RICHTEK
Maw
RICHTEK
RT6210
14
DS6210-01 February 2019www.richtek.com
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Copyright 2019 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
re-powered on. During this period of time, the SW starts
to switch since the under-voltage protection is masked
during soft-start time. When soft-start finishes, if the under-
voltage condition is removed, the converter will resume
normal operation; if the under-voltage condition, however,
still remains, that is, the FB voltage is still lower than
under-voltage threshold VTH_UVP, the under-voltage
protection will be triggered again. The cycle will repeat
until this fault condition is removed.
Output Voltage Limitation
The output voltage must be set higher than (VIN x 6.3%)
due to the limitation of the minimum on-time tON_MIN and
switching frequency. When current limiting protection is
triggered and the load current is still increasing slowly,
the output voltage will start to drop and the on-time of the
high-side MOSFET will decrease as well. When the output
voltage drops below VTH_UVP, the under-voltage protection
is triggered to turn off the internal driver to protect the
converter. If the output voltage, however, does not drop
below VTH_UVP yet when the on-time of the high-side
MOSFET has decreased to tON_MIN (~90ns), the internal
gate driver will keep switching to maintain the output
voltage, which may damage the chip under this over-current
condition.
In order to make sure the output voltage can drop below
VTH_UVP once current limiting protection is triggered, the
output voltage setting must be satisfied with the equation
below :
The output voltage at the time, when the switch has been
turned on for the minimum on-time, is
VOUT_MIN = VIN x tON x fOSC1 = 0.0315VIN
where tON_MIN = ~90ns, fOSC1 = 350kHz
The UVP is triggered when the VFB is lower than VTH_UVP,
which is 50%. That is to say VOUT_MIN should be lower
than 50% of the actual VOUT to guarantee the UVP can be
triggered under this condition.
VOUT_MIN < 0.5 x VOUT
The duty cycle limitation can be obtained.
DMIN > 0.063
For example, if the VIN = 50V, the VOUT should be set
higher than 3.15V.
External Bootstrap Diode
A 0.1μF capacitor CBOOT, where a low ESR ceramic
capacitor is typically used, is connected between the
BOOT and SW pins to provide the gate driver supply voltage
for the high-side N-MOSFET.
It is recommended to add an external bootstrap diode
from an external 3.3V supply voltage to the BOOT pin to
improve efficiency when the input voltage VIN is lower than
5.5V or duty cycle is higher than 65%. A low-cost bootstrap
diode can be used, such as IN4148 or BAT54.
Note that the external BOOT voltage must be lower than
5.5V.
Figure 6. External Bootstrap Diode
SW
BOOT
3.3V
100nF
RT6210 CBOOT
Inductor Selection
Output inductor plays a very important role in step-down
converters because it stores energy from input power rail
and releases to output load. For better efficiency, DC
resistance (DCR) of the inductor must be minimized to
reduce copper loss. In addition, since the inductor takes
up most of the PCB space, its size also matters. Low-
profile inductors can also save board space if height
limitation exists. However, low-DCR and low-profile
inductors are usually not cost effective.
On the other hand, while larger inductance may lower
ripple current, and then power loss, rise time of the inductor
current, however, increases with inductance, which
degrades the transient responses. Therefore, the inductor
design is a trade-off among performance, size and cost.
The first thing to consider is inductor ripple current. The
inductor ripple current is recommended in the range of
20% to 40% of full-load current, and thus the inductance
can be calculated using the following equation.
IN OUT OUT
MIN
SW OUT IN
VV V
L = fkI V