RT6340
Enable Control
The RT6340 provides an EN pin, as an external chip enable
control, to enable or disable the device. If V
EN
is held
below the enable threshold voltage, switching is inhibited
even if the VIN voltage is above VIN under-voltage lockout
threshold (V
UVLOH
). If V
EN
is held below 0.4V, the converter
will enter into shutdown mode, that is, the converter is
disabled. During shutdown mode, the supply current can
be reduced to I
SHDN
(1.3μA, typically). If the EN voltage
rises above the enable threshold voltage while the VIN
voltage is higher than V
UVLOH
, the device will be turned
on, that is, switching being enabled and soft-start
sequence being initiated. The EN pin has an internal pull-
up current source I
EN
(0.9μA, typically) that enables
operation of the RT6340 when the EN pin floats. The EN
pin can be used to adjust the under-voltage lockout (UVLO)
threshold and hysteresis by using two external resistors.
The RT6340 implements additional hysteresis current
source I
EN_Hys
(2.9μA, typically) to adjust the UVLO. The
I
EN_Hys
is sourced out of the EN pin when V
EN
is larger
than enable threshold voltage. When the V
EN
falls below
enable threshold voltage, the I
EN_Hys
will be stopped
sourcing out of the EN pin.
Soft-Start and Tracking Control
The soft-start function is used to prevent large inrush
currents while the converter is being powered up. The
RT6340GSP provides internal soft-start and the
RT6340GQW provides external soft-start function for inrush
currents control. The RT6340GQW provides an SS/TR pin
so that the soft-start time can be programmed by selecting
the value of the external soft-start capacitor C
SS/TR
connected from the SS/TR pin to ground or controlled by
external ramp voltage to SS/TR pin. During the start-up
sequence, the soft-start capacitor is charged by an internal
current source I
SS
(2μA, typically) to generate a soft-start
ramp voltage as a reference voltage to the PWM
comparator. The high-side MOSFET will start switching if
the V
SS/TR
is larger than 54mV, and then the voltage
difference between SS/TR pin and FB pin is larger than
75mV ( i.e. V
SS/TR
−
V
FB
> 75mV, typically) during power-
up period. If the output is pre-biased to a certain voltage
during start-up for some reason, the device will not start
switching until the voltage difference between SS/TR pin
Copyright
©
2019 Richtek Technology Corporation. All rights reserved.
EN
450µs
SS
75mV
VOUT
10% x
V
OUT
90% x V
OUT
94% x V
OUT
t
SS
and FB pin is larger than 75mV (typically). Only when
this ramp voltage is higher than the feedback voltage V
FB
,
the switching will be resumed. The FB voltage will track
the SS/TR pin ramp voltage with a SS/TR to FB offset
voltage (75mV, typically) during soft-start interval. The
output voltage can then ramp up smoothly to its targeted
regulation voltage, and the converter can have a monotonic
smooth start-up. For soft-start control, the SS pin should
never be left unconnected. After the FB pin voltage rises
above 94% of V
REF
(typically), the PGOOD pin will be in
high impedance and the V
PGOOD
will be held high. The
typical start-up waveform shown in Figure 1 indicates the
sequence and timing between the output voltage and
related voltage.
VIN
V
IN
= 4.5V to 42V
PGOOD
Figure 1 Start-Up Sequence for RT6340GQW
Power Good Indication
The RT6340GQW features an open-drain power-good
output (PGOOD) to monitor the output voltage status. The
output delay of comparator prevents false flag operation
for short excursions in the output voltage, such as during
line and load transients. Pull-up PGOOD with a resistor
to an external voltage source and it is recommended to
use pull-up resistance between the values of 1 and 10kΩ
to reduce the switching noise coupling to PGOOD pin.
The PGOOD assertion requires input voltage above 1.5V.
The power-good function is controlled by a comparator
connected to the feedback signal V
FB
. If V
FB
rises above
the power-good high threshold (V
TH_PGLH1
) (94% of the
reference voltage, typically), the PGOOD pin will be in
high impedance and V
PGOOD
will be held high after a certain
is a registered trademark of Richtek Technology Corporation.
DS6340-00
September 2019
www.richtek.com
7
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
