LTC3552-1 Linear Technology, LTC3552-1 Datasheet - Page 12

• Manufacturer Part Number: LTC3552-1
• Description: Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter
• Manufacturer: Linear Technology

www.DataSheet4U.com
OPERATIO
LTC3552-1
the P-channel switch increases with decreasing input
supply voltage (see Typical Performance Characteristics).
Therefore, the user should calculate the power dissipa-
tion when the regulator is used at 100% duty cycle with
low input voltage (see Thermal Considerations in the
Applications Information section).
APPLICATIO S I FOR ATIO
A typical LTC3552-1 application circuit is shown in
Figure 2. External component selection is driven by
the charging requirements and the switching regulators
load requirements.
Programming Charge Current
The charge current is programmed using a single resistor
from the PROG pin to ground. The charge current out of
the BAT pin is 1000 times the current out of the PROG
pin. The program resistor and the charge current are
calculated using the following equations:
Charge current out of the BAT pin can be determined
anytime by monitoring the PROG pin voltage and using
the following equation:
Programming Charge Termination
The charge cycle terminates when the charge current falls
below the programmed termination threshold. This thresh-
old is set by connecting an external resistor, R
the ITERM pin to ground. The charge termination current
12
4.5V TO
R
I
6.5V
BAT
1.575V/
V
400mA
PROG
IN
V
OUT2
=
1µF
R
=
V
PROG
PROG
C
10µF
CER
1000
OUT2
Figure 2. LTC3552-1 Basic Circuit
825Ω
I
CHG
4.7µH
U
• 1000
1.65k
V
U
C
330pF
FF2
,
I
CHG
V
ITERM
PROG
CHRG
PWR
EN
SW2
V
V
IN
OUT2
FB2
U
=
LTC3552-1
GND
1000
R
PROG
RUN1
RUN2
V
SW1
OUT1
BAT
V
V
FB1
CC
V
W
2.2µH
10µF
C
330pF
600mA
FF1
+
TERM
U
4.2V
1-CELL
Li-Ion
BATTERY
35521 F02
C
10µF
CER
V
1.8V/
800mA
OUT1
OUT1
, from
threshold (I
The termination condition is detected by using an internal
fi ltered comparator to monitor the ITERM pin. When the
ITERM pin voltage drops below 100mV* for longer than
t
current is latched off and the charger enters standby mode
where the input supply current drops to 200µA. (Note:
Termination is disabled in thermal limiting mode).
I
ITERM pin to the PROG pin, thus eliminating the need
for external resistor R
I
current is set by the equation:
When charging, transient loads on the BAT pin can cause
the ITERM pin to fall below 100mV for short periods of
time before the DC charge current has dropped to 10%
of the programmed value. The 1ms fi lter time (t
the termination comparator ensures that transient loads of
this nature do not result in premature charge cycle termi-
nation. Once the average charge current drops below the
programmed termination threshold, the charger terminates
the charge cycle and ceases to provide any current out of
the BAT pin. In this state, any load on the BAT pin must
be supplied by the battery.
*
**
Low Supply Voltage Operation
To prevent unstable operation, the regulator incorporates
an undervoltage lockout circuit which shuts down the
regulators when the V
mately 1.65V.
TERM
TERM
TERM
Any external sources that hold the ITERM pin above 100mV will prevent the LTC3552-1
from terminating a charge cycle.
These equations apply only when the ITERM pin is shorted to the PROG pin.
I
I
R
TERM
CHG
TERM
is always set to I
(typically 1ms), charging is terminated. The charge
can be set to be one tenth of I
=
=
R
=
500
TERM
R
PROG
100
I
100
TERM
TERM
V
) is set by the following equation:
V
V V
,
R
=
PROG
CHG
TERM
I
CC
CHG
10
/10, and the programmed charge
voltage drops below approxi-
=
. When confi gured in this way,
•
500
R
R
I
CHG
PROG
TERM
V
**
CHG
,
by shorting the
TERM
35521f
) on