Datasheet

LTC4089/LTC4089-5
13
40895fc
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 fl ip-fl op, turning on the
internal 1.95A power switch Q1. An amplifi er and compara-
tor monitor the current fl owing 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 amplifi er
servos the V
C
node to maintain approximately 300mV
between OUT and BAT (LTC4089). By keeping the voltage
across the battery charger low, effi ciency 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 amplifi er 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
effi cient 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/LTC4089-5 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/LTC4089-5
to handle large transient loads and wall adapter or USB
V
BUS
connect/disconnect scenarios without the need for
large bulk capacitors. The ideal diode responds within
a few microseconds and prevents the OUT pin voltage
from dropping signifi cantly 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 OUT is
suffi cient to keep a transition from input power to battery
power from causing signifi cant output voltage droop. The
ideal diode consists of a precision amplifi er that enables a
large P-channel MOSFET transistor whenever the voltage
at OUT is approximately 20mV (V
FWD
) below the voltage
at BAT. The resistance of the internal ideal diode is ap-
proximately 200m . If this is suffi cient for the application
Figure 3. LTC4089/LTC4089-5 Versus
Schottky Diode Forward Voltage Drop
CONSTANT
I
0N
CONSTANT
R
0N
CONSTANT
V
0N
V
FWD
I
MAX
FORWARD VOLTAGE (V)
4089 F03
I
FWD
0
CURRENT (A)
SLOPE: 1/R
DIO(ON)
SLOPE: 1/R
FWD
SCHOTTKY
DIODE
LTC4089
OPERATION