LTC3576 Datasheet, PDF (20/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go + Triple Step-Down DC/DCs

LTC3576/LTC3576-1
OPERATION
The input current limit is programmed by the I_LIM0 and
I_LIM1 pins or by the I²C serial port. The input current limit
has five possible settings ranging from the USB suspend
limit of 500μA up to 1A for wall adapter applications. Two
of these settings are specifically intended for use in the
100mA and 500mA USB applications. Refer to Table 1 for
current limit settings using the I_LIM0 and I_LIM1 pins and
Table 6 for current limit settings using the I²C port.
Table 1. USB Current Limit Settings Using I_LIM0 and I_LIM1
I_LIM1
I_LIM0 USB SETTING
0
0
1× Mode (USB 100mA Limit)
0
1
10× Mode (Wall 1A Limit)
1
0
Low Power Suspend (USB 500μA Limit)
1
1
5× Mode (USB 500mA Limit)
When the switching regulator is activated, the average
input current will be limited by the CLPROG programming
resistor according to the following expression:
( ) ^I_VBUS
=
I_VBUSQ
+
V_CLPROG
R_CLPROG
•
h_CLPROG + 1
where I_VBUSQ is the quiescent current of the LTC3576/
LTC3576-1, V_CLPROG is the CLPROG servo voltage in
current limit, R_CLPROG is the value of the programming
resistor and h_CLPROG is the ratio of the measured cur-
rent at V_BUS to the sample current delivered to CLPROG.
Refer to the Electrical Characteristics table for values of
h_CLPROG, V_CLPROG and I_VBUSQ. Given worst-case circuit
tolerances, the USB specification for the average input
current in 100mA or 500mA mode will not be violated,
provided that R_CLPROG is 3.01k or greater.
While not in current limit, the switching regulator’s Bat-
Track feature will set V_OUT to approximately 300mV above
the voltage at BAT. However, if the voltage at BAT is below
3.3V, and the load requirement does not cause the switch-
ing regulator to exceed its current limit, V_OUT will regulate
at a fixed 3.6V as shown in Figure 2. This “instant-on”
operation will allow a portable product to run immediately
when power is applied without waiting for the battery to
charge. If the load does exceed the current limit at V_BUS,
V_OUT will range between the no-load voltage and slightly
below the battery voltage, indicated by the shaded region
of Figure 2.
20
4.5
4.2
3.9
NO LOAD
3.6
3.3
300mV
3.0
2.7
2.4
2.4 2.7 3.0 3.3 3.6 3.9 4.2
BAT (V)
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Figure 2. V_OUT vs BAT
For very low-battery voltages, the battery charger acts like a
load and, due to limited input power, its current will tend to
pull V_OUT below the 3.6V “Instant On” voltage. To prevent
V_OUT from falling below this level, an undervoltage circuit
automatically detects that V_OUT is falling and reduces the
battery charge current as needed. This reduction ensures
that load current and voltage are always prioritized while
allowing as much battery charge current as possible. See
Over Programming the Battery Charger in the Applications
Information section.
The voltage regulation loop is compensated by the ca-
pacitance on V_OUT. A 10μF MLCC capacitor is required
for loop stability. Additional capacitance beyond this value
will improve transient response.
An internal undervoltage lockout circuit monitors V_BUS and
keeps the switching regulator off until V_BUS rises above
4.30V and is about 200mV above the battery voltage.
Hysteresis on the UVLO turns off the regulator if V_BUS
falls below 4V or to within 50mV of the battery voltage.
When this happens, system power at V_OUT will be drawn
from the battery via the ideal diode(s).
Bidirectional PowerPath Switching Regulator—
Step-Up Mode
For USB on-the-go applications, the bidirectional
PowerPath switching regulator acts as a step-up converter
to deliver power from V_OUT to V_BUS. The power from V_OUT
can come from the battery or the output of the external
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