MIC2169 Micrel Semiconductor, MIC2169 Datasheet - Page 10

• Manufacturer Part Number: MIC2169
• Description: 500 KHZ PWM SYNCHRONOUS BUCK CONTROL IC
• Manufacturer: Micrel Semiconductor

MIC2169
Voltage Setting Components
The MIC2169 requires two resistors to set the output voltage
as shown in Figure 2.
Where:
The output voltage is determined by the equation:
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:
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:
The power dissipated by the Schottky diode is:
where:
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
M9999-111803
V
V
I
P
V
V
D(avg)
DIODE
REF
F
O
DIODE(rrm)
Figure 2. Voltage-Divider Configuration
= forward voltage at the peak diode current
R2
for the MIC2169 is typically 0.8V
V
REF
MIC2169 [adj.]
I
V
OUT
V
I
D(avg)
O
REF
V
1
V
IN
Error
Amp
REF
2 80ns f
R1
R2
R1
V
F
V
0.8V
REF
S
FB
7
R1
R2
10
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
Feedback Loop Compensation
The MIC2169 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
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
shown in Figure 3. The transfer function G(s), for such a
system is:
Plotting this transfer function with the following assumed
values (L=2 H, DCR=0.009 , C
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.
Figure 3. The Output LC Filter in a Voltage Mode
G(s)
1
/
DCR s C s
OUT
2
% to 1% improvement in efficiency.
, with its electrical series resistance (ESR) as
Buck Converter
L
1 ESR s C
2
L C 1 ESR s C
DCR
“Functional Block Diagram.”
OUT
=1000 F, ESR=0.050 )
ESR
C
OUT
V
November 2003
O
Micrel