ML4895 Datasheet, PDF (7/10 Pages) Micro Linear Corporation – Synchronous Buck Controller

I_L
VIN – VOUT
L
V_OUT
L
ILPK
∆IL
IOUT = ISENSE + 1/2 ∆IL
ISENSE =
VSENSE
RSENSE
TON
TOFF
t
Figure 5. Buck Regulator Inductor Current
ML4895
For reliable operation, the inductor current rating should
exceed the value calculated by 10%-20%.
For future reference, determine the peak inductor current
at the minimum input voltage:
I_L(PEAK(MIN)) = I_SENSE(MIN) + ∆I_L(MIN)
(8)
c a fh ^{a f a f I}L PEAK MIN
= ^{VSENSE MIN}
R _SENSE
+ ∆I_{L MIN}
c a fh a f ^IL PEAK MIN
=
0.06V
R _SENSE
+ ∆I_{L MIN}
ON resistance - gate charge product provides a good
figure of merit by which to compare various MOSFETs,
the lower the figure the better. The internal gate drivers of
the ML4895 can drive over 100nC of total gate charge,
but 60nC to 70nC is a more practical limit to ensure good
switching times.
The drain-source breakdown voltage rating is determined
by the input voltage. For input voltages up to 10V, a drain
to source rating of 20V is acceptable. For input voltages
up to 15V, a drain to source rating of 30V is
recommended. For a more reliable design, look for
MOSFETs that are avalanche rated.
Now the sense resistor’s power rating can be determined.
The sense resistor must be able to carry the peak current
in the inductor during the OFF-time:
P_RSENSE = I_RMS(OFF) ² × R _SENSE
(9)
where:
In high current applications, the MOSFET’s power
dissipation often becomes a major design factor. The I²R
losses generate the largest portion of heat in the MOSFET
package. Make sure that the MOSFETs are within their
rated junction temperature at the maximum ambient
temperature by calculating the temperature rise using the
thermal resistance specifications.
FG IJ _{I 2}
H K ^RMS(OFF)
=
V
1− ^OUT
V_{IN( MAX)}
×
I2
SENSE(MAX )
+ I_SENSE(MAX)
× I_L(PEAK(MAX))
+
I²
L( PEAK( MAX))
3
The worst case power dissipation for the P-MOS switch
occurs at the minimum input voltage and is determined
as follows:
The final parameter that should be specified is the
winding resistance of the inductor. In general, the
P_P−MOS = I_RMS(ON)2 × R_DS(ON)
(11)
winding resistance should be as low as possible,
preferably in the low mΩ range. Since the inductor is in
where:
series with the load at all times, the copper losses can be
approximated by:
FHG IKJ _{IRMS(ON) 2 =}
V_OUT
V_{IN( MAX )}
×
I2
SENSE( MAX)
+ I_{SENSE( MAX )}
× I_{L(PEAK( MAX ))}
+
I²
L( PEAK( MAX ))
3
P_Cu = I_OUT ² × R_L
(10)
The worst case power dissipation for the N-MOS switch
occurs at the maximum input voltage and is determined
A good rule of thumb is to allow 2 mΩ of winding
using:
resistance per µH of inductance.
P_N−MOS = I_RMS(OFF)2 × R_DS(ON)
(12)
MOSFET SELECTION
The switching MOSFETs must be logic level types with
the ON resistance specified at V_GS = 4.5V. In general, the
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