RT6343
EN Pin for Start-Up and UVLO Adjustment
typically) charges an external capacitor to build a soft-
start ramp voltage. The internal charging current source
ISS gradually increases the voltage on CSS/TR, and the high-
side MOSFET will start switching if voltage difference
between SS/TR pin and FB pin is larger than 42mV ( i.e.
VSS/TR − VFB > 42mV, typically) during power-up period.
The FB voltage will track the SS/TR pin ramp voltage with
a SS/TR to FB offset voltage (42mV, typically) during soft-
start interval. The typical soft-start time (tSS) which is the
duration of VOUT rises from 10% to 90% of setting value is
calculated as follows :
For automatic start-up, the ENpin has an internal pull-up
current source IEN (1.2μA, typically) that enables operation
of the RT6343 when the EN pin floats. If the EN voltage
rises above the VTH_EN (1.2V, typically) and the VINvoltage
is higher than VUVLOH (4.3V, typically), the device will be
turned on, that is, switching is enabled and soft-start
sequence is initiated. If the high UVLO is required, the
EN pin can be used to adjust the under-voltage lockout
(UVLO) threshold and hysteresis. There is an additional
hysteresis current source IEN_Hys (3.4μA, typically) which
is sourced out of the EN pin when the EN pin voltage
VREF 0.8
tSS = CSS/TR
ISS
exceeds VTH_EN. When the EN pin drops below VTH_EN
,
If a heavy load is added to the output with large
capacitance, the output voltage will never enter regulation
because of UVP. Thus, the device remains in hiccup
operation. The CSS/TR should be large enough to ensure
soft-start period ends after COUT is fully charged.
the IEN_Hys is removed. Therefore, the EN pin can be
externally connected to VIN by adding two resistors, RENH
and RENL to achieve UVLO adjustment as shown in Figure
13.
According to the desired start and stop input voltage, the
resistance of REN1 and REN2 can be obtained as below :
ISS V
OUT
C
C
SS/TR
OUT
0.8I
COUT_CHG
VStart VStop
REN1
REN2
=
=
where ICOUT_CHG is the COUT charge current which is
related to the switching frequency, inductance, high-side
MOSFET peak current limit and load current.
IEN_Hys
VTH_EN
VStart VTH_EN
+ IEN
REN1
Power-Good Output
The EN pin, with high-voltage rating, supports wide input
voltage range to adjust the VIN UVLO.
The RT6343GQW features an open-drain power-good
output (PGOOD) to monitor the output voltage status. The
PGOODpin is an open-drain power-good indication output
and is to be connected to an external voltage source
through a pull-up resistor.
R
EN1
V
IN
EN
R
RT6343
GND
EN2
It is recommended to use pull-up resistance between the
values of 1 and 10kΩ to reduce the switching noise
coupling to PGOOD pin.
Figure 13. ResistiveDivider for Under-Voltage Lockout
Threshold Setting
Synchronization
Soft-Start and Tracking Control
The RT6343 can be synchronized with an external clock
ranging from 300kHz to 2.2MHz which is applied to the
RT/SYNC pin. The minimum synchronous pulse width of
the external clock must be larger than 20ns and the
amplitude should have valleys that are below 0.5V and
peaks above 2V (up to 6V). The rising edge of the SW will
be synchronized to the falling edge of the RT/SYNC pin
signal.
The RT6343GQW provides adjustable soft-start function.
The soft-start function is used to prevent large inrush
current while converter is being powered-up. The
RT6343GQW provides an SS/TR pin so that the soft-start
time can be programmed by selecting the value of the
external soft-start capacitor CSS/TR connected from the
SS/TR pin to ground or controlled by external ramp voltage
to SS/TR pin. An internal current source ISS (1.7μA,
Copyright 2019 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6343-00 October 2019
www.richtek.com
27
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