MIC2155 Datasheet, PDF (18/33 Pages) Micrel Semiconductor – 2-Phase, Single Output, PWM Synchronous Buck Control IC

Micrel, Inc.
Figure 15. Overcurrent waveform
The equations to accurately calculate the current limit
resistor value are shown below:
I_PK
=
I_OUT
2
+ ^IRIPPLE
2
I_RIPPLE
=
V_OUT × (1− D)
F_S × L
I_SET
= I_PK
−
V_OUT × T_DLY
L
R_CS
=
I_SET
× R_DSON(MAX)
I_CS(MIN)
D = Duty Cycle
F_S = Switching Frequency
L = Power inductor value
T_DLY = Current limit blanking time ~ 100ns
I_CS(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
30A _{× 6mΩ}
R_CS =
2
180μA
= 500Ω
Using the accurate method
MIC2155/2156
D=
V_OUT
= ^3.3 = 0.3
V_IN × Efficiency 12 × 0.9
I_RIPPLE
=
3.3 × (1− 0.3)
500kHz ×1.5μH
=
3.1A
I_PK
=
30
2
+
3.1
2
= 16.55A
I_SET
= 16.55 −
3.3 ×100ns
1.5μH
= 16.33A
R_CS
=
16.33 × 6mΩ
180μA
= 544Ω
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.
Output Inductor and
Winding Resistance
Q2
Figure 16. Lossless Inductor Current Sense
The voltage across capacitor C1 is:
V_S
=
I_L
⎡
⎢
× ⎢RL ×
⎢
sLo _{+ 1}
R_L
⎤
⎥
⎥
sC1× R1+ 1⎥
⎢⎣
⎥⎦
If the R1 × C1 time constant is equal to the Lo/R_L time
constant, the voltage across capacitor C1 equals the R_L
× I_L. Figure 17 is a plot of this equation and shows the
results graphically. It assumes an inductance of 1.5µH,
May 2009
18
M9999-052709-A
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