RT2658
Application Information
Non-Droop Mode Operation
The RT2658 employs current mode COT architecture that
provides ease of use, low external component count and
fast transient response. The Synchronous buck regulator
capable of delivering up to 6Acontinuous current and set
switching frequency up to 1MHz.
The RT2658 can be configured as a non-droop solution.
The benefit of a non-droop approach is that load regulation
is flat, therefore, in a system where tight DC tolerance is
desired, the non-droop approach is recommended. For
the Intel system agent application, non-droop is
recommended as the standard configuration. The non-
droop approach can be implemented by connecting a
resistor and a capacitor between the COMP and the VREF
pins. The purpose of the type II compensation is to obtain
high DC feedback gain while minimizing the phase delay
at unity gain cross over frequency of the converter.
Some feature of Current mode COT
Effective bandwidth is very high
No slope compensation necessary
Double complex zero at half the switching frequency
term of fixed frequency peak CM converter disappears.
Operation of Current Mode COT
The value of the resistor (RC) can be calculated using the
desired unity gain bandwidth of the converter, and the value
of the capacitor (CC) can be calculated by knowing where
the zero location is desired. The capacitor CP is optional,
but recommended. Its appropriate capacitance value can
be calculated using the desired pole location.
Referring to Figure 1, VCOMP is the amplified difference
between the reference voltage and the feedback voltage.
VCS is valley current sense voltage (sensing the inductor
current). The PWM comparator senses where the two
waveforms cross and triggers the on time generator.
Current Feedback
Figure 2 shows the basic implementation of the non-droop
mode using the RT2658.
V
CS
C
P
V
COMP
C
C
R
C
V
REF
VIN
COMP
VREF
VOUT
t
gm
ON
-
+
+
t
VREF
-
VREFIN
+
Time (µs)
-
+
-
HS
RCS
+
-
LS
DS(ON)
L
OUT
PWM
Figure1. Current Mode COT Waveforms
R
SW
Driver
Comparator
ESR
LS
+
R
OUT
Adaptive Constant On-Time Control
VREF
-
C
OUT
Adaptive on time generator makes the system operate in
fixed frequency when VIN or VOUT change. The technique
improves COT by making the one-shot on-time proportional
to VOUT and inversely proportional to VIN. In this way, an
on-time is chosen as approximately what it would be for
an ideal fixed-frequency PWM in similar input/output
voltage conditions.
Figure 2.Non-Droop Mode
Droop Mode Operation
The terminology for droop is the same as load line or
voltage positioning as defined in the Intel CPU VCORE
specification. Based on the actual tolerance requirement
of the application, load-line set points can be defined to
maximize either cost savings (by reducing output
capacitors) or power reduction benefits.
The on-time calculates equation of the buck converter as
shows as follows :
VOUT
tON
VIN FSW
Accurate droop voltage response is provided by the finite
gain of the droop amplifier. The equation for droop voltage
is shown below Equation :
Where
FSW is operate frequency.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS2658-01 February 2018
www.richtek.com
13
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