LTC3456 Datasheet, PDF (16/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

LTC3456
U
OPERATIO
Hot Swap Output
The LTC3456 is designed to supply power to flash memory
cards. It has a built-in Hot Swap output, HSO, which allows
memory cards to be hot swapped into and out of the
system. The Hot Swap output features short-circuit and
reverse-voltage blocking protection. Connect a 1µF (X5R
or X7R) bypass capacitor from this pin to ground.
After the V_INT and Core output voltages come into regula-
tion, the HSO pin is connected to V_INT via a 0.8Ω (typ) PMOS
switch after a delay of 0.5ms. The PMOS switch has a
120mA built-in current limit. When a flash memory card is
plugged into the system, the input bypass capacitors are
slowly charged up to 3.3V with the preset 120mA current
limit. Figure 6 shows the switching waveforms with Hot
Swap output short-circuited. As seen in the figure, the short-
circuited current out of the HSO pin is well regulated.
The LTC3456 also features reverse-voltage blocking capa-
bility for the HSO pin. In the event the HSO pin voltage rises
greater than 3.3V (V_INT pin voltage), the internal PMOS
switch is turned off and behaves like a back-to-back diode.
During shutdown, the Hot Swap output is discharged to
ground via internal pull-down resistors.
CORE REGULATOR
The core regulator produces a 0.8V to 1.8V adjustable
output from a 1.8V to 3.2V input (2-cell battery), USB port
or AC adapter supply. The core regulator utilizes a synchro-
nous N-channel MOSFET, improving efficiency and elimi-
nating the need for an external Schottky diode. The core
output voltage is measured at the FB1 pin through a resis-
tive divider network. The nominal voltage at the FB1 pin is
0.8V. The divider can be adjusted to set the output voltage
level. The FB1 voltage should always be kept below 2V.
The LTC3456 internal soft-start circuitry limits current
drawn at start-up. Soft-start is essential for input sources
with high input impedance like alkaline cells. Soft-start is
implemented by ramping up the current limit. At start-up,
the current limit is set to 25% and increased by 25% every
256µs. The final inductor current limit is reached after
1ms.
When the core regulator is turned off, the output voltage
V_CORE is discharged to ground. This is accomplished by
pulling the switching node, SW1, to ground through the
internal pull-down resistors.
V_INT
100mV/DIV
(AC COUPLED)
I_L2
500mA/DIV
V_HSO
5V/DIV
I_HSO
200mA/DIV
V_BATT = 2.4V
V_PWRON = 2V
2ms/DIV
3456 F06a
Figure 6a. Short-Circuit at the Hot Swap Output (Battery Powered)
V_INT
100mV/DIV
(AC COUPLED)
I_L3
500mA/DIV
V_HSO
5V/DIV
I_HSO
200mA/DIV
V_EXT = 5V
V_PWRON = 2V
2ms/DIV
3456 F06b
Figure 6b. Short-Circuit at the Hot Swap Output
(Wall/USB Powered)
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