LTC3555 Linear Technology, LTC3555 Datasheet - Page 24

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

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LTC3555
APPLICATIONS INFORMATION
24
Choosing the PowerPath Inductor
Because the average input current circuit does not measure
reverse current (i.e., current from SW to V
reversal in the inductor at light loads will contribute an
error to the average V
is conservative in that if the current reverses, the voltage
at CLPROG will be higher than what would represent the
actual average input current drawn. The current available
for battery charging plus system load is thus reduced but
the USB specifi cation will not be violated.
This reduction in available V
the peak-peak inductor ripple is greater than twice the
average current limit setting. For example, if the average
current limit is set to 100mA, the peak-peak ripple should
not exceed 200mA. If the input current is less than 100mA,
the measurement accuracy may be reduced. However,
this will not affect the average current loop since it will
not be in regulation.
The LTC3555 includes a current-reversal comparator which
monitors inductor current and disables the synchronous
rectifi er as current approaches zero. This comparator will
minimize the effect of current reversal on the average input
current measurement. For some low inductance values,
however, the inductor current may still reverse slightly. This
value depends on the speed of the comparator in relation
to the slope of the current waveform, given by V
is the voltage across the inductor (approximately –V
and L is the inductance value.
An inductance value of 3.3μH is a good starting value. The
ripple will be small enough for the regulator to remain in
continuous conduction at 100mA average V
At lighter loads the current-reversal comparator will dis-
able the synchronous rectifi er for currents slightly above
0mA. As the inductance is reduced from this value, the
LTC3555 will enter discontinuous conduction mode at
progressively higher loads. Ripple at V
directly proportionally to the magnitude of inductor ripple.
Transient response, however, will improve. The current
mode controller controls inductor current to exactly the
amount required by the load to keep V
transient load step requires the inductor current to change
BUS
current measurement. The error
BUS
current will happen when
OUT
OUT
in regulation. A
BUS
will increase
BUS
), current
current.
L
/L. V
OUT
L
)
to a new level. Since inductor current cannot change instan-
taneously, the capacitance on V
difference in current until the inductor current can change
to meet the new load demand. A smaller inductor changes
its current more quickly for a given voltage drive than a
larger inductor, resulting in faster transient response. A
larger inductor will reduce output ripple and current ripple,
but at the expense of reduced transient performance and
a physically larger inductor package size. For this reason
a larger C
The input regulator has an instantaneous peak current
clamp to prevent the inductor from saturating during tran-
sient load or start-up conditions. The clamp is designed so
that it does not interfere with normal operation at high loads
and reasonable inductor ripple. It is intended to prevent
inductor current runaway in case of a shorted output.
The DC winding resistance and AC core losses of the in-
ductor will affect effi ciency, and therefore available output
power. These effects are diffi cult to characterize and vary
by application. Some inductors that may be suitable for
this application are listed in Table 6.
Table 6. Recommended Inductors
INDUCTOR
TYPE
LPS4018
D53LC
DB318C
WE-TPC
Type M1
CDRH6D12
CDRH6D38
V
The style and value of capacitors used with the LTC3555
determine several important parameters such as regulator
control-loop stability and input voltage ripple. Because
the LTC3555 uses a step-down switching power supply
from V
high frequency components. It is strongly recommended
that a low equivalent series resistance (ESR) multilayer
ceramic capacitor be used to bypass V
BUS
and V
BUS
VOUT
(μH)
3.3
3.3
3.3
3.3 1.95 0.065
3.3
3.3
to V
L
OUT
will be required for larger inductor sizes.
MAX
2.26
1.55
OUT
(A)
2.2
2.2
3.5
Bypass Capacitors
I
DC
, its input current waveform contains
0.0625
0.034
0.070
0.020
MAX
DCR
0.08
(Ω)
3.9 × 3.9 × 1.7 Coilcraft
3.8 × 3.8 × 1.8
4.8 × 4.8 × 1.8 Wurth Elektronik
6.7 × 6.7 × 1.5
(L × W × H)
SIZE in mm
5 × 5 × 3
7 × 7 × 4
OUT
delivers or absorbs the
BUS
MANUFACTURER
www.coilcraft.com
Toko
www.toko.com
www.we-online.com
Sumida
www.sumida.com
. Tantalum and
3555f