MIC3230_1 Datasheet, PDF (16/19 Pages) Micrel Semiconductor – Constant Current Boost Controller for Driving High Power LEDs

Micrel, Inc.
MIC3230/1/2
provided by the MOSFET manufacturer and 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} .
The FET manufacturers’ provide a gate charge at a
specified V_GS voltage:
C_{In _ FET}
= ^QG
@V_GS
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) .
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
From Equation 26:
t_transition
≈ ^Qg
Igatedrv
=
68nC
2A
= 34ns
I_{FET _ AVE _ max} = 1.64A
V_{OUT _ max} = 28V
From Equation 25:
P_{FET _ SWITCH _ max} = 1.64A × 28V × 34ns × 500kHz = 0.78Watts
From Equation 22
P_FET = 62mW + 0.78W = 0.84W
This is about the limit for a part on a circuit board without
having to use any additional heat sinks.
Rectifier Diode
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 (28)
Eq. (29)
I_{diode _ RMS _ max} =
(1 −
D )_{⎜⎜⎛} I_IN
⎝
_
AVE
_
2
max
+
I_L
2
_ PP
12
⎟⎞
⎟
⎠
P ≈ V × I _diode
SCHOTTKY diode _ RMS _ max
P_diode ≈ 0.81W
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
OVP-Over voltage protection
Set OVP higher than the maximum output voltage by at least
one volt. To find the resistor divider values for OVP use
Equation 3 and set the OVP=30V and R8=100kΩ:
R9 = ^{100kΩ ×1.245} = 4.33kΩ
30 − 1.245
PCB Layout
1. All typologies of DC-to-DC converters have a reverse
recovery current (RRC) of the flyback or (freewheeling)
diode. Even a Schottky diode, which is advertised as having
zero RRC, it really is not zero. The RRC of the freewheeling
diode in a boost converter is even greater than in the Buck
converter. This is because the output voltage is higher than
the input voltage and the diode has to charge up to –V_OUT
during each on-time pulse and then discharge to V_F during
the off-time.
2. Even though the RRC is very short (tens of nanoseconds)
the peak currents are high (multiple amperes). The high
RRC causes a voltage drop on the ground trace of the PCB
and if the converter control IC is referenced to this voltage
drop, the output regulation will suffer.
3. It is important to connect the IC’s reference to the same
point as the output capacitors to avoid the voltage drop
caused by RRC. This is also called a star connection or
single point grounding.
4. Feedback trace: The high impedance traces of the FB
should be short.
November 2009
16
M9999-113009-B