rt8011 Richtek Technology Corporation, rt8011 Datasheet - Page 12

• Manufacturer Part Number: rt8011
• Description: 4mhz, Synchronous Step-down Regulator
• Manufacturer: Richtek Technology Corporation

RT8011/A
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don't radiate energy but generally cost more
than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs. size requirements and
any radiated field/EMI requirements.
C
The input capacitance, C
trapezoidal current at the source of the top MOSFET. To
prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used. RMS
current is given by :
This formula has a maximum at V
I
commonly used for design because even significant
deviations do not offer much relief. 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 C
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, ΔV
The output ripple is highest at maximum input voltage
since ΔI
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
www.richtek.com
12
I
Δ
RMS
RMS
IN
V
OUT
and C
= I
=
I
L
≤
OUT(MAX)
OUT
increases with input voltage. Multiple capacitors
Δ
OUT
I
L
/2. This simple worst-case condition is
⎡
⎢ ⎣
ESR
Selection
OUT
V
V
OUT
+
IN
OUT
is determined by the effective series
8fC
, is determined by :
1
OUT
V
V
OUT
IN
IN
⎤
⎥ ⎦
, is needed to filter the
−
1
IN
= 2V
OUT
, where
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
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, V
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at V
part.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation :
where V
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 6.
V
OUT
=
IN
REF
V
. At best, this ringing can couple to the output
Figure 6. Setting the Output Voltage
REF
equals to 0.8V typical.
×
⎛ +
⎜
⎝
1
RT8011/A
R2
R1
⎞
⎟
⎠
GND
V
IN
FB
large enough to damage the
DS8011/A-00 August 2006
V
OUT
R1
R2