MIC2155 Datasheet, PDF (23/33 Pages) Micrel Semiconductor – 2-Phase, Single Output, PWM Synchronous Buck Control IC

Micrel, Inc.
ΔV_OPP is the peak to peak output voltage ripple
I_PP is the peak to peak ripple current as see by the
capacitors
f_S is the per channel switching frequency
Notice the calculation is performed at 2x the switching
frequency since the capacitors see ripple current from
both phases.
For this example, using ΔV_OPP = 10mV, the minimum
C_OUT is:
C_OUT
≥
2.3
8 ×10mV × 2 × 500kHz
=
29μF
A capacitance value this low is usually not used in high
current converters because of transient output current
requirements.
For this example, 500µF total capacitance is used. It is
split up into (4) 47µF Ceramic capacitors and (2) 150µF
Aluminum Polymer capacitors
The total output ripple is a combination of the ESR and
the output capacitance. The total ripple is calculated
below:
[ ] ΔV_OUT =
⎡
⎢
⎣
8
×
I_PP
C_OUT ×
2
×
f_S
⎤²
⎥
⎦
+
I_PP
× R_ESR
2
To increase reliability, the recommended voltage rating
of capacitor should be twice the output voltage for a
tantalum and 20% greater for an aluminum electrolytic or
Ceramic.
The output capacitor RMS current is calculated below:
I_COUT(RMS) =
I_PP
12
=
2.3A
12
= 0.66A
The power dissipated in the output capacitors can be
calculated by the equation below:
( ) P_DISS(COUT) = I_COUT(RMS) ² × R_ESR
Inductor Current Sense Components
The RC circuit values that sense current across the
inductor can be calculated once the inductor is selected.
The circuit is shown in Figure 20.
Output Inductor and
Winding Resistance
Q2
Figure 20. Inductor Current Sense
MIC2155/2156
The inductor has the following values:
L = 1.0µH, R _L= 1.9mΩ
Proper sensing of the DC voltage across the inductor
requires the R_L/L time constant be equal to the R1×C1
time constant.
L _{= C1× R1}
R_L
A good range of values for C1 is 0.1µF to 1µF. For this
example C1 is chosen as 0.22µF. R1 is:
R1 = ^L =
1μH
= 2.39k
R_L × C1 1.9mΩ × 0.22μF
Input Capacitor Selection
In addition to high frequency Ceramic capacitors, a
larger bulk capacitance, either Ceramic or Al. El. should
be used to help attenuate ripple on the input and to
supply current to the input during large output current
transients. The input capacitors must be rated for the
RMS input current of the power supply. RMS input
capacitor current is determined at the maximum output
current. The graph in Figure 21 shows the normalized
RMS input ripple current vs. duty cycle. Data is
normalized to the output current.
For a 2 phase converter operating at 17% duty cycle, the
input RMS current is determined from the graph:
I_{CIN _ RMS} ≈ I_OUT × 0.24 = 7.2A
The power dissipated in the input capacitor is:
( ) P_DISS(CIN) = I_CIN(RMS) ² × R_ESR
0.6
Single Phase
0.5
0.4
0.3
2 Phase
0.2
0.1
00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY CYCLE
Figure 21. RMS Input Current vs. Duty Cycle
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 MIC2155 is regulated
by an internal 5V V_DD regulator. Logic level MOSFETs,
whose operation is specified at V_GS = 4.5V must be
used. This resistance is used to calculate the losses
May 2009
23
M9999-052709-A
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