LTC3558
20
3558f
Battery Charger Stability Considerations
The LTC3558 battery charger contains two control loops: the
constant-voltage and constant-current loops. The constant-
voltage loop is stable without any compensation when a
battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1.5µF from BAT
to GND. Furthermore, a 4.7µF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
High value capacitors with very low ESR (especially
ceramic) reduce the constant-voltage loop phase margin,
possibly resulting in instability. Ceramic capacitors up to
22µF may be used in parallel with a battery, but larger
ceramics should be decoupled with 0.2Ω to 1Ω of series
resistance.
In constant-current mode, the PROG pin is in the feedback
loop, not the battery. Because of the additional pole created
by the PROG pin capacitance, capacitance on this pin must
be kept to a minimum. With no additional capacitance on
the PROG pin, the charger is stable with program resistor
values as high as 25K. However, additional capacitance on
this node reduces the maximum allowed program resis-
tor. The pole frequency at the PROG pin should be kept
above 100kHz. Therefore, if the PROG pin is loaded with a
capacitance, C
PROG
, the following equation should be used
to calculate the maximum resistance value for R
PROG
:
R
C
PROG
PROG
≤
1
210
5
π ••
APPLICATIONS INFORMATION
Average, rather than instantaneous, battery current may be
of interest to the user. For example, if a switching power
supply operating in low-current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
fi lter can be used on the PROG pin to measure the average
battery current as shown in Figure 6. A 10k resistor has
been added between the PROG pin and the fi lter capacitor
to ensure stability.
USB Inrush Limiting
When a USB cable is plugged into a portable product,
the inductance of the cable and the high-Q ceramic input
capacitor form an L-C resonant circuit. If there is not much
impedance in the cable, it is possible for the voltage at
the input of the product to reach as high as twice the
USB voltage (~10V) before it settles out. In fact, due to
the high voltage coeffi cient of many ceramic capacitors
(a nonlinearity), the voltage may even exceed twice the
USB voltage. To prevent excessive voltage from damag-
ing the LTC3558 during a hot insertion, the soft connect
circuit in Figure 7 can be employed.
In the circuit of Figure 7, capacitor C1 holds MP1 off
when the cable is fi rst connected. Eventually C1 begins
to charge up to the USB input voltage applying increasing
gate support to MP1. The long time constant of R1 and
C1 prevents the current from building up in the cable too
fast thus dampening out any resonant overshoot.
3558 F06
C
FILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
R
PROG
LTC3558
PROG
GND
10k
Figure 6. Isolated Capacitive Load on PROG Pin and Filtering
R1
40k
5V USB
INPUT
3558 F07
C1
100nF
C2
10µF
MP1
Si2333
USB CABLE
V
CC
GND
LTC3558
Figure 7. USB Soft Connect Circuit
