RT8253A データシートの表示（PDF） - Richtek Technology
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RT8253A Datasheet PDF : 15 Pages
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Application Information
The RT8253A is a single phase buck PWM converter with
internal N-MOSFET switches. It provides single feedback
loop, current mode control with fast transient response.
An internal 0.8V reference allows the output voltage to be
precisely regulated for low output voltage applications. A
fixed switching frequency (340kHz) oscillator is integrated
to eliminate external component count. The RT8253A also
supports programmable soft start function by an external
capacitor. Protection features include over current
protection, under voltage protection and input Under
Voltage Lockout (UVLO).
PWM Operation
The RT8253A utilizes DEM control to improve light load
efficiency. Depending on the load current, the controller
automatically operates in Diode Emulation Mode (DEM)
or in Continuous Conduction Mode (CCM) with fixed
frequency PWM.
At light load condition, the RT8253A automatically operates
in diode emulation mode to reduce switching frequency
to improve efficiency. As the output current decreases from
heavy load condition, the inductor current decreases, and
eventually the inductor valley current decreases to zero,
which is the boundary between continuous conduction
mode and discontinuous conduction mode. By emulating
the behavior of diodes, the low side MOSFET allows only
partial negative current to flow when the inductor
freewheeling current becomes negative. As the load current
further decreases, it takes longer and longer to discharge
the output capacitor to the level that allows the next UGATE
on-time to begin. When the output current increases from
light load to heavy load, the switching frequency increases
to the CCM value as the inductor current reaches the
continuous conduction condition. The controller will then
operate in continuous conduction mode with 340kHz fixed
PWM switching frequency.
Output Voltage Setting
Connect a resistive voltage divider at the FB pin between
VOUT and GND to adjust the respective output voltage
between 0.8V and 20V (Figure 1). Choose R2 to be
approximately 10kΩ, and solve for R1 using the
equation :
RT8253A
VOUT
=
VFB
x
⎛⎜1+
⎝
⎛
⎜⎝
R1
R2
⎞
⎟⎠
⎞
⎟
⎠
where VFB is 0.8V (typ.).
VOUT
R1
FB
RT8253A
R2
GND
Figure 1. Setting VOUT with a Resistive Voltage Divider
External Bootstrap Diode & Capacitor
The bootstrap capacitor must be 0.1μF and located between
the BOOT pin and SW pin. This capacitor provides the
gate driver voltage for the high side MOSFET and should
be a high quality ceramic type with X7R or X5R grade
dielectric for temperature stability.
An external bootstrap diode may enhance the efficiency
of the regulator and it is recommended to add one between
an external 5V source and the BOOT pin (Figure 2). The
applicable conditions of the external bootstrap diode are
as follows :
z Input voltage is lower than 5.5V; and
z Duty cycle is high: D = VOUT >65%
VIN
In these cases, the external 5V source can be a fixed 5V
from the system or the output of the RT8253A. Note that
the external boot voltage must be lower than 5.5V.
5V
BOOT
RT8253A
SW
0.1µF
Figure 2. External Bootstrap Diode & Capacitor
Chip Enable and Disable
The EN pin is the RT8253A enable input. Drive EN below
the precise input falling edge trip level to place the RT8253A
in its low power shutdown state. The RT8253A quiescent
current drops to lower than 3μA while in shutdown. When
DS8253A-02 March 2011
www.richtek.com
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