LTC3559/LTC3559-1
18
3559fb
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.
the current from building up in the cable too fast thus
dampening out any resonant overshoot.
Buck Switching Regulator General Information
The LTC3559/LTC3559-1 contain two 2.25MHz constant-
frequency current mode switching regulators that provide
up to 400mA each. Both switchers can be programmed
for a minimum output voltage of 0.8V and can be used
to power a microcontroller core, microcontroller I/O,
memory or other logic circuitry. Both regulators support
100% duty cycle operation (dropout mode) when the
input voltage drops very close to the output voltage and
are also capable of operating in Burst Mode operation for
highest effi ciencies at light loads (Burst Mode operation
is pin selectable). The switching regulators also include
soft-start to limit inrush current when powering on, short
circuit current protection, and switch node slew limiting
circuitry to reduce radiated EMI.
A single MODE pin sets both regulators in Burst Mode
operation or pulse skip operating mode while each regula-
tor is enabled individually through their respective enable
pins EN1 and EN2. The buck regulators input supply (PV
IN
)
should be connected to the battery pin (BAT). This allows
the undervoltage lockout circuit on the BAT pin to disable
the buck regulators when the BAT voltage drops below
2.45V. Do not drive the buck switching regulators from
a voltage other than BAT. A 2.2μF decoupling capacitor
from the PV
IN
pin to GND is recommended.
Buck Switching Regulator
Output Voltage Programming
Both switching regulators can be programmed for output
voltages greater than 0.8V. The output voltage for each
buck switching regulator is programmed using a resistor
divider from the switching regulator output connected to
the feedback pins (FB1 and FB2) such that:
V
OUT
= 0.8(1 + R1/R2)
Typical values for R1 are in the range of 40k to 1M. The
capacitor C
FB
cancels the pole created by feedback re-
sistors and the input capacitance of the FB pin and also
helps to improve transient response for output voltages
much greater than 0.8V. A variety of capacitor sizes can
be used for C
FB
but a value of 10pF is recommended for
3559 F06
C
FILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
R
PROG
LTC3559/
LTC3559-1
PROG
GND
10k
Figure 6. Isolated Capacitive Load on PROG Pin and Filtering
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 damaging
the LTC3559/LTC3559-1 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 voltage applying increasing gate support
to MP1. The long time constant of R1 and C1 prevents
Figure 7. USB Soft Connect Circuit
R1
40k
5V USB
INPUT
3559 F07
C1
100nF
C2
10μF
MP1
Si2333
USB CABLE
V
CC
GND
LTC3559/
LTC3559-1
