Datasheet
SS SS
1.25V
t = C x
7.5μA
SS
FB
V
V =
1.56
R4
43k
R3
59k
V1
Output of external
DC DC converter
Vout
1.5V/3A
Vin
2.5V to 5.5V
PVIN
PVIN
EN
FB
C1
22mF
L1
1mH
C2
22mF
R1
140k
SW
TLV62090
SW
Power Good
AVIN
R2
160k
SS
R3
500k
AGND
PG
VOS
DEF
CP
CN
C3
10nF
1
2
13
4
5
6
PGND PGND
7
8
9
10
11
12
3
14 15
16
R3 R1
= x 1.56
R4 R2
TLV62090
www.ti.com
SLVSBB9B –MARCH 2012–REVISED APRIL 2012
Where
R
DS(on)
= High side FET on-resistance
R
L
= DC resistance of the inductor
V
OUT(max)
= nominal output voltage plus maximum output voltage tolerance
Softstart (SS)
To minimize inrush current during start up, the device has an adjustable softstart depending on the capacitor
value connected to the SS pin. The device charges the softstart capacitor with a constant current of typically 7.5
µA. The feedback voltage follows this voltage with a fraction of 1.56 until the internal reference voltage of 0.8 V is
reached. The softstart operation is completed once the voltage at the softstart capacitor has reached typically
1.25 V. The soft-start time can be calculated using Equation 3. The larger the softstart capacitor the longer the
softstart time. The relation between softstart voltage and feedback voltage can be estimated using Equation 4.
(3)
(4)
This is also the case for the fixed output voltage option having the internal regulation voltage. Leaving the
softstart pin floating sets the minimum start-up time.
Start-up Tracking (SS)
The softstart pin can also be used to implement output voltage tracking with other supply rails. The internal
reference voltage follows the voltage at the softstart pin with a fraction of 1.56 until the internal reference voltage
of 0.8 V is reached. The softstart pin can be used to implement output voltage tracking as shown in Figure 20.
Figure 20. Output Voltage Tracking
In Figure 20, the output V2 tracks the voltage applied to V1. The voltage tracks simultaneously when following
conditions are met:
(5)
As the fraction of R3/R4 becomes larger the voltage V1 ramps up faster than V2, and if it gets smaller then the
ramp is slower than V2. R4 needs to be determined first using Equation 6.
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