Datasheet

LTC4099
26
4099fd
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 current limit modes. The input current will
be comprised of two components, the current that is used
to deliver power to V
OUT
, and the quiescent current of the
switching regulator. To ensure that the USB specification
is strictly met, 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.
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 power path switching regulator are both fairly nar
-
row,
the LTC4099 was designed for a specific inductance
value of 3.3µH. Some inductors which may be suitable
for this application are listed in Table 9.
Table 9. Recommended Inductors for the LTC4099
INDUCTOR
TYPE
L
(µH)
MAX
I
DC
(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
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
T
ype M1
3.3 1.95 0.065
4.8 ×
4.8
× 1.8
Würth Elektronik
www.we-online.com
CDRH6D12
CDRH6D38
3.3
3.3
2.2
3.5
0.063
0.020
6.7
× 6.7
× 1.5
7 × 7
× 4
Sumida
www.sumida.com
applicaTions inForMaTion
V
BUS
and V
OUT
Bypass Capacitors
The style and value of the capacitors used with the
LTC4099 determine several important parameters such as
regulator control
loop stability and input voltage ripple.
Because the LTC4099 uses a step-down switching power
supply from
V
BUS
to V
OUT
, its input current waveform
contains high frequency components. It is strongly rec-
ommended that
a low equivalent series resistance (ESR)
multilayer ceramic capacitor be used to bypass
V
BUS
.
Tantalum and aluminum capacitors are not recommended
because of their high ESR. The value of the capacitor on
V
BUS
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 connected directly across the
USB power bus. If the overvoltage protection circuit is
used to protect
V
BUS
, then its soft-starting nature can
be exploited and a larger
V
BUS
capacitor can be used
if desired.
To prevent large
V
OUT
voltage steps during transient load
conditions, it is also recommended that a ceramic ca-
pacitor be used to bypass
V
OUT
. The output capacitor is
used in the compensation of the switching regulator. At
least 10µF with low ESR are required on
V
OUT
. 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 available
,
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 perfor
-
mance 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