Accurate Constant-Current, Constant-Voltage 20A Power Supply Ensures Safe Charging of Supercaps and Li-Ion Batteries

design features
Accurate Constant-Current, Constant-Voltage
20A Power Supply Ensures Safe Charging of
Supercaps and Li-Ion Batteries
Josh Caldwell
Many applications require a power supply that can
accurately regulate a voltage and accurately limit output
current, but there are remarkably few solutions that can
do both with a single IC. System designers must typically
trade off accuracy in one feature for accuracy in the
other by choosing between a high gain, high accuracy
voltage regulator with a crude current limit or a high
accuracy current regulator with a crude voltage clamp.
SINGLE-CELL LITHIUM-ION
BATTERY CHARGER PROVIDES
10A OF CHARGING CURRENT
Safety concerns and thermal limitations
of charging lithium-ion batteries mean
the charger must be able to carefully
control charging currents and voltages.
Ideally, a microcontroller can accurately
throttle back the charging current during
the initial and top-off charging phases.
This forces the use of a current regulation scheme that has precision adjustable
current control, thermal limiting capabilities, and an accurate voltage limit.
The LT®3741 simplifies the design of
constant-current, constant-voltage regulators by combining an accurate current
regulator and an accurate voltage regulator in a single IC, thus eliminating power
system design trade-offs. The LT3741 is
a synchronous buck DC/DC controller
designed to regulate output currents up
to 20A and output voltages up to 34V,
with a current regulation accuracy of
±6% and a voltage accuracy of ±1.5%.
Near-ideal constant voltage and constant
current regulation is possible because of
the LT3741’s average current mode control architecture. As seen in Figure 1, the
transition between the voltage and current
loop is seamless and extremely sharp.
Figure 1. VOUT vs IOUT for a 200W, 10V/20A
constant-current, constant-voltage step-down
converter
Figure 2. A 10A single-cell lithium-ion battery charger
A unique topology allows the LT3741 to
both sink and source current. Precise load
current control is achieved with analog
control pins CTRL1 and CTRL2. The switching frequency can be programmed from
200kHz to 1MHz and synchronized to an
external clock from 300kHz to 1MHz.
VIN
EN/UVLO
12
1µF
µCONTROLLER
CTRL1
10
VOUT (V)
8
HG
RT
SYNC
82.5k
LG
RHOT
45.3k
8 10 12 14 16 18 20 22
IOUT (A)
CTRL2
SENSE+
SS
SENSE–
FB
1nF
VC
82.5k
8.2nF
VIN
24V
M1
L1
2.2µH
1%
5mΩ
VOUT
4.2V, 10A MAXIMUM
+
D1
22µF
3.6V
M2
10Ω
GND
RNTC
470k
33µF
SW
VCC_INT
VREF
2.2µF
2 VIN = 18V
VOUT = 10V
ILIMIT = 20A
0
0 2 4 6
220nF
CBOOT
LT3741
6
4
The LT3741 easily meets these requirements. Figure 2 shows the LT3741 configured as a lithium-ion battery charger with
the maximum current limit set at 10A and
the voltage limit set at 4.2V. Charging
current is independent of the output voltage and can be adjusted down to 0A via
CTRL1. The voltage divider from VREF to
10Ω
22nF
30.1k
D1: Philips Semiconductor PMEG4002EB
L1: Vishay IHLP4040DZER2R2M01
M1: Renesas RJK0365DPA
M2: Renesas RJK0346DPA
RNTC: Vishay NTCS0805E3474JXT
12.1k
October 2010 : LT Journal of Analog Innovation | 23
CTRL2 provides the thermal limit control
using a temperature dependent resistor.
EN/UVLO
1µF
With the sharp transition between current
and voltage control, the LT3741 delivers
system reliability and safety by allowing
the battery to be charged with constant
current up to the voltage regulation point.
Efficiency for this solution is about 93%.
82.5k
RHOT
45.3k
THERMALLY DERATING
THE LOAD CURRENT
Supercapacitors are replacing lead-acid
batteries in a number of applications from
rapid-charge power cells for cordless tools
to short-term backup power for microprocessors to vehicle and mobile defense
applications. Although each of these
applications reaps different benefits from
using a supercapacitor, they all require
careful control of the charging current and
voltage limiting to prevent system-wide
damage or damage to the supercapacitor.
The charging power source must provide
an accurately regulated current source to
the supercapacitor, regardless of output voltage while providing an accurate
voltage limit to prevent overcharging.
24 | October 2010 : LT Journal of Analog Innovation
M1
100nF
L1
1.0µH
SW
LT3741
VCC_INT
10Ω
22µF
M2
10nF
SENSE+
SS
SENSE–
FB
VC
47.5k
4.7nF
+
10Ω
VOUT
5V
20A
SUPERCAP*
GND
CTRL2
330µF
×3
D1
LG
CTRL1
50k
R1
2.5mΩ
33nF
38.3k
D1: Philips Semiconductor PMEG4002EB
12.1k
L1: Vishay IHLP4040DZER1R0M01
M1: Renesas RJK0365DPA
M2: Renesas RJK0346DPA
RNTC: Vishay NTCS0805E3474JXT
*5F SUPERCAP, Illinois Capacitor 505DCN5R4M
Figure 3. A 20A supercapacitor charger with 5V regulated output
charging current within a completely
discharged supercapacitor. In Figure 4,
the output voltage is plotted verses output current for this charger, showing
the LT3741 maintaining current regulation into a virtually shorted output.
STRONG GATE DRIVERS
AND HIGH CURRENT LDO
Modern high current switching Power
MOSFETs are most efficient when driven
with low resistance drivers to reduce
transitional losses. The LT3741 contains
Figure 4. Output voltage vs load current for a 5V/20A
supercapacitor charger
very strong gate drivers. The LG and
HG PMOS pull-up driver on-resistance is
typically 2.3Ω. The on-resistance of the
LG and HG NMOS pull-down drivers are typically less than 1.3Ω. While the gate drivers
reduce losses, the LT3741 is also capable
of driving two high current MOSFETs in
parallel where load currents exceed 20A.
The LT3741 utilizes a 5V internal high
current low dropout voltage regulator to
provide up to 50mA to the gate drivers.
Figure 5. Efficiency and power loss vs load current
for the 20A supercapacitor charger
6
100
5
95
4
90
20
85
15
3
2
30
25
EFFICIENCY
80
10
POWER LOSS
1 VIN = 20V
VOUT = 5V
ILIMIT = 20A
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26
IOUT (A)
75
70
VIN = 20V
VOUT = 5V
0
5
10
15
LOAD CURRENT (A)
20
25
5
0
POWER LOSS (W)
Figure 3 shows a 20A supercapacitor
charger with a 5V regulated output voltage. Utilizing a wide input-commonmode range error amplifier for current
regulation, the LT3741 provides accurate
charging currents through a broad-range
of output voltages including a short on
the output. This is essential to prevent
excessive heat dissipation and limit the
HG
EFFICIENCY (%)
SUPERCAPACITOR CHARGER
VREF
RNTC
470k
VOUT (V)
Proper thermal management is essential
with any high power regulator to both
protect the load and reduce the chance of
system-wide damage. The LT3741 uses the
CTRL2 pin to reduce the regulated inductor current. Whenever CTRL2 is lower than
the analog control voltage on the CTRL1
pin, the regulated current is reduced. The
temperature derating is programmed
using a temperature dependent resistor
divider from the VREF pin to ground.
RT
SYNC
100µF
CBOOT
2.2µF
VIN
10V TO 36V
VIN
EN/UVLO
design features
EN/UVLO
EN/UVLO
1µF
82.5k
RT
SYNC
HG
100nF
CBOOT
VREF
2.2µF
RHOT
45.3k
RNTC
470k
10nF
22µF
M1
L1
8.2µH
SW
LT3741
VCC_INT
LG
CONTROL
INPUT
CTRL1
GND
CTRL2
SENSE+
SS
SENSE–
FB
VC
VIN
36V
VIN
30.1k
3.9nF
10mΩ
VOUT
100µF 20V
5A
D1
22µF
10Ω
10Ω
M2
10nF
187k
D1: Philips Semiconductor PMEG4002EB
L1: Vishay IHLP5050FDER8R2M01
M1: Vishay Si7884BDP
M2: Vishay SiR470DP
RNTC: Vishay NTCS0805E3474JXT
12.1k
Figure 6. A 100W 20V/5A constant-current, constant-voltage step-down converter
A 100W 20V/5A CONSTANTCURRENT/CONSTANT-VOLTAGE
STEP-DOWN CONVERTER
exceeding the input common mode range
of the current control loop error amplifier.
The LT3741 may be used as a generalpurpose power solution where accurate
output current limit is required. Figure 6
shows a 500kHz, 100W, 20V/5A constantcurrent, constant-voltage converter.
Average current mode control keeps the
LT3741 stable and allows it to readily
to meet any output voltage or current
requirements. For additional protection,
the LT3741 utilizes a common mode lockout circuit that prevents the output from
COMPACT SOLUTION
Figure 7. DC1602A high power
constant-current, constant-voltage
demo circuit
The LT3741 is available in a 20-pin
exposed pad TSSOP or 20-pin 4mm × 4mm
exposed pad QFN, creating a complete,
uncompromising power solution that
can takes up a mere 1.5in2. The part
is designed specifically for use with
low inductance, high saturation current inductors, further reducing board
area and profile height. Figure 7 shows
a demonstration circuit that produces a
6V/20A constant-current, constant-voltage
output. The components in this particular
design have standard footprints, making
it easy to switch them out to adjust the
output current limit and regulated voltage.
CONCLUSION
The LT3741 offers accurate current and
voltage regulation for constant-current
and constant-voltage applications with
nearly ideal voltage and current regulation characteristics. The combination of
a high gain current control-loop and an
equally high gain voltage control loop
relaxes the tolerance requirements of
other power supply components, thus
reducing overall cost, complexity and
board size. Average current mode control allows the use of low value, low
cost, high saturation current inductors to
further reduce overall board footprint.
With the demands of today’s battery
and supercapacitor chargers, and system
requirements for high accuracy current
limit and voltage regulation, the LT3741
provides a versatile power solution. n
October 2010 : LT Journal of Analog Innovation | 25
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