Datasheet

LTC4160/LTC4160-1
21
41601fa
APPLICATIONS INFORMATION
Bidirectional PowerPath Switching Regulator CLPROG
Resistor and Capacitor Selection
As described in the Bidirectional PowerPath Switching
Regulator – Step-Down Mode section, the resistor on the
CLPROG pin determines the average V
BUS
input current
limit. In step-down mode the switching regulator’s V
BUS
input current limit can be set to either the 1x mode (USB
100mA), the 5x mode (USB 500mA) or the 10x mode. The
V
BUS
input current will be comprised of two components,
the current that is used to drive V
OUT
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. Recall that:
I
VBUS
= I
VBUSQ
+ V
CLPROG
/R
CLPPROG
• (h
CLPROG
+1).
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.
Bidirectional PowerPath Switching Regulator Inductor
Selection
Because the V
BUS
voltage range and V
OUT
voltage range
of the PowerPath switching regulator are both fairly nar-
row, the LTC4160/LTC4160-1 were designed for a specific
inductance value of 3.3μH. Some inductors which may be
suitable for this application are listed in Table 3.
Table 3. Recommended PowerPath Inductors for the
LTC4160/LTC4160-1
INDUCTOR
TYPE
L
(μH)
MAX IDC
(A)
MAX DCR
(Ω)
SIZE IN mm
(L x W x H) MANUFACTURER
LPS4018 3.3 2.2 0.08 3.9 x 3.9 x 1.7 Coilcraft
www.coilcraft.com
D53LC
DB318C
3.3
3.3
2.26
1.55
0.034
0.070
5 x 5 x 3
3.8 x 3.8 x 1.8
Toko
www.toko.com
WE-TPC
Type M1
3.3 1.95 0.065 4.8 x 4.8 x 1.8 Wurth Electronik
www.we-online.com
CDRH6D12
CDRH6D38
3.3
3.3
2.2
3.5
0.063
0.020
6.7 x 6.7 x 1.5
7 x 7 x 4
Sumida
www.sumida.com
Bidirectional PowerPath Switching Regulator V
BUS
and V
OUT
Bypass Capacitor Selection
The type and value of capacitors used with the LTC4160/
LTC4160-1 determine several important parameters such
as regulator control-loop stability and input voltage ripple.
Because the LTC4160/LTC4160-1 use a bidirectional
switching regulator between V
BUS
and V
OUT
, the V
BUS
current waveform contains high frequency components.
It is strongly recommended that a low equivalent series
resistance (ESR) multilayer ceramic capacitor (MLCC) 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 inrush current limit specification for USB devices is
calculated in terms of the total number of Coulombs needed
to charge the V
BUS
bypass capacitor to 5V. The maximum
inrush charge for USB On-The-Go devices is 33μC. This
places a limit of 6.5μF of capacitance on V
BUS
assuming
a linear capacitor. However, most ceramic capacitors have
a capacitance that varies with bias voltage. The average
capacitance needs to be less than 6.5μF over a 0V to 5V bias
voltage range to meet the inrush current-limit specification.
A 10μF capacitor in a 0805 package, such as the Murata
GRM21BR71A106KE51L would be a suitable V
BUS
bypass
capacitor. If more capacitance is required for better noise
performance and stability, it should be connected directly
to the V
BUS
pin when using the overvoltage protection
circuit. This extra capacitance will be soft-connected over
a couple of milliseconds to limit inrush current and avoid
excessive transient voltage drops on V
BUS
.
To prevent large V
OUT
voltage steps during transient load
conditions, it is also recommended that an MLCC be used
to bypass V
OUT
. The output capacitor is used in the com-
pensation of the switching regulator. At least 10µF with
low ESR are required on V
OUT
. Additional capacitance will
improve load transient performance and stability.
MLCCs typically have exceptional ESR performance.
MLCCs combined with a tight board layout and an unbroken
ground plane will yield very good performance and low
EMI emissions.