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LTC4098/LTC4098-1
22
40981fc
CLPROG Resistor and Capacitor
As described in the Bat-Track Input Current Limited Step
Down Switching Regulator section, the resistor on the
CLPROG pin determines the average input current limit
in each of the six current limit modes. The input cur-
rent will be comprised of two components, the current
that is used to drive VOUT and the quiescent current of
the switching regulator. To ensure that the total average
input current remains below the USB specification, both
components of input current should be considered. The
Electrical Characteristics table gives the typical values for
quiescent currents in all settings as well as current limit
programming accuracy. To get as close to the 500mA or
100mA specifications as possible, a precision resistor
should be used.
Higher or lower input currents may be programmed with
the use of a resistor other than the recommended 3.01k.
The resistor value can be calculated using the formula:
RCLPROG =
VCLPROG
INOM IVBUSQ
hCLPROG + 1
(
)
where INOM is the desired nominal input current limit. Note
that this resistor will affect all input current limit settings
and may cause the LTC4098/LTC4098-1 to exceed USB
current specifications.
An averaging capacitor is required in parallel with the
resistor so that the switching regulator can determine
the average input current. This capacitor also provides
the dominant pole for the feedback loop when current
limit is reached. To ensure stability, the capacitor on
CLPROG should be 0.1μF or larger.
Choosing the Inductor
Because the input voltage range and output voltage range
of the PowerPath switching regulator are both fairly nar-
row, the LTC4098/LTC4098-1 were designed for a specific
inductance value of 3.3μH. Some inductors which may be
suitable for this application are listed in Table 3.
VBUS and VOUT Bypass Capacitors
The style and value of capacitors used with the LTC4098/
LTC4098-1 determine several important parameters such
as regulator control loop stability and input voltage
ripple. Because the LTC4098/LTC4098-1 use 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 recommended 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 specification allows a maximum of 10μF to be con-
nected directly across the USB power bus. If additional
capacitance is required for noise performance, it may be
connected directly to the VBUS pin when using the OVP
feature of the LTC4098/LTC4098-1. This extra capacitance
will be soft-connected over several milliseconds to limit
inrush current and avoid excessive transient voltage
drops on the bus.
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
Table 3. Recommended Inductors for the LTC4098/LTC4098-1
INDUCTOR TYPE
L
(μH)
MAX IDC
(A)
MAX DCR
(
Ω)
SIZE IN mm
(L
× W × H)
MANUFACTURER
LPS4018
3.3
2.2
0.08
3.9
× 3.9 × 1.7
Coilcraft
www.coilcraft.com
D53LC
DB318C
3.3
2.26
1.55
0.034
0.070
5
× 5 × 3
3.8
× 3.8 × 1.8
Toko
www.toko.com
WE-TPC Type M1
3.3
1.95
0.065
4.8
× 4.8 × 1.8
Würth Elektronik
www.we-online.com
CDRH6D12
CDRH6D38
3.3
2.2
3.5
0.0625
0.020
6.7
× 6.7 × 1.5
7
× 7 × 4
Sumida
www.sumida.com
APPLICATIONS INFORMATION