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| 型號： | MIC2169MM |
| 廠商： | Micrel Semiconductor,Inc. |
| 英文描述： | 500 KHZ PWM SYNCHRONOUS BUCK CONTROL IC |
| 中文描述： | 500千赫的PWM同步降壓控制IC |
| 文件頁數(shù)： | 8/14頁 |
| 文件大?。?/td> | 160K |
| 代理商： | MIC2169MM |
MIC2169
Micrel
M9999-111803
8
November 2003
low-side switches. For applications where V
IN
< 5V, the
internal V
DD
regulator operates in dropout mode, and it is
necessary that the power MOSFETs used are sub-logic level
and are in full conduction mode for V
GS
of 2.5V. For applica-
tions when V
IN
> 5V; logic-level MOSFETs, whose operation
is specified at V
GS
= 4.5V must be used.
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 and in calculating the
value of current-sense (CS) resistor. Total gate charge is the
charge required to turn the MOSFET on and off under
specified operating conditions (V
DS
and V
GS
). The gate
charge is supplied by the MIC2169 gate-drive circuit. At
500kHz switching frequency and above, the gate charge can
be a significant source of power dissipation in the MIC2169.
At low output load, this power dissipation is noticeable as a
reduction in efficiency. The average current required to drive
the high-side MOSFET is:
I
Q
f
G[high-side](avg)
G
S
=
×
where:
I
G[high-side](avg)
= average high-side MOSFET gate
current.
Q
G
= total gate charge for the high-side MOSFET taken from
manufacturer
’
s data sheet for V
GS
= 5V.
The low-side MOSFET is turned on and off at V
DS
= 0
because the freewheeling diode is conducting during this
time. The switching loss for the low-side MOSFET is usually
negligible. Also, the gate-drive current for the low-side
MOSFET is more accurately calculated using CISS at V
DS
=
0 instead of gate charge.
For the low-side MOSFET:
I
C
V
f
G[low-side](avg)
ISS
GS
S
=
×
×
Since the current from the gate drive comes from the input
voltage, the power dissipated in the MIC2169 due to gate
drive is:
(
A convenient figure of merit for switching MOSFETs is the on
resistance times the total gate charge R
DS(ON)
×
Q
G
. Lower
numbers translate into higher efficiency. Low gate-charge
logic-level MOSFETs are a good choice for use with the
MIC2169.
Parameters that are important to MOSFET switch selection
are:
Voltage rating
On-resistance
Total gate charge
The voltage ratings for the top and bottom MOSFET are
essentially equal to the input voltage. A safety factor of 20%
should be added to the V
DS
(max) of the MOSFETs to account
for voltage spikes due to circuit parasitics.
P
V
I
I
GATEDRIVE
IN
G[high-side](avg)
G[low-side](avg)
=
+
)
The power dissipated in the switching transistor is the sum of
the conduction losses during the on-time (P
CONDUCTION
) and
the switching losses that occur during the period of time when
the MOSFETs turn on and off (P
AC
).
P
P
P
SW
CONDUCTION
AC
=
+
where:
P
I
R
CONDUCTION
SW(rms)
SW
2
=
×
P
P
P
AC
AC(off)
AC(on)
=
+
R
SW
= on-resistance of the MOSFET switch
D
duty cycle
V
V
O
IN
=
Making the assumption the turn-on and turn-off transition
times are equal; the transition times can be approximated by:
t
C
V
C
V
I
T
ISS
GS
OSS
IN
G
=
×
+
×
where:
C
ISS
and C
OSS
are measured at V
DS
= 0
I
G
= gate-drive current (1A for the MIC2169)
The total high-side MOSFET switching loss is:
P
(V
V ) I
t
f
AC
IN
PK
T
S
=
+
×
×
where:
t
T
= switching transition time (typically 20ns to 50ns)
V
D
= freewheeling diode drop, typically 0.5V
f
S
it the switching frequency, nominally 500kHz
The low-side MOSFET switching losses are negligible and
can be ignored for these calculations.
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.
L
V
(V max
f
S
×
×
V
)
V max
0.2 I
max
(
OUT
OUT
OUT
=
×
)
)
)
where:
f
S
= switching frequency, 500kHz
0.2 = ratio of AC ripple current to DC output current
V
IN
(max) = maximum input voltage
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