MIC2169B_10 Datasheet, PDF (13/25 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

Micrel, Inc.
The voltage rating of capacitor should be twice the
voltage for a tantalum and 20% greater for aluminum
electrolytic.
The output capacitor RMS current is calculated below:
I_{C OUT ( 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. A tantalum input capacitor’s
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. 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:
( ) 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 )}
Voltage Setting Components
The MIC2169B requires two resistors to set the output
voltage as shown in Figure 6.
Error
Amp
R1
FB
5
R2
MIC2169B
V_REF
0.8V
Figure 6. Voltage-Divider Configuration
MIC2169B
The output voltage is determined by the equation:
V_O
=
V_REF
× ⎜⎛1+
⎝
R1 _⎟⎞
R2 _⎠
where
V_REF for the MIC2169B is typically 0.8V
A typical value of R1 can be between 3kΩ and 10kΩ. If
R1 is too large, it may allow noise to be introduced into
the voltage feedback loop. If R1 is too small, in value, it
will decrease the efficiency of the power supply,
especially at light loads. Once R1 is selected, R2 can be
calculated using:
R2 = ^{VREF × R1}
V_O − V_REF
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 50ns. 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 × 50ns × 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, D1, 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
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.
April 2010
13
M9999-041210-B