Datasheet

LTC4089-3
13
40893f
For more information www.linear.com/4089-3
The battery charger will reduce its current as needed to
ensure that the USB specification is not exceeded. If the
load current is greater than the current limit, the output
voltage will drop to just under the battery voltage where
the ideal diode circuit will take over and the excess load
current will be drawn from the battery.
In USB applications, the minimum value for R
CLPROG
should
be 2.1k. This will prevent the input current from exceeding
500mA due to LTC4089-3 tolerances and quiescent cur-
rents. A 2.1k CLPROG resistor will give a typical current
limit of 476mA in high power mode (HPWR = 1) or 95mA
in low power mode (HPWR = 0).
When SUSP is driven to a logic high, the input power
path is disabled and the ideal diode from BAT to OUT will
supply power to the application.
High Voltage Step Down Regulator
The power delivered from HVIN to HVOUT is controlled
by a 750kHz constant frequency, current mode step down
regulator. An external P-channel MOSFET directs this
power to OUT and prevents reverse conduction from OUT
to HVOUT (and ultimately HVIN).
A 750kHz oscillator enables an RS flip-flop, turning on
the internal 1.95A power switch Q1. An amplifier and
comparator monitor the current flowing between the HVIN
and SW pins, turning the switch off when this current
reaches a level determined by the voltage at V
C
. An error
amplifier servos the V
C
node to maintain approximately
300mV between OUT and BAT. By keeping the voltage
across the battery charger low, efficiency is optimized
because power lost to the battery charger is minimized
and power available to the external load is maximized. If
the BAT pin voltage is less than approximately 3.3V, then
the error amplifier will servo the V
C
node to provide a
constant HVOUT output voltage of about 3.6V. An active
clamp on the V
C
node provides current limit. The V
C
node
is also clamped to the voltage on the HVEN pin; soft-start
is implemented by a voltage ramp at the HVEN pin using
an external resistor and capacitor.
An internal regulator provides power to the control circuitry.
This regulator includes an undervoltage lockout to prevent
switching when HVIN is less than about 4.7V. The HVEN
pin is used to disable the high voltage regulator. HVIN
input current is reduced to less than 2µA and the external
P-channel MOSFET disconnects HVOUT from OUT when
the high voltage regulator is disabled.
The switch driver operates from either the high voltage
input or from the BOOST pin. An external capacitor and
diode are used to generate a voltage at the BOOST pin that
is higher than the input supply. This allows the driver to
fully saturate the internal bipolar NPN power switch for
efficient operation.
When HVOUT is below 3.95V the operating frequency
is reduced. This frequency foldback helps to control the
regulator output current during start-up and overload.
Ideal Diode from BAT to OUT
The LTC4089-3 has an internal ideal diode as well as a
controller for an optional external ideal diode. If a battery
is the only power supply available, or if the load current
exceeds the programmed input current limit, then the
battery will automatically deliver power to the load via an
ideal diode circuit between the BAT and OUT pins. The
ideal diode circuit (along with the recommended 4.7µF
capacitor on the OUT pin) allows the LTC4089-3 to handle
large transient loads and wall adapter or USB V
BUS
con-
nect/disconnect scenarios without the need for large bulk
capacitors. The ideal diode responds within a few micro-
seconds and prevents the OUT pin voltage from dropping
significantly below the BAT pin voltage. A comparison of
the I-V curve of the ideal diode and a Schottky diode can
be seen in Figure 3.
If the input current increases beyond the programmed
input current limit additional current will be drawn from
the battery via the internal ideal diode. Furthermore, if
power to IN (USB V
BUS
) or HVIN (high voltage input) is
removed, then all of the application power will be provided
by the battery via the ideal diode. A 4.7µF capacitor at
OPERATION