MIC2168A Datasheet, PDF (12/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

Micrel
MIC2168A
The maximum value of ESR is calculated:
R_ESR
≤
ΔV_OUT
I_PP
where:
V_OUT = peak-to-peak output voltage ripple
I_PP = peak-to-peak inductor ripple current
The peak input current is equal to the peak inductor
current, so:
ΔV_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:
The total output ripple is a combination of the ESR
output capacitance. The total ripple is calculated below:
I_CIN (rms) ≈ I_OUT (max) × D × (1 − D)
ΔV_OUT =
⎜⎜⎝⎛
I_PP × (1− D)
C_OUT × f_S
⎟⎟⎠⎞²
+
(I_PP
×
R_ESR
)²
where:
D = duty cycle
C_OUT = output capacitance value
f_S = switching frequency
The power dissipated in the input capacitor is:
P_{DISS(CIN )} = I_CIN (rms)² × R_{ESR(CIN )}
Voltage Setting Components
The MIC2168A requires two resistors to set the output
voltage as shown in Figure 2.
The voltage rating of capacitor should be twice the
voltage for a tantalum and 20% greater for an aluminum
electrolytic. The output capacitor RMS current is
calculated below:
I_COUT(rms)
=
I_PP
12
The power dissipated in the output capacitor is:
Figure 2. Voltage-Divider Configuration
where:
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 capacitor’s ESR.
V_REF for the MIC2168A is typically 0.8V
The output voltage is determined by the equation:
V_O
=
V_REF
× ⎜⎛1 +
⎝
R1 _⎟⎞
R2 _⎠
January 2010
12
M9999-011510