Datasheet

( ) ( )
( )
( )
( )
f
OUT
RIPPLE
V 20 mV 1 D 20 mV L
ESR
2 V I 2 V
´ ´ - ´ ´
= -
´
( )
( )
( )
(
)
( )
f
OUT OUT
IN max
TRIP
IND peak
DS on IN max
V
1
R L
V V V
I
V´
- ´
= + ´
( )
( )
(
)
( )
( )
( )
(
)
( )
f f
OUT OUT OUT OUT
IN max IN max
IND ripple IN max OUT max IN max
1 3
I I
V V V V V V
L
V V
´
´ ´
- ´ - ´
= = ´
( )
OUT
V 2.0
R1 R2
2.0
-
= ´
TPS51123
SLUS890E –DECEMBER 2008–REVISED JANUARY 2013 www.ti.com
UVLO Protection
TPS51123 has VREG5 under voltage lock out protection (UVLO). When the VREG5 voltage is lower than UVLO
threshold voltage both switch mode power supplies are shut off. This is non-latch protection. When the VREG3
voltage is lower than (VO2 - 1 V), both switch mode power supplies are also shut off
Thermal Shutdown
TPS51123 monitors the temperature of itself. If the temperature exceeds the threshold value (typically 150°C),
TPS51123 is shut off including LDOs. This is non-latch protection.
External Parts Selection
The external components selection is much simple in D-CAP™ Mode.
1. Determine output voltage
The output voltage is programmed by the voltage-divider resistor, R1 and R2, as shown in Figure 36. R1 is
connected between VFBx pin and the output, and R2 is connected betwen the VFBx pin and GND.
Recommended R2 value is from 10 kΩ to 20 kΩ. Determine R1 using equation as below.
(5)
2. Choose the Inductor
The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum
output current. Larger ripple current increases output ripple voltage and improves S/N ratio and helps stable
operation.
(6)
The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak
inductor current before saturation. The peak inductor current can be estimated as follows.
(7)
3. Choose the Output Capacitor(s)
Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. Determine ESR to meet
required ripple voltage. A quick approximation is as shown in Equation 8. This equation is based on that required
output ripple slope is approximately 20 mV per T
SW
(switching period) in terms of VFB terminal voltage.
where
• D is the duty cycle
• the required output ripple slope is approximately 20 mV per t
SW
(switching period) in terms of VFB terminal
voltage (8)
4. Choose the Low-Side MOSFET
It is highly recommended that the low-side MOSFET should have an integrated Schottky barrier diode, or an
external Schottky barrier diode in parallel to achieve stable operation.
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