DS8817A Datasheet, PDF (12/21 Pages) Richtek Technology Corporation – Dual-Phase PWM Controller with PWM-VID Reference

RT8817A
Application Information
The RT8817A is a dual-phase synchronous Buck PWM
controller with integrated drivers which is optimized for
high performance graphic microprocessor and computer
applications. A COT (Constant-On-Time) PWM controller
and two MOSFET drivers with internal bootstrap diodes
are integrated so that the external circuit can be easily
designed and the number of component is reduced.
The topology solves the poor load transient response timing
problems of fixed-frequency mode PWM and avoids the
problems caused by widely varying switching frequencies
in conventional constant on-time and constant off-time
PWM schemes. The IC supports dynamic mode transition
function with various operating states, which include single
phase with CCM, dual-phase with CCM, single phase with
diode emulation mode and dual-phase with diode emulation
mode operation. These different operating states make
the system efficiency as high as possible.
The RT8817A provides a PWM-VID dynamic control
operation in which the feedback voltage is regulated and
tracks external input reference voltage. It also features
complete fault protection functions including over voltage,
under voltage and current limit.
Remote Sense
The RT8817A uses the remote sense path (VSNS and
RGND) to overcome voltage drops in the power lines by
sensing the voltage directly at the end of GPU. Normally,
to protect remote sense path disconnecting, there are
two resistors (R_Local) connecting between local sense path
and remote sense path. That is, in application with remote
sense, the R_Local is recommended to be 10Ω to 100Ω. If
no need of remote sense, the R_Local is recommended to
be 0Ω.
V_IN
BOOT
UGATE
PHASE
LGATE
Local Sense Path
V_OUT
RGND
VSNS
R_Local+
R_Local-
GPU⁺
GPU^-
Remote Sense Path
Figure 3. Output Voltage Sensing
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
www.richtek.com
12
PWM Operation
The RT8817A integrates a Constant-On-Time (COT) PWM
controller, and the controller provides the PWM signal
which relies on the output ripple voltage comparing with
internal reference voltage as shown in Figure 4. Referring
to the function block diagram of TON Genx, the
synchronous UGATE driver is turned on at the beginning
of each cycle. After the internal one-shot timer expires,
the UGATE driver will be turned off. The pulse width of
this one-shot is determined by the converter’ s input
voltage and the output voltage to keep the frequency fairly
constant over the input voltage and output voltage range.
Another one-shot sets a minimum off-time.
V_OUT
V_PEAK
V_OUT
V_VALLEY
V_REF
t
0
t_ON
Figure 4. Constant On-Time PWM Control
On-Time Control
The on-time one-shot comparator has two inputs. One
input monitors the output voltage, while the other input
samples the input voltage and converts it to a current.
This input voltage proportional current is used to charge
an internal on-time capacitor. The on-time is the time
required for the voltage on this capacitor to charge from
zero volts to V_OUT, thereby making the on-time of the high-
side switch directly proportional to output voltage and
inversely proportional to input voltage. The implementation
results in a nearly constant switching frequency without
the need for a clock generator.
T_ON
=
2

V_OUT
V_IN 
 3.2p
0.5

R_TON
and then the switching frequency F_S is :
F_S = V_OUT /  V_IN  T_ON 
R_TON is a resistor connected from the V_IN to TON pin.
The recommend operation frequency range is 250kHz to
750kHz.
is a registered trademark of Richtek Technology Corporation.
DS8817A-00 September 2016