QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 920
STAND ALONE LI-ION CHARGER WITH SYNCHRONOUS BUCK REGULATOR
4
flowing. Continue slowly increasing the battery
simulator power supply, thus simulating the Li-
Ion battery accepting charge. As the battery
simulator approaches the float voltage of
4.200V, as measured by V2, the charge current
will begin to drop as the charger begins the
constant voltage portion of the charge cycle.
When the charger is in the constant voltage por-
tion of the charge cycle, small changes in the
simulator power supply voltage will result in
relatively large changes in charge current. The
PROG terminal can be used to indicate charge
current level at all times during the charge cycle
with 1 Volt indicating 100% of the programmed
current. When the charge current drops below
the End-of-Charge threshold level of 50mA, the
Charge, LED will go OFF indicating the battery
is near full charge. The charge cycle will con-
tinue until the 4.5 hour timer ends.
The one Ohm resistor in series with the ceramic
input capacitor on Vcc minimizes the ringing
and overshoot that appears at the input when
the input voltage is hot switched. Without the
series resistor, serious damage to the LTC4080
can occur. See Figure 2. Pads are also included
for an additional input capacitor. Adding a tan-
talum capacitor at this location will also mini-
mize voltage ringing and overshoot.
Buck Regulator
Select one of the four output voltages using
jumpers JP4 through JP7. (With no jumpers
installed, the output voltage is set to the refer-
ence voltage of 800mV.) With jumpers JP3 in
the fixed frequency position, JP1 and JP2 in
the ON position, connect a suitable load resis-
tor between VOUT and GND terminals. The
maximum load current is approximately
300mA. Verify that the output voltage, as
measured by V4 is between the limits as
shown in Table 1. A scope can also be used to
measure the regulator output ripple voltage.
When evaluating output ripple, observe proper
scope probe measuring technique as shown in
Figure 3. For high efficiency at light load condi-
tions, move jumper J3 to the BURST position.
In Burst mode, the output ripple voltage is
higher (20 - 40mV) and the maximum output
current is approximately 50mA.
See LTC4080 Data Sheet for additional information.
Figure 2. Input Voltage Transient When Input
Power is Hot Switched
V
OUT
GND
Figure 3. Scope Probe Placement for Measur-
ing Output Ripple Voltage
