MIC2155_0911 Datasheet, PDF (24/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
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
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 two-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
MIC2155/2156
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
during the MOSFET’s conduction time. If operating at
4.5V_IN, without connecting V_DD to V_IN, the gate drive
voltage to the high-side FET could be as low as 3.2V.
MOSFETs with low V_GS enhanced gates should be used
in this situation.
It is important to note the on-resistance of a MOSFET
increases at high junction temperature. A 75°C rise in
junction temperature will increase the channel resistance
of the MOSFET by 40% 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_DS and V_GS). The gate charge is supplied by the
MIC2155 gate drive circuit. Gate charge can be a
significant source of power dissipation in the controller
due to the high switching frequencies and generally
large MOSFETs that are driven. At low output load this
power dissipation is noticeable as a reduction in
efficiency.
November 2009
24
M9999-111209-B