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MIC2182 Datasheet, PDF (18/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller Final Information
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MIC2182
For the low-side switch (N-channel MOSFET), the dc power
dissipation is:
Pswitch2(dc) = RDS(on)2 × ISW 2(rms)2
Since the ac switching losses for the low side MOSFET is
near zero, the total power dissipation is:
Plow-side MOSFET(max) = Pswitch2(dc)
The total power dissipation for the high side MOSFET is:
PhighsideMOSFET(max) = PSWITCH 1(dc) + PAC
External Schottky Diode
An external freewheeling diode is used to keep the inductor
current flow continuous while both MOSFETs are turned off.
This dead time prevents current from flowing unimpeded
through both MOSFETs and is typically 80ns The diode
conducts twice during each switching cycle. Although the
average current through this diode is small, the diode must be
able to handle the peak current.
ID(avg) = IOUT × 2 × 80ns × fS
The reverse voltage requirement of the diode is:
Vdiode(rrm) = VIN
The power dissipated by the Schottky diode is:
Pdiode = ID(avg) × VF
where:
VF = forward voltage at the peak diode current
The external Schottky diode, D2, is not necessary for circuit
operation since the low-side MOSFET contains a parasitic
body diode. The external diode will improve efficiency and
decrease high frequency noise. If the MOSFET body diode is
used, it must be rated to handle the peak and average current.
The body diode has a relatively slow reverse recovery time
and a relatively high forward voltage drop. The power lost in
the diode is proportional to the forward voltage drop of the
diode. As the high-side MOSFET starts to turn on, the body
diode becomes a short circuit for the reverse recovery period,
dissipating additional power. The diode recovery and the
TIME
Figure 12. Switch Output Noise
With and Without Shottky Diode
Micrel
circuit inductance will cause ringing during the high-side
MOSFET turn-on.
An external Schottky diode conducts at a lower forward
voltage preventing the body diode in the MOSFET from
turning on. The lower forward voltage drop dissipates less
power than the body diode. The lack of a reverse recovery
mechanism in a Schottky diode causes less ringing and less
power loss. Depending on the circuit components and oper-
ating conditions, an external Schottky diode will give a 1/2%
to 1% improvement in efficiency. Figure 12 illustrates the
difference in noise on the VSW pin with and without a
Schottky diode.
Output Capacitor Selection
The output capacitor values are usually determined by the
capacitors ESR (equivalent series resistance). Voltage rating
and RMS current capability are two other important factors in
selecting the output capacitor. Recommended capacitors are
tantalum, low-ESR aluminum electrolytics, and OS-CON.
The output capacitor’s ESR is usually the main cause of
output ripple. The maximum value of ESR is calculated by:
RESR
≤
∆VOUT
IPP
where:
VOUT = peak to peak output voltage ripple
IPP = peak to peak inductor ripple current
The total output ripple is a combination of the ESR and the
output capacitance. The total ripple is calculated below:
( ) ∆VOUT =


IPP × (1−
COUT ×
D)
fS 
2
+
IPP
× RESR
2
where:
D = duty cycle
COUT = output capacitance value
fS = switching frequency
The voltage rating of capacitor should be twice the output
voltage for a tantalum and 20% greater for an aluminum
electrolytic or OS-CON.
The output capacitor RMS current is calculated below:
ICOUT (rms) =
IPP
12
The power dissipated in the output capacitor is:
PDISS(COUT ) = ICOUT (rms)2 × RESR(COUT )
Input Capacitor Selection
The input capacitor should be selected for ripple current
rating and voltage rating. Tantalum input capacitors may fail
when subjected to high inrush currents, caused by turning the
input supply on. Tantalum input capacitor voltage rating
should be at least 2 times the maximum input voltage to
maximize reliability. Aluminum electrolytic, OS-CON, and
multilayer polymer film capacitors can handle the higher
inrush currents without voltage derating.
MIC2182
18
June 2000
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