MIC2199 MICREL [Micrel Semiconductor], MIC2199 Datasheet - Page 10

• Manufacturer Part Number: MIC2199
• Manufacturer: MICREL [Micrel Semiconductor]

MIC2199
Applications Information
Following applications information includes component se-
lection and design guidelines.
Inductor Selection
Values for inductance, peak, and RMS currents are required
to select the output inductor. The input and output voltages
and the inductance value determine the peak-to-peak induc-
tor ripple current. Generally, higher inductance values are
used with higher input voltages. Larger peak-to-peak ripple
currents will increase the power dissipation in the inductor
and MOSFETs. Larger output ripple currents will also require
more output capacitance to smooth out the larger ripple
current. Smaller peak-to-peak ripple currents require a larger
inductance value and therefore a larger and more expensive
inductor. A good compromise between size, loss and cost is
to set the inductor ripple current to be equal to 20% of the
maximum output current.
The inductance value is calculated by the equation below.
where:
The peak-to-peak inductor current (AC ripple current) is:
The peak inductor current is equal to the average output
current plus one half of the peak-to-peak inductor ripple
current.
The RMS inductor current is used to calculate the I
in the inductor.
Maximizing efficiency requires the proper selection of core
material and minimizing the winding resistance. The high
frequency operation of the MIC2199 requires the use of ferrite
materials for all but the most cost sensitive applications.
Lower cost iron powder cores may be used but the increase
in core loss will reduce the efficiency of the power supply. This
is especially noticeable at low output power. The winding
resistance decreases efficiency at the higher output current
levels. The winding resistance must be minimized although
this usually comes at the expense of a larger inductor.
The power dissipated in the inductor is equal to the sum of the
core and copper losses. At higher output loads, the core
losses are usually insignificant and can be ignored. At lower
output currents, the core losses can be a significant contribu-
MIC2199
f
0.2 = ratio of AC ripple current to DC output current
V
L
I
I
I
S
PP
PK
INDUCTOR(rms)
IN(max)
=
= switching frequency
=
=
V
IN(max)
I
V
V
OUT(max)
OUT
OUT
= maximum input voltage
V
IN(max)
×
×
×
(V
(V
f
S
+
=
IN(max)
IN(max)
×
I
0.5 I
OUT(max)
0.2 I
×
f
×
S
×
−
−
PP
×
OUT(max)
V
V
L
OUT
OUT
×
1
)
)
+
1
3
⎛
⎜
⎝
I
OUT(max)
I
P
2
×R losses
⎞
⎟
⎠
2
10
tor. Core loss information is usually available from the mag-
netics vendor.
Copper loss in the inductor is calculated by the equation
below:
The resistance of the copper wire, R
temperature. The value of the winding resistance used should
be at the operating temperature.
where:
Current-Sense Resistor Selection
Low inductance power resistors, such as metal film resistors
should be used. Most resistor manufacturers make low
inductance resistors with low temperature coefficients, de-
signed specifically for current-sense applications. Both resis-
tance and power dissipation must be calculated before the
resistor is selected. The value of R
the maximum output current and the maximum threshold
level. The power dissipated is based on the maximum peak
output current at the minimum overcurrent threshold limit.
The maximum overcurrent threshold is:
The maximum power dissipated in the sense resistor is:
MOSFET Selection
External N-Channel logic-level power MOSFETs must be
used for the high- and low-side switches. The MOSFET gate-
to-source drive voltage of the MIC2199 is regulated by an
internal 5V V
operation is specified at V
It is important to note the on-resistance of a MOSFET
increases with increasing temperature. A 75°C rise in junc-
tion temperature will increase the channel resistance of the
MOSFET by 50% to 75% of the resistance specified at 25°C.
This change in resistance must be accounted for when
calculating MOSFET power dissipation.
Total gate charge is the charge required to turn the MOSFET
on and off under specified operating conditions (V
V
circuit. At 500kHz switching frequency, the gate charge can
be a significant source of power dissipation in the MIC2199.
At low output load this power dissipation is noticeable as a
R
GS
WINDING(hot)
). The gate charge is supplied by the MIC2199 gate drive
T
T
R
(usually specified by the manufacturer)
P
R
I
P
OVERCURRENT(max)
INDUCTORCu
HOT
20°C
D(R
WINDING(20°C)
SENSE
SENSE
= temperature of the wire under operating load
= ambient temperature
=
DD
=
I
)
R
OUT(max)
=
55mV
WINDING(20 C)
regulator. Logic-level MOSFETs, whose
I
OVERCURRENT(max)
=
is room temperature winding resistance
I
INDUCTOR(rms)
=
95mV
R
GS
CS
°
= 4.5V must be used.
× +
(
1 0.0042 (T
SENSE
WINDING
2
×
2
R
×
is chosen based on
WINDING
R
CS
November 2004
×
, increases with
HOT
−
DS
Micrel
T
20 C
and
°
)
)