LTC3455_15 Datasheet, PDF (16/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Low or Bad Battery Protection (200ms Timeout)
The 200ms reset timer is also used to prevent starting
the LTC3455/LTC3455-1 when there is insufficient exter-
nal power or insufficient battery voltage to regulate the
outputs. When first turned on, the internal 200ms timer
starts. If only Switcher 1 is enabled (ON2 is low) and its
output does not reach 90% of its final value within 200ms,
Switcher 1 is shut down even if the ON pin is held low or if
the PWRON pin is held high (the V_MAX pin will remain on
as long as ON is low or PWRON is high). This automatic
shutdown feature prevents possible damage to a defec-
tive or overdischarged Li-Ion battery. If ON2 is tied to
V_MAX so that Switcher 2 is also turned on at startup, then
both outputs must reach 90% of their final values within
200ms. Once the output(s) are in regulation, the timer is
reset for a full 200ms.
Schottky Diode Selection/WALLFB Resistor Selection
When a 5V wall adapter is used, power is provided to the
V_MAX pin through a Schottky diode. The most important
specification in picking this diode is its reverse leakage
current. When the LTC3455/LTC3455-1 are turned on but
wall power is not present, the Schottky will leak current to
ground through the WALLFB resistor divider (see Figure
5). This leakage current should be minimized (by pick-
ing an appropriate low-leakage Schottky diode) as it can
dramatically reduce Burst Mode efficiency at light loads.
In addition, a high leakage current can also false trip the
WALLFB pin and turn on the LTC3455/LTC3455-1 even if
wall power is not available. To help prevent this false turn-
on, use the WALLFB resistor values shown in Figure 5.
The diode forward voltage drop should be around 500mV
or less at its maximum rated current to allow charging
even when the wall adapter voltage is lower than normal.
Some manufacturers have recently introduced Schottky
diodes optimized for a very low forward drop, but their
reverse leakage currents can be more than 100μA at
room temperature, and over 1mA at high temperatures.
These diodes are not recommended for use with the
LTC3455/LTC3455-1, but if they are used operation at
high temperature should be checked thoroughly to avoid
problems due to excessive diode leakage current.
Three good diode choices are the MBRM110E (1A, 10V),
MBR120ESF (1A, 20V), and the MBRA210E (2A, 10V).
All are available in very small packages from ON Semi-
conductor (www.onsemi.com), have reverse leakage cur-
rents under 1μA at room temperature, and have forward
drops of around 500mV at their maximum rated current
(1A or 2A).
10
V_MAX
V_MAX
LTC3455/
LTC3455-1
I_LEAKAGE
WALLFB ¹¹
WALL 5V
3.32K
1.24K
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Figure 5. Schottky Leakage Current Path
Switching Regulator General Information
The LTC3455/LTC3455-1 contain two 1.5MHz constant-
frequency current mode switching regulators that operate
with efficiencies up to 96%. Switcher 1 provides up to
400mA at 1.5V/1.8V (to power a microcontroller core),
and Switcher 2 provides up to 500mA at 3V/3.3V (to power
microcontroller I/O, memory and other logic circuitry).
Both converters support 100% duty cycle operation (low
dropout mode) when the input voltage drops very close
to the output voltage, and both are capable of operating
in Burst Mode operation for highest efficiencies at light
loads (Burst Mode operation is pin selectable). Switcher 2
has independent ON/OFF control, but operates only when
Switcher 1 is also enabled and in regulation. If both are
enabled at power-up, Switcher 2 is allowed to turn on only
after Switcher 1 has reached 90% of its final value. This
power-up delay ensures proper supply sequencing and
reduces the peak battery current at startup. If the output
of Switcher 1 drops to below 85% of its programmed
output voltage, Switcher 2 will turn off. This ensures that
any problems with the core supply will shut down the rest
of the system.
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