MIC2159 Datasheet, PDF (11/17 Pages) Micrel Semiconductor – SYNCHRONOUS-itty™ Step-Down Converter IC

Micrel
Note that ∆Vout should be kept within the +/- 3% of
nominal limits to prevent the loop switching between
hysteretic and voltage mode control.
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. Aluminium
electrolytic, OS-CON, and multilayer polymer film
capacitors can handle the higher inrush currents without
voltage de-rating. 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:
∆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 )}
Voltage Setting Components
The MIC2159 requires two resistors to set the output
voltage as shown in Figure 2.
Figure 2. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_O
= V_REF
⋅ ⎜⎛1 +
⎝
R1 _⎟⎞
R2 _⎠
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:
MIC2159
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) = VIN
The power dissipated by the Schottky diode is:
P_DIODE = I_D(avg) x 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. Depending on
the circuit components and operating conditions, an
external Schottky diode will give a 1/2% to 1%
improvement in efficiency.
Feedback Loop Compensation
The MIC2159 controller comes with an internal
transconductance error amplifier used for compensating
the voltage feedback loop by placing a capacitor (C1) in
series with a resistor (R1) and another capacitor C2 in
parallel from the COMP pin to ground. See “MIC2159
Block Diagram.”
Power Stage
The power stage of a voltage mode controller has an
inductor, L1, with its winding resistance (DCR)
October 2006
11
M9999-101206