the power switch (same
as across the flyback diode because V_OUT is an ac
ground). When the power switch turns off, the drain to
source capacitance and parasitic inductance will cause a
high frequency ringing at the switch node. A snubber
circuit as shown in the application schematic may be
required if ringing is present at the switch node. A critically
damped circuit at the switch node is where R equals the
characteristic impedance of the switch node.
January 2010
16
M9999-011510-A