LTC3576 Datasheet, PDF (33/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go + Triple Step-Down DC/DCs

LTC3576/LTC3576-1
APPLICATIONS INFORMATION
Bidirectional PowerPath Switching Regulator
CLPROG Resistor and Capacitor Selection
As described in the Bidirectional Switching Regulator—
Step-Down Mode section, the resistor on the CLPROG
pin determines the average V_BUS input current limit when
the switching regulator is 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 USB
specification is strictly met, both components of the 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 pos-
sible, 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 input voltage range and output voltage range
of the PowerPath switching regulator are both fairly nar-
row, the LTC3576/LTC3576-1 were designed for a specific
inductance value of 3.3μH. Some inductors which may be
suitable for this application are listed in Table 7.
Table 7. Recommended PowerPath Inductors for the LTC3576
MAX MAX
INDUCTOR L I_DC DCR SIZE IN mm
TYPE
(μH) (A) (Ω) (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 0.034 5 × 5 × 3 Toko
3.3 1.55 0.070 3.8 × 3.8 × 1.8 www.toko.com
WE-TPC
Type M1
3.3 1.95 0.065 4.8 × 4.8 × 1.8 Wurth Electronik
www.we-online.com
CDRH6D12 3.3 2.2 0.063 6.7 × 6.7 × 1.5 Sumida
CDRH6D38 3.3 3.5 0.020 7 × 7 × 4 www.sumida.com
Bidirectional PowerPath Switching Regulator V_BUS
and V_OUT Bypass Capacitor Selection
The type and value of capacitors used with the LTC3576/
LTC3576-1 determine several important parameters such
as regulator control-loop stability and input voltage ripple.
Because the LTC3576/LTC3576-1 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 several
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.
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