RT6218A/B
Abnormal case
detected (UV)
VOUT
t
HICCUP_ON
= C
SS
x 0.85 / I
SS
t
HICCUP_OFF
= C
SS
x 6.3 / I
SS
t
EN
SS = 1.9V
SS
V1
FB
LX
t
T1
T2
T3
External Bootstrap Capacitor
PG
Connect a 0.1F low-ESR ceramic capacitor between
the BOOT and LX pins. This bootstrap capacitor
supplies for the gate driver of the high-side N-channel
MOSFET switch.
Over-Temperature Protection (Thermal Shutdown)
The RT6218A/B includes an over-temperature
protection (OTP) circuitry to prevent overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature
exceeds a thermal shutdown threshold T
SD
. Once the
junction temperature cools down by a thermal
shutdown hysteresis (T
SD
), the IC will resume normal
operation with a complete soft-start.
Soft-Start
The RT6218A/B provides adjustable soft-start function.
When the EN pin becomes high, the SS charge current
(I
SS
) begins charging the capacitor which is connected
from the SS pin to GND (C
SS
).The soft-start function is
used to prevent large inrush current while converter is
being powered-up. The soft-start timing can be
programmed by the external capacitor CSS between
SS and GND. An internal current source I
SS
(6A)
charges an external capacitor to build a soft-start ramp
voltage. The VFB voltage will track the internal ramp
voltage during soft-start interval. The typical soft-start
time is calculated as follows :
Soft-Start time t
SS
= 55s + C
SS
x 0.55 / 11A + C
SS
x
0.86 / I
SS
t
SS
= SS rising to FB settled (T2 + T3)
T1 : EN delay, from EN go high to SS start rising, T1 = 55μs;
T2 : speed up SS, from SS rising to FB start rising, T2 = C
SS
x 0.55 / 11μA;
T3 : normal SS, from FB rising to settled, T3 = C
SS
x 0.86 / I
SS
;
V1 : offset voltage between SS and FB, V1 = 700mV;
PG go high after SSOK (SS = 1.9V)
Power Good Indication
The PGOOD pin is an open-drain output and is
connected to an external pull-up resistor. It is controlled
by a comparator, which the feedback signal V
FB
is fed
to. If V
FB
is above 90% of the internal reference voltage
after soft-start finished, the PGOOD pin will be in high
impedance and V
PGOOD
will be held high. Otherwise,
the PGOOD output will be pulled low.
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6218A/B-01
October
2018
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
