MIC2168A Datasheet, PDF (13/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

Micrel
MIC2168A
A typical value of R1 can be between 3k and 10k . 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}
VO − VREF
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 15ns. 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.
The reverse voltage requirement of the diode is:
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 MIC2168A 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 “Functional
Block Diagram.”
Power Stage
The power stage of a voltage mode controller has an
inductor, L1, with its winding resistance (DCR)
connected to the output capacitor, C_OUT, with its
electrical series resistance (ESR) as shown in Figure 3.
V_DIODE(rms) = 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
Figure 3. The Output LC Filter in a Voltage-Mode
Buck Converter
The transfer function G(s), for such a system is:
G_(S)
−
⎜⎜⎝⎛
DCR
×
s
×
C
(1 + ESR × s × C
+ s² × L × C + 1+
ESR
×
s
×
C
⎟⎟⎠⎞
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.
Plotting this transfer function with the following assumed
values (L = 2μH, DCR = 0.009Ω, C_OUT = 1000μF, ESR =
0.025Ω) gives lot of insight as to why one needs to
compensate the loop by adding resistor and capacitors
on the COMP pin. Figures 4 and 5 show the gain curve
and phase curve for the above transfer function.
January 2010
13
M9999-011510