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MIC2155 Datasheet(PDF) 18 Page - Micrel Semiconductor

Part # MIC2155
Description  2-Phase, Single Output, PWM Synchronous Buck Control IC
PDF  33 Pages
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Manufacturer  MICREL [Micrel Semiconductor]
Direct Link  http://www.micrel.com
Logo MICREL - Micrel Semiconductor

MIC2155 Datasheet(HTML) 18 Page - Micrel Semiconductor

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Micrel, Inc.
MIC2155/2156
May 2009
18
M9999-052709-A
(408) 944-0800
Figure 15. Overcurrent waveform
The equations to accurately calculate the current limit
resistor value are shown below:
2
I
2
I
I
RIPPLE
OUT
PK
+
=
L
F
)
D
1
(
V
I
S
OUT
RIPPLE
×
−
×
=
L
T
V
I
I
DLY
OUT
PK
SET
×
−
=
)
MIN
(
CS
)
MAX
(
DSON
SET
CS
I
R
I
R
×
=
D = Duty Cycle
FS = Switching Frequency
L = Power inductor value
TDLY = Current limit blanking time ~ 100ns
ICS(min) = 180µA
Example:
Consider a 12V to 3.3V @ 30A converter with 1.5µH
power inductor and 90% efficiency at full load. Each
channel will supply 15A at a 500kHz (MIC2155)
switching frequency. The on-resistance of the low side
MOSFET is 6mΩ.
Using the simple method
Ω
=
μ
Ω
×
=
500
A
180
m
6
2
A
30
RCS
Using the accurate method
Ω
=
μ
Ω
×
=
=
μ
×
−
=
=
+
=
=
μ
×
−
×
=
=
×
=
×
=
544
A
180
m
6
33
.
16
R
A
33
.
16
H
5
.
1
ns
100
3
.
3
55
.
16
I
A
55
.
16
2
1
.
3
2
30
I
A
1
.
3
H
5
.
1
kHz
500
)
3
.
0
1
(
3
.
3
I
3
.
0
9
.
0
12
Efficiency
V
D
CS
SET
PK
RIPPLE
IN
OUT
3
.
3
V
Using the simple method here would result in a current
limit point lower than expected.
This equation sets the minimum current limit point of the
converter, but maximum will depend on the actual
inductor value and on resistance of the MOSFET under
current limit conditions. This could be in the region of
50% higher and should be considered to ensure that all
the power components are within their thermal limits
unless thermal protection is implemented separately.
Inductor Current Sensing
Current sharing between the two phases is achieved by
sensing the inductor current in each phase. Lossless
inductor current sensing is used, which has the
advantages of lower power loss and lower cost – over
using a discrete resistor in series with the inductor.
The inductor sense circuit is shown in Figure 16. It
extracts the voltage drop across the inductor’s DC
winding resistance.
Q2
Output Inductor and
Winding Resistance
Figure 16. Lossless Inductor Current Sense
The voltage across capacitor C1 is:
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎣
⎡
+
×
+
×
×
=
1
1
R
1
sC
1
R
sLo
RL
I
V
L
L
S
If the R1 × C1 time constant is equal to the Lo/RL time
constant, the voltage across capacitor C1 equals the RL
× IL. Figure 17 is a plot of this equation and shows the
results graphically. It assumes an inductance of 1.5µH,



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