Description
+
–
DEMO MANUAL
DC1977A-A/DC1977A-B
LTC4121EUD/LTC4121EUD-4.2
400mA Synchronous Buck
Battery Charger
DC1977A-A LTC4120EUD-4.2 (Fixed Output)
DC1977A-B LTC4121EUD (Adjustable Output)
The LTC4121 and LTC4121-4.2 feature constant-current–
constant-voltage charging capability suitable for lithiumion or lead-acid cells. The LTC4121-4.2 supports charging
Demonstration Board DC1977A showcases the
LTC4121-4.2 and LTC4121 40V, 400mA synchronous-
buck battery charger integrated circuit. The DC1977A
a single lithium-ion cell with a cell voltage of 4.2V. The
LTC4121 may be programmed to charge battery voltages
up to 18V with a resistive divider.
supports the maximum-power-point tracking (MPPT)
feature of the LTC4121EUD to optimize power delivery
from photovotalic cells or highly resistive sources.
performance summary
SYMBOL PARAMETER CONDITIONS
IN DC1977A Input Voltage I(IN) < 800mA 4.4 40 V
PV
IN
DC1977A PV Cell Input I(IN) < 800mA 5 40.5 V
V(BAT) DC1977A BAT Pin Voltage R11 = 1.40MΩ, R12 = 1.05MΩ 2.5 4.25 V
I(BAT) DC1977A BAT Pin Current V(BAT) = 3.7V; DC1977A; (R7) = 3.01kΩ; JP1 (“MPPT”) = ‘OFF’ 383 402 421 mA
Note: Reference designators refer to Schematic on p. 7.
Specifications are at TA = 25°C
Design files for this circuit board are available at
http://www.linear.com/demo/DC1977
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Analog
Devices, Inc. All other trademarks are the property of their respective owners.
MIN TYP MAX
UNITS
Demo BoarD application
High Efficiency, Wide Input Voltage Range Charging with LTC4121
MPPT1
MPPT2
PROG
IN
RUN
LTC4121
MPPT
PROG
FREQ
C
IN
R
RUN1
10µF
261k
R
R
RUN2
787k
324k
R
V
IN
V
+ 200mV
BAT
TO 40V
121k
R
3.01k
GND
INTV
BOOST
SW
CHGSNS
BAT
FBG
LTC4121 Efficiency vs V
CC
C
BOOST
22nF
R
FB1
1.05M
FB
R
FB2
1.40M
C
INTVCC
2.2µF
LPS4018-333ML
+
Li-Ion
dc1977a F01
C
BAT
47µF
100
95
90
85
EFFICIENCY (%)
80
75
V
BAT
70
5
= 4.1V
10 15 20 25 30 35 40
VIN (V)
R
R
PROG
PROG
IN
= 6.04k
= 3.01k
DC1977A F02
dc1977afb
1
DEMO MANUAL
DC1977A-A/DC1977A-B
assemBly test proceDure
Refer to Figure 1 for the proper measurement equipment
setup and jumper settings and follow the procedure below.
1. Set JP1 (“MPPT”) to ‘ON’, set PS1 to 3.6V and turn
on.
2. Connect PS2 to point A, set to 15V and turn on.
3. Verify that VM1 indicates 3.3V to 3.9V, and then verify
that AM1 indicates 387mA to 417mA. Verify that VM2
shows 14.5V to 15.1V. There is only a series diode
between PV+ and the VIN pin of the LTC4121. The
purpose of this diode is prevent backfeeding a PV
cell, if connected. A single diode Vf is insufficient to
activate the MPPT feature, and the LTC4121 delivers
full charge current to the battery.
4. Turn PS2 off, move connection to point B, and turn
PS2 on.
5. Verify that VM1 indicates 3.3V to 3.9V, and then verify
that AM1 indicates 387mA to 417mA. Verify that VM2
shows 12.6V to 13.2. The source impedance of the
power supply is now ≈ 16Ω. But this impedance still
allows delivering full charge current without engaging
the MPPT feature.
6. Turn PS2 off, move connection to Point C, and turn
PS2 on.
7. Verify that VM1 indicates 3.3V to 3.9V, and then Verify
that AM1 indicates 105mA to 115mA. Verify that VM2
shows 10.6V to 11.3V. The source impedance of the
power supply is now 98Ω. The MPPT feature has
engaged and the charge delivered to the battery has
been reduced to allow V
to stay at the programmed
IN
MPPT point.
8. Set JP1 (“MPPT”) to ‘OFF’, test is finished.
PS2
15V POWER SUPPLY
0.25A
PS1
3.6V BIPOLAR SUPPLY
1A
AM2
+
–
+
–
– +
–
3.6Ω
BC
82.4Ω
2W
AM1
15.8Ω
2W
+
VM2
–
+–
–
+
VM1
–
Figure1. DC1977A Equipment Setup
Note: All connections from equipment should be kelvin-connected directly to the board pins
which they are connected on this diagram. All input or output leads should be twisted pair.
A
DC1977a F03
2
dc1977afb
theory of operation
V
1.227V
DEMO MANUAL
DC1977A-A/DC1977A-B
The LTC4121EUD-4.2/LTC4121EUD is a 4.4V ~ 40V input
buck topology battery charger with maximum power point
tracking (MPPT) for use with PV cells or highly resistive
power supplies. The buck-topology charger uses current
mode control for stable operation.
LTC4121EUD ENABLE
The LTC4121 can be enabled or disabled via the RUN pin,
and this functionality can be accessed via JP2, the RUN
jumper. When JP2 is in the “ENABLE” position, R3 and
R4 ensure that the LTC4121 is not enabled until Vin is
greater than 4.4V.
Note: Do not float the LTC4121 RUN pin. Operate
the demo board with JP2 in either the DISABLED or
ENABLED position.
Buck Charger
The heart of the LTC4121EUD is the buck-topology bat
tery charger. The buck-topology charger is a synchronous,
current-mode-control regulator with N-channel FETs. The
use of N-channel FETs minimizes conduction losses, and
requires only a single external 0.022µF capacitor to generate the high-side gate drive.
The LTC4121EUD can charge up to four Li-Ion cells in
series, and supports a maximum battery voltage of 18V.
The LTC4121EUD-4.2 is optimized for charging a single
Li-Ion cell to a fixed cell voltage of 4.2V.
The current in the buck inductor passes through a small
on-die resistor for current measurement, and then goes
back out to the BAT pin. The battery is connected to the
BAT pin; this allows the LTC4121EUD to measure not only
the cycle-by-cycle current, but also the average current.
The cycle-by-cycle current is used by the current-mode
buck regulator, and the average current is the battery
charge current as programmed by R
= 3.01kΩ, so I(BAT) = 402mA provided that the
R
PROG
MPPT function does not reduce the current.
. On DC1977A,
PROG
-
The buck regulator acts as a current source when the
battery is in the constant-current charging region and as
a classic voltage output buck regulator when the battery
is in the constant-voltage charging region.
The battery charge current is programmed by
R
=3.01kΩ. The equation for R
PROG
R
The LTC4121EUD provides a switching frequency select
pin, FREQ, to select between 750kHz and 1.5MHz; this
function is accessed by JP4, the “FREQ” jumper.
Note: Do not float the LTC4121 FREQ pin. Operate the
demo board with JP3 in either the 750kHz or 1.5MHz
position.
Figure2 shows various nodes of interest with V
and the switching frequency at 750kHz (T = 1.333µs), The
duty cycle is 86% for V(BAT) = 3.6V, not the 72% duty
expected from a buck regulator. When the battery voltage
is 3.6V, the charger is in constant-current mode, so the
control loop is forcing the output of the buck regulator
to the voltage necessary to push 400mA into the battery.
This “effective” voltage, 5 • 0.86 = 4.3V, is the voltage
necessary to ensure that a 400mA average current is flowing through the on-die sense resistor.
Figure3 shows the same nodes as Figure2, but with
VIN = 40V. The switching frequency is still 750kHz. The
duty cycle is ≈ 200ns/1.333µs, or 15%, but the period is
2.7µs. This is because the LTC4121 minimum on time was
greater than that needed to achieve 4.3V, and the LTC4121
starts to pulse skip to get the necessary average duty
cycle. The average duty cycle is 300ns/2.667µs=11%.
This produces an output voltage of 4.3V, so the battery
still charges at 400mA.
Maximum Power Point Tracking (MPPT)
The LTC4121EUD provides a maximum-power-point
tracking (MPPT) function for use with PV cells or highly
PROG
= h
PROG
•
PROG
I
CHG
= 986 •
PROG
0.4
is:
= 3.01kΩ
= 5V,
IN
dc1977afb
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