MIC2169_09 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

MIC2169
Error
Amp
R1
FB
7
R2
MIC2169 [adj.]
V_REF
0.8V
Figure 2. Voltage-Divider Configuration
Where:
V_REF for the MIC2169 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 × 15ns × 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 cur-
rent. The body diode has a relatively slow reverse recovery
time and a relatively high forward voltage drop. The power
Micrel
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 upon the circuit
components and operating conditions, an external Schottky
diode will give a ¹/₂% to 1% improvement in efficiency.
Feedback Loop Compensation
The MIC2169 controller comes with an internal transcon-
ductance 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 induc-
tor, 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
×
(1+ ESR × s
C+ 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=0.025Ω) 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.
30
30
7.5
-15
-37.5
-80 _-80
100
100
1.10³
1 ^.104
f
1 ^.105
1 ^.106
1000000
Figure 4. The Gain Curve for G(s)
M9999-032409
10
March 2009