DS8817A Datasheet, PDF (18/21 Pages) Richtek Technology Corporation – Dual-Phase PWM Controller with PWM-VID Reference

RT8817A
5μs delay is used in OVP detection circuit to prevent false
trigger.
Output Under-Voltage Protection (UVP)
The output voltage can be continuously monitored for under-
voltage protection. When the output voltage is less than
40% of its set voltage, under voltage protection is triggered
and then all UGATE and LGATE gate drivers are forced
low. There is a 3μs delay built in the UVP circuit to prevent
false transitions. During soft-start, the UVP blanking time
is equal to PGOOD blanking time.
MOSFET Gate Driver
The RT8817A 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
RT8817A 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 RT8817A embeds high current gate
drivers to obtain high efficiency power conversion.
MOSFET Selection
The majority of power loss in the step-down power
conversion is due to the loss in the power MOSFETs. For
low voltage high current applications, the duty cycle of
the high-side MOSFET is small. Therefore, the switching
loss of the high-side MOSFET is of concern. Power
MOSFETs with lower total gate charge are preferred in
such kind of application.
However, the small duty cycle means the low-side
MOSFET is on for most of the switching cycle. Therefore,
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the conduction loss tends to dominate the total power
loss of the converter. To improve the overall efficiency, the
MOSFETs with low R_DS(ON) are preferred in the circuit
design. In some cases, more than one MOSFET are
connected in parallel to further decrease the on-state
resistance. However, this depends on the low-side
MOSFET driver capability and the budget.
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  V_OUT
f_SW  k 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

is a registered trademark of Richtek Technology Corporation.
DS8817A-00 September 2016