LTC3456_15 Datasheet, PDF (24/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

LTC3456
APPLICATIO S I FOR ATIO
We can choose a low resistance 7.8µH or slightly higher
value inductor. We can choose a 450mA, 10µH inductor
(Murata LQH32CN100).
We need to check the ripple current when the core output
is powered from the AC adapter or the USB. L1 is used to
power the output in this case too.
Substituting V_OUT = 1.8V, V_IN(MAX) = 5.5V , L1 = 10µH,
f = 1MHz in above equation gives:
∆I_L1
=
1.8V • ₍5.5V –
5.5V • 10µH •
1.8V₎
1MHz
=
120mA
L1 gives a reasonable value of ripple current when pow-
ered from both battery and USB or AC adapter.
The main output needs to be 3.3V and the maximum load
current is 200mA. Hot Swap current is derived from the
same main converter.
When powered from the 2 AA cell, the 3.3V output is
generated via the L2 boost inductor. The inductor current
ripple, ∆I_L2, for Boost mode operation is given by:
( ) _∆IL2
=
V_IN • V_OUT – V_IN
V_OUT • f(L2)
A reasonable starting value of inductor ripple current is
∆I_L2 = 150mA. Substituting V_OUT = 3.3V, V_IN(MIN) = 1.8V,
∆I_L2 = 150mA, f = 1MHz in above equation gives:
L2
=
1.8V • ₍3.3V – 1.8V₎
3.3V • 150mA • 1MHz
=
5.4µH
We can choose a low resistance 4.7µH, 650mA inductor
(Murata LQH32CN4R7M53).
When powered from the AC adapter or the USB, the 3.3V
output is generated via the L3 buck inductor. The inductor
current ripple (∆I_L3) for Buck mode operation is given by:
( ) _∆IL3
=
V_OUT • V_IN – V_OUT
V_IN • f(L3)
A reasonable starting value of inductor ripple current is
∆I_L3 = 100mA. Substituting V_OUT = 3.3V, V_IN(MAX) = 5.5V,
∆I_L3 = 100mA, f = 1MHz in above equation gives:
L3
=
3.3V • ₍5.5V – 3.3V₎
5.5V • 100mA • 1MHz
=
13.2µH
We can choose a 450mA, 10µH inductor (Murata
LQH32CN100K53).
A 4.7µF to 22µF (X5R or X7R) ceramic output capacitor is
sufficient for most applications. They have a low ESR and
result in a low output ripple.
Figure 18 shows the complete circuit along with the
efficiency curves.
3456fa
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