MIC2155_0911 Datasheet, PDF (19/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
Using the accurate method:
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 _{= 16.55A}
2
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:
MIC2155/2156
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,
RL = 0.01Ω (-40dB), C1=0.1µF and R1=1.5k. The time
constants are equal and diverge at the same rate. The
overall impedance, H(s), equals RL for all frequencies.
50
40
30
L/R
20
10
0
-10
-20
-30 _H(s)
RC
-40
-50
10 100 1k 10k 100k 1M
FREQUENCY (Hz)
Figure 17. Current Sense Gain/Phase Plot
Current Sharing
The schematic in Figure 18 illustrates the current sharing
scheme. The error amplifier in Channel 1, E/A 1,
monitors the output voltage and adjusts the duty cycle of
Channel 1 to regulate that voltage. The inputs of
transconductance error amplifier, E/A 2, are connected
to the current sense points of each channel. The error
amplifier regulates Channel 2 current by monitoring the
current sense point of Channel 1 and forcing the current
sense point of Channel 2 to be equal.
Any offset or difference in current between the two
channels is caused by tolerances in the inductance,
DCR, and tolerances of R1, C1, R2 and C2. Additionally,
voltage offset in E/A 2 may cause variations in output
current sharing. At lower currents, these variations may
force the current of Channel 2 to be 0.
A nominal 10mV offset inhibits the Channel 2 low-side
MOSFET until the output current increases to the
magnitude where the voltage across C1 is 10mV. This
prevents the low-side MOSFET of Channel 2 from
sinking current to ground during startup or during low
current operation.
November 2009
19
M9999-111209-B