LTC4088 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4088' PDF : 24 Pages View PDF
LTC4088
Applications Information
Table 3. Recommended Inductors for the LTC4088
L MAX IDC MAX DCR SIZE IN mm
INDUCTOR TYPE (µH) (A)
(Ω)
(L × W × H)
LPS4018
3.3 2.2
0.08 3.9 × 3.9 × 1.7
D53LC
DB318C
3.3 2.26
3.3 1.55
WE-TPC Type M1 3.3 1.95
0.034
0.070
0.065
5×5×3
3.8 × 3.8 × 1.8
4.8 × 4.8 × 1.8
CDRH6D12
CDRH6D38
3.3
2.2
0.0625 6.7 × 6.7 × 1.5
3.3 3.5
0.020
7×7×4
MANUFACTURER
Coilcraft
www.coilcraft.com
Toko
www.toko.com
Wurth Elektronik
www.we-online.com
Sumida
www.sumida.com
VBUS and VOUT Bypass Capacitors
The style and value of capacitors used with the LTC4088
determine several important parameters such as regula-
tor control-loop stability and input voltage ripple. Be-
cause the LTC4088 uses a step-down switching power
supply from VBUS to VOUT, its input current waveform
contains high frequency components. It is strongly
recommended that a low equivalent series resistance
(ESR) multilayer ceramic capacitor be used to bypass
VBUS. Tantalum and aluminum capacitors are not rec-
ommended because of their high ESR. The value of the
capacitor on VBUS directly controls the amount of input
ripple for a given load current. Increasing the size of this
capacitor will reduce the input ripple. The USB specifica-
tion allows a maximum of 10µF to be connected directly
across the USB power bus. If additional capacitance is
required for noise performance, a soft-connect circuit
may be required to limit inrush current and avoid exces-
sive transient voltage drops on the bus (see Figure 5).
To prevent large VOUT voltage steps during transient
load conditions, it is also recommended that a ceramic
capacitor be used to bypass VOUT. The output capacitor
is used in the compensation of the switching regula-
tor. At least 10µF with low ESR are required on VOUT.
Additional capacitance will improve load transient
performance and stability.
Multilayer ceramic chip capacitors typically have excep-
tional ESR performance. MLCCs combined with a tight
board layout and an unbroken ground plane will yield very
good performance and low EMI emissions.
There are several types of ceramic capacitors avail-
able each having considerably different characteristics.
For example, X7R ceramic capacitors have the best voltage
and temperature stability. X5R ceramic capacitors have
apparently higher packing density but poorer performance
over their rated voltage and temperature ranges. Y5V
ceramic capacitors have the highest packing density,
but must be used with caution, because of their extreme
nonlinear characteristic of capacitance versus voltage. The
actual in-circuit capacitance of a ceramic capacitor should
be measured with a small AC signal and DC bias as is
expected in-circuit. Many vendors specify the capacitance
verse voltage with a 1VRMS AC test signal and, as a result,
over state the capacitance that the capacitor will present
in the application. Using similar operating conditions as
the application, the user must measure or request from
the vendor the actual capacitance to determine if the
selected capacitor meets the minimum capacitance that
the application requires.
Overprogramming the Battery Charger
The USB high power specification allows for up to 2.5W
to be drawn from the USB port. The switching regulator
transforms the voltage at VBUS to just above the voltage
at BAT with high efficiency, while limiting power to less
than the amount programmed at CLPROG. The charger
should be programmed (with the PROG pin) to deliver the
maximum safe charging current without regard to the USB
specifications. If there is insufficient current available to
charge the battery at the programmed rate, it will reduce
charge current until the system load on VOUT is satisfied
and the VBUS current limit is satisfied. Programming the
charger for more current than is available will not cause
the average input current limit to be violated. It will merely
allow the battery charger to make use of all available
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