rt8059 Richtek Technology Corporation, rt8059 Datasheet - Page 8

• Manufacturer Part Number: rt8059
• Description: 1.5mhz, 1a, High Efficiency Pwm Step-down Dc/dc Converter
• Manufacturer: Richtek Technology Corporation

RT8059
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
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 Setting
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
www.richtek.com
8
IN
L
increases with input voltage. Multiple capacitors
. At best, this ringing can couple to the output
Figure 1. Setting Output Voltage
RT8059
GND
FB
IN
large enough to damage the
V
OUT
R1
R2
For adjustable voltage mode, the output voltage is set by
an external resistive voltage divider according to the
following equation :
where V
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to ΔI
resistance of C
discharge C
by the regulator to return V
During this recovery time, V
overshoot or ringing which would indicate a stability
problem.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P
where T
the ambient temperature, and θ
thermal resistance.
For recommended operating condition specifications of
the RT8059, the maximum junction temperature is
and T
thermal resistance, θ
23-5 packages, the thermal resistance, θ
on a standard JEDEC 51-3 single-layer thermal test board.
The maximum power dissipation at T
calculated by the following formula :
P
TSOT-23-5 package
V
D(MAX)
D(MAX)
OUT
A
=
is the ambient temperature. The junction to ambient
= (T
J(MAX)
REF
=
V
REF
LOAD
(125°C
J(MAX)
is the internal reference voltage (0.6V typ.)
OUT
is the maximum junction temperature, T
(1
(ESR), where ESR is the effective series
+
generating a feedback error signal used
OUT
− T
R2
R1
− 25°C) / (255°C/W) = 0.392W for
. ΔI
OUT
A
JA
)
) / θ
, is layout dependent. For TSOT-
LOAD
immediately shifts by an amount
JA
OUT
also begins to charge or
OUT
JA
to its steady-state value.
is the junction to ambient
can be monitored for
DS8059-03 April 2011
A
=
JA
25°C
, is 255°C/W
can be
125°C
A
is