RT8004A 查看數據表（PDF） - Richtek Technology
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RT8004A Datasheet PDF : 13 Pages
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RT8004A
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is
commonly used for design because even significant
deviations do not result in much difference. Note that ripple
current ratings from capacitor manufacturers are often
based on only 2000 hours of life which makes it advisable
to further derate the capacitor, or choose a capacitor rated
at a higher temperature than required. Several capacitors
may also be paralleled to meet size or height requirements
in the design.
The selection of COUT is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients, as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later section.
The output ripple, ΔVOUT, is determined by :
ΔVOUT
≤
ΔIL
⎢⎣⎡ESR
+
1
8fCOUT
⎤
⎥⎦
The output ripple is highest at maximum input voltage
since ΔIL increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and
RMS current handling requirements. Dry tantalum, special
polymer, aluminum electrolytic and ceramic capacitors are
all available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density but it is important to only
use types that have been surge tested for use in switching
power supplies. Aluminum electrolytic capacitors have
significantly higher ESR but can be used in cost-sensitive
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capacitors
have excellent low ESR characteristics but can have a
high voltage coefficient and audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
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10
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, VIN. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at VIN large enough to damage the
part.
Output Voltage Programming
The output voltage is set by an external resistive voltage-
divider according to the following equation :
VOUT = 0.8V (1+ R1 )
R2
The resistive voltage-divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
VOUT
R1
FB
RT8004A
R2
GND
Figure 1. Setting the Output Voltage
Frequency Synchronization
The RT8004A’ s internal oscillator can be synchronized
to an external clock signal. During synchronization, the
high side MOSFET turn-on is locked to the falling edge of
the external frequency source. The synchronization
frequency range is from 300kHz to 4MHz. Synchronization
only occurs if the external frequency is greater than the
frequency set by the external resistor. Because slope
compensation is generated by the oscillator’ s RC circuit,
the external frequency should be set 25% higher than the
frequency set by the external resistor to ensure that
adequate slope compensation is present.
Soft-Start
The EN/SS pin provides a means to shut down the
RT8004A as well as a timer for soft-start. Pulling the EN/
SS pin below 0.5V places the RT8004A in a low quiescent
current shutdown state (IQ < 1μA).
The RT8004A contains an internal soft-start clamp that
DS8004A-03 March 2011
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