MIC2168 Datasheet, PDF (10/14 Pages) Micrel Semiconductor – 1MHZ PWM SYNCHRONOUS BUCK CONTROL IC

MIC2168
Voltage Setting Components
The MIC2168 requires two resistors to set the output voltage
as shown in Figure 2.
Error
Amp
R1
FB
7
R2
MIC2168 [adj.]
V_REF
0.8V
Figure 2. Voltage-Divider Configuration
Where:
V_REF for the MIC2168 is typically 0.8V
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:
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 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.
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, D1, is not necessary for circuit
operation since the low-side MOSFET contains a parasitic
body diode. The external diode will improve efficiency and
Micrel
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 compo-
nents and operating conditions, an external Schottky diode
will give a ¹/₂% to 1% improvement in efficiency.
Feedback Loop Compensation
The MIC2168 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. The transfer function G(s), for such a
system is:
L
DCR
V_O
ESR
C_OUT
Figure 3. The Output LC Filter in a Voltage Mode
Buck Converter
G(s)
=


DCR
×
s
×
C
(1+ ESR × s
+ s² × L × C
×
+
C)
1+
ESR
×
s
×
C


Plotting this transfer function with the following assumed
values (L=2 µH, DCR=0.009Ω, C_OUT=1000µF, ESR=0.050Ω)
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.
M9999-111803
10
November 2003