LTC3455 Datasheet, PDF (15/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
APPLICATIO S I FOR ATIO
Low or Bad Battery Protection (200ms Timeout)
The 200ms reset timer is also used to prevent starting the
LTC3455 when there is insufficient external 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 defective 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 is 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 picking an appro-
priate 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 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,
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 Semiconduc-
tor (www.onsemi.com), have reverse leakage currents
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
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 contains two 1.5MHz constant-frequency
current mode switching regulators that operate with effi-
ciencies 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 600mA at 3V/3.3V (to power microcon-
troller I/O, memory and other logic circuitry). Both con-
verters 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 sytem.
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