LM3560 Datasheet, PDF (36/42 Pages) Texas Instruments – LM3560 Synchronous Boost Flash Driver w/ Dual 1A High-Side Current Sources (2A Total Flash Current)

Applications Information
OUTPUT CAPACITOR SELECTION
The LM3560 is designed to operate with at least a 10 µF ce-
ramic output capacitor. When the boost converter is running
the output capacitor supplies the load current during the boost
converters on-time. When the NMOS switch turns off the in-
ductor energy is discharged through the internal PMOS
switch, supplying power to the load and restoring charge to
the output capacitor. This causes a sag in the output voltage
during the NFET on-time and a rise in the output voltage dur-
ing the NFET off-time. The output capacitor is therefore cho-
sen to limit the output ripple to an acceptable level depending
on load current and input/output voltage differentials and also
to ensure the converter remains stable.
For proper operation the output capacitor must be at least a
10 µF ceramic. Larger capacitors such as a 22 µF or multiple
capacitors in parallel can be used if lower output voltage ripple
is desired. To estimate the output voltage ripple considering
the ripple due to capacitor discharge (ΔV_Q) and the ripple due
to the capacitors ESR (ΔV_ESR) use the following equations:
For continuous conduction mode, the output voltage ripple
due to the capacitor discharge is:
In ceramic capacitors the ESR is very low so a close approx-
imation is to assume that 80% of the output voltage ripple is
due to capacitor discharge and 20% from ESR. Table 19 lists
different manufacturers for various output capacitors and their
case sizes suitable for use with the LM3560.
INPUT CAPACITOR SELECTION
Choosing the correct size and type of input capacitor helps
minimize the input voltage ripple caused by the switching of
the LM3560’s boost converter, and reduces noise on the
boost converters input terminal that can feed through and
disrupt internal analog signals. In the Typical Application Cir-
cuit a 10 µF ceramic input capacitor works well. It is important
to place the input capacitor as close as possible to the
LM3560’s input (IN) terminals. This reduces the series resis-
tance and inductance that can inject noise into the device due
to the input switching currents. Table 19 lists various input
capacitors that are recommended for use with the LM3560.
The output voltage ripple due to the output capacitors ESR is
found by:
TABLE 19. Recommended Input/Output Capacitors (X5R Dielectric)
Manufacturer
TDK Corporation
TDK Corporation
Murata
Part Number
C1608JB0J106M
C2012JB1A106M
GRM21BR61A106KE19
Value
10 µF
10 µF
10 µF
Case Size
0603 (1.6mm×0.8mm×0.8mm)
0805 (2mm×1.25mm×1.25mm)
0805 (2mm×1.25mm×1.25mm)
Voltage Rating
6.3V
10V
10V
INDUCTOR SELECTION
The LM3560 is designed to use a 1 µH or 2.2 µH inductor.
Table 20 lists various inductors and their manufacturers that
can work well with the LM3560. When the device is boosting
(V_OUT > V_IN) the inductor will typically be the largest area of
efficiency loss in the circuit. Therefore, choosing an inductor
with the lowest possible series resistance is important. Addi-
tionally, the saturation rating of the inductor should be greater
than the maximum operating peak current of the LM3560.
This prevents excess efficiency loss that can occur with in-
ductors that operate in saturation. For proper inductor oper-
ation and circuit performance, ensure that the inductor
saturation and the peak current limit setting of the LM3560
are greater than I_PEAK in the following calculation:
where ƒ_SW = 2MHz, and efficiency can be found in the Typical
Performance Characteristics plots.
TABLE 20. Recommended Inductors
Manufacturer
TOKO
TOKO
TDK
TDK
L
2.2 µH
1 µH
1 µH
1 µH
Part Number
Dimensions
(L×W×H)
FDSD0312- 3 mm×3.2 mm×1.2
H-2R02M
mm
FDSD0312- 3 mm×3.2 mm×1.2
H-1R0M
mm
VLS252012T-1R0N 2 mm×2.5 mm×1.2
mm
VLS4012ET-1R0N 4 mm x 4 mm x 1.2
mm
I_SAT
2.3A
3.4A
2.45A
2.8A
R_DC
105 mΩ
43 mΩ
73 mΩ
50 mΩ
35
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