RT8120 查看數據表（PDF） - Richtek Technology
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RT8120 Datasheet PDF : 17 Pages
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RT8120
resistance. However, this depends on the low side
MOSFET driver capability and the budget.
It is recommended to bypass low side MOSFET with a
snubber circuit (R = 1Ω, C = 3.3nF).
Compensation Network Design
The RT8120 is a voltage mode controller and requires
external compensation to have an accurate output voltage
regulation with fast transient response. The RT8120 uses
a high gain operational transconductance amplifier (EOTA)
as the error amplifier. As Figure 4 shows, the EOTA works
as the voltage controlled current source. The calculation
of the transconductance is shown below :
GM =
ΔIOUT
ΔVM
,
where
ΔVM
=
(VIN+ )
−
(VIN− )
and ΔVCOMP = ΔIOUT x ZOUT
VIN+
+
GM
VIN-
-
Figure 5 shows a typical buck control loop using a Type II
compensator. The control loop consists of the power stage,
PWM comparator and a compensator. The PWM
comparator compares VCOMP with the oscillator (OSC)
sawtooth wave to provide a Pulse-Width Modulated (PWM)
with an amplitude of VIN at the PHASE node. The PWM
wave is smoothed by the output filter LOUT and COUT. The
output voltage (VOUT) is sensed and fed to the inverting
input of the error amplifier.
The modulator transfer function is the small-signal transfer
function of VOUT/VCOMP (output voltage over the error
amplifier output). This transfer function is dominated by a
DC gain, a double pole, and an ESR zero as shown in
Figure 6.
IOUT
VCOMP
ZOUT
Figure 4. Operational Transconductance Amplifier, EOTA
VIN
ΔVOSC
PWM
Comparator
+
-
Driver
Logic
UGATE
PAHSE
LGATE
VREF
+
GM
FB
-
COMP
Q1
LOUT
Q2
COUT
VOUT
RFB1
RFB2
VCOMP
CC
CP
RC
Figure 5. Typical Voltage Mode Buck Converter Control Loop
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS8120-08 September 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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