LTC3576 Datasheet, PDF (19/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go + Triple Step-Down DC/DCs

LTC3576/LTC3576-1
OPERATION
Bidirectional PowerPath Switching Regulator—
Step-Down Mode
The power delivered from V_BUS to V_OUT is controlled by
a 2.25MHz constant frequency bidirectional switching
regulator operating in step-down mode. V_OUT drives the
combination of the external load (step-down switching
regulators 1, 2 and 3) and the battery charger. To meet the
maximum USB load specification, the switching regulator
contains a measurement and control system that ensures
that the average input current remains below the level
programmed at CLPROG.
If the combined load does not cause the switching regu-
lator to reach the programmed input current limit, V_OUT
will track approximately 0.3V above the battery voltage.
By keeping the voltage across the battery charger at this
low level, power lost to the battery charger is minimized.
Figure 1 shows the power flow in step-down mode.
If the combined external load plus battery charge current
is large enough to cause the switching regulator to reach
the programmed input current limit, the battery charger
will reduce its charge current by precisely the amount
necessary to enable the external load to be satisfied. Even
if the battery charge current is programmed to exceed the
allowable USB current, the USB specification for average
input current will not be violated; the battery charger will
reduce its current as needed. Furthermore, if the load cur-
rent at V_OUT exceeds the programmed power from V_BUS,
load current will be drawn from the battery via the ideal
diode(s) even when the battery charger is enabled.
The current out of CLPROG is a precise fraction of the V_BUS
current. When a programming resistor and an averaging
capacitor are connected from CLPROG to GND, the volt-
age on CLPROG represents the average input current of
the switching regulator. As the input current approaches
the programmed limit, CLPROG reaches 1.18V and power
delivered by the switching regulator is held constant.
TO AUTOMOTIVE,
FIREWIRE, ETC.
V_IN
HIGH VOLTAGE
STEP-DOWN
SW
V_C
SWITCHING
REGULATOR
FB
OVSENS
6
OVGATE
5
OVERVOLTAGE PROTECTION
6V
s2
+–
26
V_C
27
WALL
V_OUT
–
3.6V
BAT + 0.3V 4.3V
+
Bat-Track HV CONTROL
ACPR
28
TO USB
OR WALL
ADAPTER
35 ^VBUS
34 ^VBUS
I_SWITCH/N
PWM AND
GATE DRIVE
V_BUS
VOLTAGE
CONTROLLER
–
+
5V
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
IDEAL
DIODE
+
OmV
–
15mV ^+–
SW
36
V_OUT
33
IDGATE
31
CLPROG –
1
+
1.18V
0.3V
+–
3.6V
BAT
32
AVERAGE V_BUS INPUT
CURRENT LIMIT
CONTROLLER
V_OUT VOLTAGE
CONTROLLER
3576 F01
USB INPUT
BATTERY POWER
HV INPUT
Figure 1. PowerPath Block Diagram—Power Available from USB/Wall Adapter
3.5V TO
(BAT + 0.3V)
TO SYSTEM LOAD
OPTIONAL EXTERNAL
IDEAL DIODE PMOS
+ _{SINGLE CELL}
Li-Ion
3576f
19