DS8817A Datasheet, PDF (15/21 Pages) Richtek Technology Corporation – Dual-Phase PWM Controller with PWM-VID Reference

RT8817A
The soft-start time can be calculated as:
t_SS

(C_SS
 V_REFIN )
I_SS
where I_SS = 50μA (typ.), V_REFIN is the voltage of REFIN
pin, and C_SS is the external capacitor placed from OCSET/
SS pin to GND.
VCC
I_SS
V_REFIN
OCSET/SS
C_SS ^SS
V_OUT
t_SS
Figure 8. External Soft-Start Time Setting
Power Good Output (PGOOD)
The PGOOD pin is an open-drain output, and it requires
a pull-up resistor. During soft-start, the PGOOD is held
low and is allowed to be pulled high after V_OUT achieved
over UVP threshold and under OVP threshold. In
additional, if any protection is triggered during operation,
the PGOOD will be pulled low immediately.
PWM VID and Dynamic Output Voltage Control
The RT8817A features a PWM VID input for dynamic
output voltage control as shown in Figure 9, which
reduces the number of device pin and enables a wide
dynamic voltage range. The output voltage is determined
by the applied voltage on the REFIN pin. The PWM duty
cycle determines the variable output voltage at REFIN.
PWM IN
VID
VREF
R_REF1
R_{REFADJ REFADJ}
C_REFADJ
R_BOOT
RGND
Buffer
RGND
REFIN
R_STANDBY
R_REF2
C_REFIN
Standby
Q1
Mode Control
RGND
RGND
RGND
Figure 9. PWM VID Analog Circuit Diagram
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS8817A-00 September 2016
With the external circuit and VID control signal, the
controller provides three operation modes shown as
Figure 10.
VREF
REFIN
PWM VID
BOOT
MODE
NORMAL
MODE
BOOT
MODE _STANDBY
MODE
STANDBY
CONTROL
Figure 10. PWM VID Time Diagram
Boot Mode
VID is not driven, and the buffer output is tri-state. At this
time, turn off the switch Q1 and connect a resistor divider
as shown in Figure 9 that can set the REFIN voltage to be
V_BOOT as the following equation :
V_BOOT
=
V_VREF


R_REF1

R_REF2
R_REF2

R_BOOT

where V_VREF = 2V (typ.)
Choose R_REF2 to be approximately 10kΩ, and the R_REF1
and R_BOOT can be calculated by the following equations :
  _{RREF1  RBOOT}
 ^{RREF2 }
V_VREF  V_BOOT
V_BOOT
  _{RREF1 } R_REF2 
V_VREF  V_BOOT
V_BOOT
 R_BOOT
  _RBOOT
 ^{RREF2 }
V_VREF  V_BOOT
V_BOOT
 R_REF1
Standby Mode
An external control can provide a very low voltage to
meet V_OUT operating in standby mode. If the VID pin is
floating and switch Q1 is enabled as shown in Figure 9,
the REFIN pin can be set for standby voltage according
to the calculation below :
V_STANDBY = V_VREF

R_REF1

R_REF2 // R_STANDBY
R_BOOT  (R_REF2 // R_STANDBY
)
By choosing R_REF1, R_REF2, and R_BOOT, the R_STANDBY can
be calculated by the following equation :
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
15