MIC24420 Datasheet, PDF (20/34 Pages) Micrel Semiconductor – 2.5A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
improves efficiency due to its lower forward voltage drop
as compared to the internal parasitic diode in the
MOSFET. It may also decrease high frequency noise
because the schottky diode junction does not suffer from
reverse recovery.
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. Depending on the circuit
components and operating conditions, an external
Schottky diode may give up to 1% improvement in
efficiency.
Compensation
The voltage regulation, filter and power stage sections
are shown in Figure 12. The error amplifier regulates the
output voltage and compensates the voltage regulation
loop. It is a simplified type III compensator utilizing two
compensating zeros and two poles. Figure 12 also
shows the transfer function for each section.
Compensation is necessary to insure the control loop
has adequate bandwidth and phase margin to properly
respond to input voltage and output current transients.
High gain at DC and low frequencies is needed for
accurate output voltage regulation. Attenuation near the
switching frequency prevents switching frequency noise
from interfering with the control loop.
The output filter contains a complex double pole formed
by the capacitor and inductor and a zero from the output
capacitor and its ESR. The transfer function of the filter
is:
MIC24420/MIC24421
s
1+
Gfilter(s) =
ωz
s
s²
1+
+
Q ωo ωo
Where:
ωz =
1
C_O ⋅ R_ESR
ωo =
1
C_O ⋅L_O
Q = R ⋅ ^CO
L
The Modulator gain is proportional to the input voltage
and inversely proportional to the internal ramp voltage
generated by the oscillator. The peak-to-peak ramp
voltage is 1V.
Gmod
=
⎜⎜⎝⎛
V_IN
V_RAMP
⎟⎟⎠⎞
The output voltage divider attenuates V_OUT and feeds it
back to the error amplifier. The divider gain is:
H = ^R4 = ^VREF
R1 + R4 V_OUT
June 2012
Figure 12. Voltage Loop and Transfer Functions
20
M9999-062012-C