MIC3230 Datasheet, PDF (16/20 Pages) Micrel Semiconductor – Constant Current Boost Controller for Driving High Power LEDs

Micrel, Inc.
MIC3230/1/2
I _{FET _ RMS} =
D_{⎜⎜⎝⎛} I _IN
2
_ AVE
+
I2
L _ PP
12
⎟⎞
⎟⎠
From Equation 26:
t_transition
≈
Qg
Igatedrv
=
68nC
2A
= 34ns
I_{FET _ AVE _ max} = 1.64A
The switching losses occur during the switching
transitions of the FET. The transition times, t_transition, are
the times when the FET is turning off and on. There are
two transition times per period, T. It is important not to
confuse T (the period) with the transition time, t_transition.
V_{OUT _ max} = 28V
From Equation 25:
P_{FET _ SWITCH _ max} = 1.64A × 28V × 34ns × 500kHz = 0.78Watts
From Equation 22
Eq. (24)
T= ¹
Fsw
P_FET = 62mW + 0.78W = 0.84W
This about the limit for a part on a circuit board without
Eq. (25)
having to use any additional heat sinks.
P_{FET _ SWITCH _ max} = I_{FET _ AVE _ max} ×V_{OUT _ max} × t_{transition _ max} × F_SW
To find t_{transition _ max} :
Rectifier Diode
A Schottky Diode is best used here because of the lower
Eq. (26)
t_{transition _ max}
≈
Qg
Igatedrv
forward voltage and the low reverse recovery time. The
voltage stress on the diode is the max V_OUT and
where Qg is the total gate charge of the external
MOSFET provided by the MOSFET manufacturer and
therefore a diode with a higher rating than max V_OUT
should be used. An 80% de-rating is recommended
here as well.
the Qg should chosen at a V_GS≈10V. This is not an
exact value, but is more of an estimate of t_{transition _ max} .
Eq (28)
I_{diode _ RMS _ max} =
(1 −
D )_{⎜⎜⎛} I_IN
⎝
_
AVE
2
_ max
+
I_L
2
_ PP
12
⎟⎞
⎟
⎠
The FET manufacturers’ provide a gate charge at a
specified V_GS voltage:
C_{In _ FET}
=
Q_G
@ V_GS
Eq. (29)
P ≈ V × I _diode
SCHOTTKY diode _ RMS _ max
P_diode ≈ 0.81W
This is the FET’s input capacitance. Select a FET with
R_DS(on) and Q_G such that the external power is below
about 0.7W for a SO-8 or about 1W for a PowerPak
(FET package). The Vishay Siliconix Si7148DP in a
PowerPak SO-8 package is one good choice. The
internal gate driver in the MIC3230/1/2 is 2A. From the
Si7148DP data sheet:
R_{DS(on)_25°C}=0.0145Ω
Total gate Charge=68nC (typical)
The R_DS(on) (temp) is a function of temperature. As the
temperature in the FET increases so does the R_DS(on).
To find R_DS(on) (temp) use Equation 27, or simply
double the R_DS(on) (25^oC) for R_DS(on) (125^oC) .
MIC3230 power losses
The power losses in the MIC3230are:
Eq.(30) P_MIC3230 = Q_gate ×V_gate × F + I_Q ×Vin
where Q_gate is the total gate charge of the external
MOSFET. V_gate is the gate drive voltage of the
MIC3230. F is the switching frequency. I_Q is the
quiescent current of the MIC3230 found in the electrical
characterization table. I_Q = 3.2mA . V_IN is the voltage at
the V_IN pin of the MIC3230. From Eq.(30)
P_MIC3230 = 68nF ×12 × 500kHz + 3.2mA ×14 = 0.45W
Eq. (27) R_DS(on) (temp) = R_DS(on) (25^o C) × (1.007^{(Temp−25o )} )
The R_DS(on) (temp) at 125°C is:
R_DSon (125^o C) = 0.0145 × (1.007^{(125∗ −25o )} ) ≈ 30mΩ
From Equation 23: P_{FET _COND} = 1.64² × 30mΩ = 62mW
January 2009
16
M9999-011409-A