Datasheet

LTC4090/LTC4090-5
13
4090fc
OPERATION
USB Input Current Limit
The input current limit and charge control circuits of the
LTC4090/LTC4090-5 are designed to limit input current as
well as control battery charge current as a function of I
OUT
.
OUT drives the external load and the battery charger.
If the combined load at OUT does not exceed the pro-
grammed input current limit, OUT will be connected to IN
through an internal 215mΩ P-channel MOSFET.
If the combined load at OUT exceeds the programmed input
current limit, the battery charger will reduce its charge cur-
rent by the amount necessary to enable the external load
to be satisfi ed while maintaining the programmed input
current. Even if the battery charge current is set to exceed
the allowable USB current, a correctly programmed input
current limit will ensure that the USB specifi cation is never
violated. Furthermore, load current at OUT will always be
prioritized and only excess available current will be used
to charge the battery.
The input current limit, I
CL
, can be programmed using the
following formula:
I
CL
=
1000
R
CLPROG
•V
CLPROG
⎛
⎝
⎜
⎞
⎠
⎟
=
1000V
R
CLPROG
where V
CLPROG
is the CLPROG pin voltage (typically 1V)
and R
CLPROG
is the total resistance from the CLPROG pin
to ground. For best stability over temperature and time,
1% metal fi lm resistors are recommended.
The programmed battery charge current, I
CHG
, is de-
fi ned as:
I
CHG
=
50,000
R
PROG
•V
PROG
⎛
⎝
⎜
⎞
⎠
⎟
=
50,000V
R
PROG
Input current, I
IN
, is equal to the sum of the BAT pin output
current and the OUT pin output current. V
CLPROG
will track
the input current according to the following equation:
I
IN
=I
OUT
+I
BAT
=
V
CLPROG
R
CLPROG
•1000
In USB applications, the maximum value for R
CLPROG
should be 2.1k. This will prevent the input current from
exceeding 500mA due to LTC4090/LTC4090-5 tolerances
and quiescent currents. A 2.1k CLPROG resistor will give
a typical current limit of 476mA in high power mode
(when HPWR is high) or 95mA in low power mode (when
HPWR is low).
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 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).
An oscillator, with frequency set by R
T
, enables an RS fl ip-
fl op, turning on the internal power switch. An amplifi er and
comparator monitor the current fl owing between 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 (LTC4090). By keeping the voltage
across the battery charger low, effi ciency is optimized be-
cause 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 (LTC4090). 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 generating a voltage ramp at the HVEN
pin using an external resistor and capacitor.
The switch driver operates from either the high voltage
input or from the BOOST pin. An external capacitor and
internal 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.
To further optimize effi ciency, the high voltage buck regu-
lator automatically switches to Burst Mode
®
operation in
light load situations. Between bursts, all circuitry associated
with controlling the output switch is shut down reducing
the input supply current.
Burst Mode is a registered trademark of Linear Technology Corporation