RT6200
Application Information
The RT6200 is a high voltage buck converter that can
support the input voltage range from 4.5V to 36V and the
output current can be up to 0.6A.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
V
f L
VOUT
V
IN
OUT
1
IL =
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
highest efficiency operation. However, it requires a large
inductor to achieve this goal.
V
OUT
R1
FB
RT6200
GND
R2
For the ripple current selection, the value of ΔIL= 0.4(IMAX
)
will be a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the
ripple current stays below the specified maximum, the
inductor value should be chosen according to the following
equation :
Figure 1. Output Voltage Setting
For adjustable voltage mode, the output voltage is set by
an external resistive voltage divider according to the
following equation :
V
f I
V
OUT
V
IN(MAX)
OUT
R1
R2
L =
1
VOUT = VFB 1
L(MAX)
Where VFB is the feedback reference voltage (0.8V typ.).
Inductor Core Selection
The inductor type must be selected once the value for L
is known. Generally speaking, high efficiency converters
can not afford the core loss found in low cost powdered
iron cores. So, the more expensive ferrite or
mollypermalloy cores will be a better choice.
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and PHASE pin. This capacitor provides the
gate driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and the BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65%. The bootstrap diode can be a
low cost one such as 1N4148 or BAT54.
The selected inductance rather than the core size for a
fixed inductor value is the key for actual core loss. As the
inductance increases, core losses decrease. Unfortunately,
increase of the inductance requires more turns of wire
and therefore the copper losses will increase.
The external 5V can be a 5V fixed input from system or a
Ferrite designs are preferred at high switching frequency
due to the characteristics of very low core losses. So,
design goals can focus on the reduction of copper loss
and the saturation prevention.
5V output of the RT6200.
5V
BOOT
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design
current is exceeded. The previous situation results in an
abrupt increase in inductor ripple current and consequent
output voltage ripple.
10nF
RT6200
PHASE
Figure 2. External Bootstrap Diode
Do not allow the core to saturate!
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6200-04 August 2015
www.richtek.com
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