Datasheet

V
IN
- V
OUT
L
=
2'i
L
DT
S
t
L
i'
OUT
I
S
T
S
DT
L
V
OUT
L
- V
OUT
V
IN
D =
V
OUT
+ V
D
V
IN
+ V
D
- V
SW
D =
V
OUT
V
IN
FB
GND
SW
EN
VOUT
L1
D1
R1
R2
VINA/VIND
C1 C4
U1
C2
GND
EN
2
6
4, 5
1
3
R3
Chf
2.2 PF
VIN
C3
GND
22 PF
20k
10k
2.2 PF
22 PF
22 nF
(opt.)
1.0 PH
3.3 PH
(³;´YHUVLRQ)
1.8V
LMR10515
SNVS728C –OCTOBER 2011–REVISED APRIL 2013 www.ti.com
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature
exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature
drops to approximately 150°C.
Figure 22. Typical Application Schematic
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
O
) to input voltage (V
IN
):
The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to
calculate a more accurate duty cycle. Calculate D by using the following formula:
V
SW
can be approximated by:
V
SW
= I
OUT
x R
DSON
The diode forward drop (V
D
) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the
V
D
, the higher the operating efficiency of the converter. The inductor value determines the output ripple current.
Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the
inductor value will decrease the output ripple current.
One must ensure that the minimum current limit (1.8A) is not exceeded, so the peak current in the inductor must
be calculated. The peak current (I
LPK
) in the inductor is calculated by:
I
LPK
= I
OUT
+ Δi
L
Figure 23. Inductor Current
In general,
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