LTC3456 Datasheet, PDF (14/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

LTC3456
U
OPERATIO
USB Powered
The LTC3456 is designed to be powered from the USB as
shown in Figure 4. The LTC3456 has an internal current-
limited 0.5Ω (typ) PMOS switch with preset 0.1A and 0.5A
current limits. The LTC3456 interfaces with the USB
controller bus via logic pins USBHP and SUSPEND.
The USBHP pin is used to set the USB current limit to either
100mA or 500mA. This pin has a weak 2.5µA pull-down
current source to ensure that Low Power mode is in effect
during start-up. If the USBHP pin is held low, it limits the
input power drawn from the USB port. The USB port can
supply the Core output (1.8V at 200mA) effortlessly in Low
Power mode (USBHP = 0V). However, the loading on the
3.3V output must be held below 100mA (typ) when the
USB is in low power mode. If the loading on 3.3V output
is increased beyond 100mA (typ), the USB port is unable
to supply the load current and the LTC3456 will switch
between the battery and the USB port. This results in
undesirable switching noise and increased voltage ripple
at the 3.3V output.
Pulling the SUSPEND pin logic high disables all USB
functionality. The USB switch connected between USB
and V_EXT behaves like a back-to-back diode whenever
SUSPEND is pulled high. In Suspend mode the device
limits the current drawn from the USB pin to 100µA (typ).
The minimum voltage to a USB-powered device may drop
as low as 4.35V due to cable and connector drops. The
LTC3456 has an internal voltage monitor that checks the
USB supply voltage and cuts off the USB power if the USB
voltage falls below 4V. There is 75mV of hysteresis built-
in the USB voltage monitor.
When the IC is enabled, the USB pin is connected to the
V_EXT pin via the PMOS switch. The V_EXT pin gets charged
by the preset 0.1A or 0.5A current limit determined by the
state of the USBHP pin. As the V_EXT pin voltage rises above
4V, the V_INT, Core output, Main output and Hot Swap
outputs power-up in that sequence.
–
4V
+
V_EXT
V_EXT
9
10µF
500mA
100mA
USBHP
USBHP
12
SUSPEND
SUSPEND
18
LTC3456
EN
BUCK 1
SW1
5
L1
10µH
SW2_BK
BUCK 2
13
V_INT
15
V_MAIN
16
L3
10µH
USB
4.35V TO 5.5V
USB
R
10
4.7µF
+ USB_RDY
–
1Ω
4V –
+^VOS
HSO
17
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Figure 4. USB-Powered LTC3456
10µF
CORE OUTPUT
1.8V
200mA
V_INT
3.3V
22µF
MAIN OUTPUT
3.3V
1µF ^150mA
Hot Swap OUTPUT
3.3V
1µF ^50mA
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