LTC3552 Datasheet, PDF (13/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

LTC3552
APPLICATIO S I FOR ATIO
A typical LTC3552 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:
1000V
1000V
R_PROG = _ICHG , I_CHG = _RPROG
Charge current out of the BAT pin can be determined
anytime by monitoring the PROG pin voltage and using
the following equation:
I_BAT
=
V_PROG
R_PROG
• 1000
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_TERM, from
the ITERM pin to ground. The charge termination current
threshold (I_TERM) is set by the following equation:
I_TERM
=
100V
R_TERM
=
I_CHG
10
• ^RPROG ,
R_TERM
R_TERM
=
100V
I_TERM
The termination condition is detected by using an internal
filtered comparator to monitor the ITERM pin. When the
ITERM pin voltage drops below 100mV* for longer than
t_TERM (typically 1ms), charging is terminated. The charge
current is latched off and the charger enters standby mode
where the input supply current drops to 200µA. (Note:
Termination is disabled in trickle charging and thermal
limiting modes).
I_TERM can be set to one tenth of I_CHG by shorting the
ITERM pin to the PROG pin, thus eliminating the need
for external resistor R_TERM. When configured in this way,
I_TERM is always set to I_CHG/10, and the programmed charge
current is set by the equation:
I_CHG
=
500V
R_PROG
,R_PROG
=
500V **
I_CHG
* Any external sources that hold the ITERM pin above 100mV will prevent the LTC3552
from terminating a charge cycle.
** These equations apply only when the ITERM pin is shorted to the PROG pin.
V_IN
4.5V TO 6.5V
V_OUT2
2.5V/400mA
1µF
C_OUT2
4.7µF
CER
619Ω
1.24k
4.7µH
LTC3552
V_IN
RUN1
ITERM
RUN2
PROG
BAT
CHRG
PWR
EN
SW2
V_CC
POR
MODE/SYNC
SW1
C_FF2
68pF
R4
887k
V_FB2
R3
280k
V_FB1
GND
800mA
+
10µF
4.2V
1-CELL Li-Ion
BATTERY
2.2µH
R2
604k
R1
301k
C_FF1
33pF
V_OUT1
1.8V/800mA
C_OUT1
10µF
CER
3552 F02
Figure 2. LTC3552 Basic Application Circuit
3552f
13