RT8055GQW Richtek USA Inc, RT8055GQW Datasheet - Page 10

• Manufacturer Part Number: RT8055GQW
• Description: IC DCDC CONV STP-DN SYNC 10WDFN
• Manufacturer: Richtek USA Inc
• Type: Step-Down (Buck), PWM - Current Moder

Specifications of RT8055GQW

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 V ~ 5 V
• Current - Output: 3A
• Frequency - Switching: 300kHz ~ 2MHz
• Voltage - Input: 2.6 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-WFDFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

RT8055
This keeps the maximum output current relatively constant
regardless of duty cycle.
Short Circuit Protection
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. A
current runaway detector is used to monitor inductor
current. As current increasing beyond the control of current
loop, switching cycles will be skipped to prevent current
runaway from occurring.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔI
and decreases with higher inductance.
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. However, it requires a large inductor to achieve this
goal.
For the ripple current selection, the
will be a reasonable starting point. The
current occurs at the highest V
ripple current stays below the specified maximum, the
inductor value should be chosen according to the following
equation :
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit.
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 :
www.richtek.com
10
L =
I
Δ
RMS
IN
I =
L
⎡
⎢
⎣
and C
f
=
⎡
⎢
⎣
× Δ
V
I
V
f L
OUT(MAX)
OUT
×
OUT
I
L(MAX)
OUT
⎤ ⎡
⎥ ⎢
⎦ ⎣
×
Selection
⎤ ⎡
⎥ ⎢
⎦ ⎣
1
×
V
−
V
OUT
V
1
IN
V
−
OUT
IN
V
IN(MAX)
V
⎤
⎥
⎦
V
OUT
V
OUT
IN
IN
L
, is needed to filter the
increases with higher V
IN
⎤
⎥
⎦
−
. To guarantee that the
1
value of ΔI
largest ripple
L
= 0.4(I
MAX
IN
)
This formula has a maximum at V
I
commonly used for design because even significant
deviations do not offer much relief. 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
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
Δ
RMS
V
OUT
= I
L
≤
OUT
increases with input voltage. Multiple capacitors
Δ
I
L
/2. This simple worst-case condition is
⎡
⎢ ⎣
ESR
OUT
+
OUT
is determined by the effective series
8fC
, is determined by :
1
OUT
⎤
⎥ ⎦
DS8055-03 March 2011
IN
= 2V
OUT
, where