RT8205A RICHTEK [Richtek Technology Corporation], RT8205A Datasheet - Page 25

• Manufacturer Part Number: RT8205A
• Description: High Efficiency, Main Power Supply Controllers for Notebook Computers
• Manufacturer: RICHTEK [Richtek Technology Corporation]

In non-CPU applications, the output capacitor's size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple :
where V
Organic semiconductor capacitor(s) or specialty polymer
capacitor(s) are recommended.
For low input-to-output voltage differentials (VIN / VOUTx
< 2), additional output capacitance is required to maintain
stability and good efficiency in ultrasonic mode.
The amount of overshoot due to stored inductor energy
can be calculated as :
where I
Although Mach Response
provides many advantages such as ease-of-use, minimum
external component configuration, and extremely short
response time, due to not employing an error amplifier in
the loop, a sufficient feedback signal needs to be provided
by an external circuit to reduce the jitter level. The required
signal level is approximately 15mV at the comparing point.
This generates V
node. The output capacitor ESR should meet this
requirement.
Output Capacitor Stability
Stability is determined by the value of the ESR zero relative
to the switching frequency. The point of instability is given
by the following equation :
Do not put high-value ceramic capacitors directly across
the outputs without taking precautions to ensure stability.
Large ceramic capacitors can have a high- ESR zero
frequency and cause erratic, unstable operation. However,
it is easy to add enough series resistance by placing the
capacitors a couple of inches downstream from the
inductor and connecting VOUTx or the FBx divider close
to the inductor.
Copyright
DS8205A/B/C-06 July 2012
ESR
V
f
ESR
SOAR
=
≤
©
PEAK
≤
P-P
2
2012 Richtek Technology Corporation. All rights reserved.
L
× ×
IR
2 C
is the peak-to-peak output voltage ripple.
π
is the peak inductor current.
×
×
(I
I
ESR C
LOAD(MAX)
V
PEAK
OUT
P-P
Ripple
1
×
×
)
2
V
= (V
×
OUT
OUTx
L
TM
OUT
DRV
≤
f
/ 2) x 15mV at the output
SW
4
TM
dual ramp valley mode
Unstable operation manifests itself in two related and
distinctly different ways: double-pulsing and feedback loop
instability.
Double-pulsing occurs due to noise on the output or
because the ESR is so low that there is not enough voltage
ramp in the output voltage signal. This “fools” the error
comparator into triggering a new cycle immediately after
the 300ns minimum off-time period has expired. Double-
pulsing is more annoying than harmful, resulting in nothing
worse than increased output ripple. However, it may
indicate the possible presence of loop instability, which
is caused by insufficient ESR.
Loop instability can result in oscillations at the output
after line or load perturbations that can trip the overvoltage
protection latch or cause the output voltage to fall below
the tolerance limit.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully observe
the output-voltage-ripple envelope for overshoot and ringing.
It helps to simultaneously monitor the inductor current
with an AC current probe. Do not allow more than one
cycle of ringing after the initial step-response under- or
overshoot.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum operation junction temperature. The maximum
power dissipation depends on the thermal resistance of
IC package, PCB layout, the rate of surroundings airflow
and temperature difference between junction to ambient.
The maximum power dissipation can be calculated by
following formula :
P
Where T
temperature, T
the junction to ambient thermal resistance.
For recommended operating conditions specification, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance θ
WQFN-24L 4x4 packages, the thermal resistance θ
52°C/W on the standard JEDEC 51-7 four layers thermal
D(MAX)
is a registered trademark of Richtek Technology Corporation.
= ( T
J(MAX)
J(MAX)
A
is the ambient temperature and the θ
is the maximum operation junction
- T
A
) / θ
JA
RT8205A/B/C
JA
is layout dependent. For
www.richtek.com
JA
JA
25
is
is