DS8813D Datasheet, PDF (20/23 Pages) Richtek Technology Corporation – Multi-Phase PWM Controller with PWM-VID Reference

RT8813D
V_X
V_TSNS
R_OTSET
TSNS
R_NTC
1V
C⁺MP
-
V_H
TALERT
Internal OTP
Figure 13. External OTP Setting
R_OTSET can be determined using the following equation :
R_OTSET = R_NTC,TC  V_X 1
where R_NTC,T°C is the thermistor's resistance at OTP trigger
temperature.
The standard formula for the resistance of the NTC
thermistor as a function of temperature is given by :
    _{RNTC,TC}
=
R_25C  e^{β}
1
T273

1
298
 ^{}
where R_25°C is the thermistor's nominal resistance at room
temperature 25°C, β (beta) is the thermistor's material
constant in Kelvins, and T is the thermistor's actual
temperature in Celsius.
MOSFET Gate Driver
The RT8813D integrates high current gate drivers for the
MOSFETs to obtain high efficiency power conversion in
synchronous Buck topology. A dead-time is used to prevent
the crossover conduction for high side and low side
MOSFETs. Because both the two gate signals are off
during the dead-time, the inductor current freewheels
through the body diode of the low side MOSFET. The
freewheeling current and the forward voltage of the body
diode contribute power losses to the converter. The
RT8813D employs adaptive dead-time control scheme to
ensure safe operation without sacrificing efficiency.
Furthermore, elaborate logic circuit is implemented to
prevent cross conduction. For high output current
applications, two power MOSFETs are usually paralleled
to reduce R_DS(ON). The gate driver needs to provide more
current to switch on/off these paralleled MOSFETs. Gate
driver with lower source/sink current capability results in
longer rising/falling time in gate signals and higher
switching loss. The RT8813D embeds high current gate
drivers to obtain high efficiency power conversion.
Inductor Selection
Inductor plays an importance role in step-down converters
because the energy from the input power rail is stored in
it and then released to the load. From the viewpoint of
efficiency, the DC Resistance (DCR) of inductor should
be as small as possible to minimize the copper loss. In
additional, the inductor occupies most of the board space
so the size of it is important. Low profile inductors can
save board space especially when the height is limited.
However, low DCR and low profile inductors are usually
not cost effective.
Additionally, higher inductance results in lower ripple
current, which means the lower power loss. However, the
inductor current rising time increases with inductance value.
This means the transient response will be slower. Therefore,
the inductor design is a trade-off between performance,
size and cost.
In general, inductance is designed to let the ripple current
ranges between 20% to 40% of full load current. The
inductance can be calculated using the following equation :
L_min
=
V_IN
F_SW  k
 V_OUT
 I_{OUT_rated}

V_OUT
V_IN
where k is the ratio between inductor ripple current and
rated output current.
Input Capacitor Selection
Voltage rating and current rating are the key parameters
in selecting input capacitor. Generally, input capacitor has
a voltage rating 1.5 times greater than the maximum input
voltage is a conservatively safe design.
The input capacitor is used to supply the input RMS
current, which can be approximately calculated using the
following equation :
I_RMS = I_OUT 
V_OUT
V_IN
 ^{}1
V_OUT
V_IN

The next step is to select proper capacitor for RMS current
rating. Use more than one capacitor with low Equivalent
Series Resistance (ESR) in parallel to form a capacitor
bank is a good design. Besides, placing ceramic capacitor
close to the drain of the high side MOSFET is helpful in
reducing the input voltage ripple at heavy load.
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is a registered trademark of Richtek Technology Corporation.
DS8813D-00 September 2016