MIC2199_10 Datasheet, PDF (11/14 Pages) Micrel Semiconductor – 300kHz 4mm × 4mm Synchronous Buck Converter

Micrel, Inc.
For the low-side switch (N-Channel MOSFET), the DC power
dissipation is:
P_SWITCH2(dc) = R_DS(on)2 × I_{SW 2(rms)2}
Since the AC switching losses for the low side MOSFET is
near zero, the total power dissipation is:
P_{low-side MOSFET(max)} = P_SWITCH2(dc)
The total power dissipation for the high side MOSFET is:
P_{high−sideMOSFET(max)} = P_{SWITCH 1(dc)} +P_AC
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.
I_D(avg) = I_OUT × 2 × 80ns × f_S
The reverse voltage requirement of the diode is:
V_DIODE(rrm) = V_IN
The power dissipated by the Schottky diode is:
P_DIODE = I_D(avg) × V_F
where:
V_F = 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 cur-
rent. 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 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 operating condi-
tions, an external Schottky diode will give a 1/2% to 1%
improvement in efficiency.
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.
MIC2199.
The output capacitor’s ESR is usually the main cause of output
ripple. The maximum value of ESR is calculated by:
R_ESR
≤
∆V_OUT
I_PP
where:
V_OUT = peak-to-peak output voltage ripple
I_PP = 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:
( ) ^∆V_OUT ⁼


I_PP
×
(1
−
D)


2
+
 ^C_OUT ^{× f}_S 
I_PP
× R_ESR
2
where:
D = duty cycle
C_OUT = output capacitance value
f_S = 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:
I_COUT(rms)
=
I_PP
12
The power dissipated in the output capacitor is:
P_{DISS(COUT )} = I_COUT(rms)2 ×R_{ESR(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.
The input voltage ripple will primarily depend on the input
capacitors ESR. The peak input current is equal to the peak
inductor current, so:
∆V_IN = I_{INDUCTOR(peak)} ×R_{ESR(CIN )}
The input capacitor must be rated for the input current ripple.
The RMS value of input capacitor current is determined at
the maximum output current. Assuming the peak-to-peak
inductor ripple current is low:
I_CIN(rms)≈ I_OUT(max) × D × (1 − D)
The power dissipated in the input capacitor is:
P_{DISS(CIN )} = I_CIN(rms)2 ×R_{ESR(CIN )}
January 2010
11
M9999-011310