EN
(V)
Low
“>2.4V”
=> High
“>2.4V”
=> High
“>2.4V”
=> High
“>2.4V”
=> High
“>2.4V”
=> High
ENC
(V)
Low
Low
“>2V”
=> High
“>2V”
=> High
“>2V”
=> High
“>2V”
=> High
RT8223P
Table 3. Power Up Sequencing
ENTRIP1 ENTRIP2 REF VREG5 VREG3
X
X
Off
Off
Off
X
X
On
On
On
SMPS1
Off
Off
SMPS2
Off
Off
Off
Off
On
On
On
Off
Off
Off
On
On
On
On
Off
On
On
Off
On
On
On
On
Off
On
On
On
On
On
On
On
Output Voltage Setting (FBx)
Connect a resistor voltage-divider at the FBx pin between
VOUTx and GND to adjust the respective output voltage
between 2V and 5.5V (Figure 4). Referring to Figure 4 as
an example, choose R2 to be approximately 10kΩ, and
solve for R1 using the equation :
VOUTX
=
VFBX
×
⎛⎜1+
⎝
⎛
⎜⎝
R1
R2
⎞
⎟⎠
⎞
⎟
⎠
where VFBX is 2V.
VIN
UGATEx
PHASEx
LGATEx
VOUTx
FBx
VOUTx
R1
R2
Figure 4. Setting VOUTX with a Resistor Voltage Divider
Output Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as shown in
the following equation :
L = tON × (VIN − VOUTx )
LIR ×ILOAD(MAX)
where LIR is the ratio of the peak to peak ripple current to
the average inductor current.
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
(IPEAK) :
IPEAK = ILOAD(MAX) + ⎡⎣(LIR/2)×ILOAD(MAX) ⎤⎦
The calculation above shall serve as a general reference.
To further improve the transient response, the output
inductance can be reduced even further. This needs to be
considered along with the selection of the output capacitor.
Output Capacitor Selection
The capacitor value and ESR determine the amount of
output voltage ripple and load transient response. Thus,
the capacitor value must be greater than the largest value
calculated from below equations :
VSAG
=
(ΔILOAD )2
×L × (K ×
VOUTx
VIN
+
tOFF(MIN) )
2 × COUT
×
VOUTx
×
⎡
⎢K
⎣
×
⎛
⎜
⎝
VIN
− VOUTx
VIN
⎞⎤
⎟⎥
⎠⎦
−
tOFF(MIN)
VSOAR
=
(ΔILOAD )2 ×L
2 × COUT × VOUTx
DS8223P-01 June 2011
www.richtek.com
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