RT8205A RICHTEK [Richtek Technology Corporation], RT8205A Datasheet - Page 24

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

RT8205A/B/C
Output Voltage Setting (FBx)
Connect FBx directly to GND or VREG5 to enable the
fixed, SMPS output voltages (3.3V and 5V). Connect a
resistor voltage-divider at the FBx between the VOUTx
and GND to adjust the respective output voltage between
2V and 5.5V (Figure 6). Choose R2 to be approximately
10kΩ, and solve for R1 using the equation :
where V
VREG5 connects to VOUT1 through an internal switch
only when VOUT1 is above the VREG5 automatic switch
threshold (4.66V). VREG3 connects to VOUT2 through
an internal switch only when VOUT2 is above the VREG3
automatic switch threshold (3.06V). This is the most
effective way when the fixed output voltages are used.
Once VREGx is supplied from VOUTx, the internal linear
regulator turns off. This reduces internal power dissipation
and improves efficiency when the VREGx is powered with
a high input voltage.
Output Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or L
follows :
where L
the average inductor current.
Copyright
www.richtek.com
24
L =
V
OUTx
Figure 6. Setting VOUTx with a Resistor-Divider
T
ON
L
©
= V
IR
IR
FBx
2012 Richtek Technology Corporation. All rights reserved.
×
UGATEx
PHASEx
LGATEx
is the ratio of the peak to peak ripple current to
IR
×
VOUTx
(V
FBx
PGND
is 2V (typ.).
) determine the inductor value as shown as
I
LOAD(MAX)
GND
IN
FBx
−
×
⎡
⎢
⎣
V
1
OUTx
+ ⎜
⎛
⎝
R2
R1
)
⎞
⎟
⎠
⎤
⎥
⎦
V
IN
R1
R2
V
OUTx
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice, although powdered iron is
inexpensive and can work well at 200kHz. The core must
be large enough not to saturate at the peak inductor current
(I
This inductor ripple current also impacts transient-response
performance, especially at low VIN − VOUTx differences.
Low inductor values allow the inductor current to slew
faster, replenishing charge removed from the output filter
capacitors by a sudden load step. The peak amplitude of
the output transient V
transient. The V
maximum duty factor, which can be calculated from the
on-time and minimum off-time :
Where minimum off-time (T
is from Table 1.
Output Capacitor Selection
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements, yet
have high enough ESR to satisfy stability requirements.
Also, the capacitance value must be high enough to
absorb the inductor energy going from a full-load to no-
load condition without tripping the OVP circuit.
For CPU core voltage converters and other applications
where the output is subject to violent load transients, the
output capacitor's size depends on how much ESR is
needed to prevent the output from dipping too low under a
load transient. Ignoring the sag due to finite capacitance :
ESR
=
V
I
PEAK
PEAK
SAG
2 C
×
is a registered trademark of Richtek Technology Corporation.
) :
= I
≤
OUT
LOAD(MAX)
( I
I
Δ
LOAD(MAX)
LOAD
×
V
P-P
V
OUTx
)
SAG
2
+ [(L
× ×
L
×
SAG
also features a function of the
⎡
⎢
⎣
IR
K
⎛
⎜
⎝
/ 2) x I
K
⎛
⎜
⎝
is also a function of the output
V
V
OFF(MIN)
IN
OUTx
V
DS8205A/B/C-06 July 2012
IN
−
LOAD(MAX)
V
V
IN
OUTx
+
) = 300ns (typ.) and K
T
OFF(MIN)
⎞
⎟
⎠
]
−
T
OFF(MIN)
⎞
⎟
⎠
⎤
⎥
⎦