LT1620/LT1621
Rail-to-Rail Current
Sense Amplifier
FEATURES
s
s
s
DESCRIPTIO
s
s
s
s
s
Accurate Output Current Programming
Usable in Charging Applications Up to 32V Output
Programmable Load Current Monitor for End-of-
Charging-Cycle Notification (16-Pin Version)
Dual Function IC (LT1621) Allows Convenient
Integration of Load and Input Current Sensing
Level-Shifted Current Sense Output for Current Mode
PWM Controllers
Can be Used for NiCd, NiMH, Lead-Acid and Lithium-
Ion Battery Charging
Greater than 96% Efficiency Possible in Charger
Applications
High Output Currents Possible: > 10A
Easily Obtained
The LT
®
1620 simplifies the design of high performance,
controlled current battery charging circuits when used in
conjunction with a current mode PWM controller IC.
The LT1620 regulates average output current independent
of input and output voltage variations. Output current can
be easily adjusted via a programming voltage applied to
the LT1620’s PROG pin.
Most current mode PWM controllers have limited output
voltage range because of common mode limitations on the
current sense inputs. The LT1620 overcomes this restric-
tion by providing a level-shifted current sense signal,
allowing a 0V to 32V output voltage range.
The 16-pin version of the LT1620 contains a program-
mable low charging current flag output. This output flag
can be used to signal when a Li-Ion battery charging cycle
is nearing completion.
The LT1621 incorporates two fully independent current
control circuits for dual loop applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATI
s
s
s
s
S
High Current Battery Chargers
High Output Voltage DC/DC Converters
Constant Current Sources
Overcurrent Fault Protectors
TYPICAL APPLICATI
(V
BATT
+ 0.5V) TO 32V
V
IN
LTC1435
SYNCHRONOUS
BUCK
REGULATOR
I
TH
SENSE
–
+
V
IN
22µF
35V
×
2
I
BATT
TO 4A
V
BATT
1.43M
0.1%
SW
27µH
FB
110k
0.1%
0.025Ω
INTV
CC
0.1µF
6
V
CC
1
8
SENSE
AVG
2
7
I
OUT
PROG
LT1620MS8
3
GND
4
5
IN
–
IN
+
0.1µF
3k
1%
15.75k
1%
22µF
35V
EFFICIENCY (%)
+
LT1620/21 • F01
SIMPLIFIED SCHEMATIC. SEE FIGURE 2 FOR COMPLETE SCHEMATIC
Figure 1. Low Dropout, High Current Li-Ion Battery Charger
U
Efficiency
100
V
IN
= 24V
V
BATT
= 16V
95
V
BATT
= 12V
90
V
BATT
= 6V
85
80
75
0
1
3
4
2
BATTERY CHARGE CURRENT (A)
5
1620/21 • TA02
UO
UO
1
LT1620/LT1621
ABSOLUTE
AXI U
RATI GS
(Referenced to Ground) (Note 1)
Sense Amplifier Input Common Mode .......– 0.3V to 36V
Operating Ambient Temperature Range
Commercial ............................................ 0°C to 70°C
Industrial ............................................ – 40°C to 85°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
Power Supply Voltage: V
CC
..........................– 0.3V to 7V
Programming Voltage:
PROG, PROG2 ............ – 0.3V to V
CC
+ 0.3V (7V Max)
I
OUT
, SENSE, AVG, AVG2,
MODE Voltage ................ – 0.3V to V
CC
+ 0.3V (7V Max)
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SENSE 1
I
OUT
2
GND 3
IN
–
4
8
7
6
5
AVG
PROG
V
CC
IN
+
SENSE 1
NC 2
I
OUT
3
NC 4
GND 5
MS8 PACKAGE
S8 PACKAGE
8-LEAD PLASTIC MSOP
8-LEAD PLASTIC SO
θ
JA
= 250°C/W (MS)
θ
JA
= 120°C/W (S)
MODE 6
NC 7
IN
–
8
ORDER PART NUMBER
LT1620CS8
LT1620IS8
LT1620CMS8
MS8 PART MARKING
BC
Consult factory for Military grade parts.
GN PACKAGE
16-LEAD PLASTIC SSOP
θ
JA
= 149°C/W
ORDER PART NUMBER
LT1620CGN
LT1620IGN
ELECTRICAL CHARACTERISTICS
V
IN
+
= 16.8V, V
CC
= 5V, V
IOUT
= 2V, T
A
= 25°C unless otherwise noted.
SYMBOL PARAMETER
Supply
V
CC
5V Supply Voltage
I
CC
DC Active Supply Current
LT1620GN
DC Active Supply Current
LT1620S8, LT1620MS8, 1/2 LT1621GN
DC Active Supply Current
LT1620S8, LT1620MS8, 1/2 LT1621GN
Current Sense Amplifier
V
CM
Input Common Mode Range
Differential Input Voltage Range
V
ID
(IN
+
– IN
–
)
V
OSSENSE
Input Offset - Measured at
×1
Output
(V
SENSE
)
CONDITIONS
q
SENSE = AVG = PROG = PROG2 = V
CC
4.5V
≤
V
CC
≤
5.5V, IN
+
– IN
–
= 100mV
SENSE = AVG = PROG = V
CC
4.5V
≤
V
CC
≤
5.5V, IN
+
– IN
–
= 100mV
SENSE = AVG = PROG = V
CC
4.5V
≤
V
CC
≤
5.5V, IN
+
– IN
–
= 0mV
0V
≤
V
CM
≤
32V
V
CC
≤
V
CM
≤
32V
V
ID
= 80mV
2
U
U
W
W W
U
W
TOP VIEW
16 AVG
15 NC
14 PROG
13 PROG2
12 AVG2
11 V
CC
10 NC
9
IN
+
TOP VIEW
PROG A 1
AVG A 2
SENSE A 3
I
OUT A
4
GND B 5
IN
–
B 6
IN
+
B 7
V
CC B
8
16 V
CC A
15 IN
+
A
14 IN
–
A
13 GND A
12 I
OUT B
11 SENSE B
10 AVG B
9
PROG B
GN PACKAGE
16-LEAD PLASTIC SSOP
θ
JA
= 149°C/W
ORDER PART NUMBER
LT1621CGN
LT1621IGN
MIN
4.5
TYP
5.0
2.8
2.3
MAX
5.5
3.8
4.0
3.3
3.7
1.9
2.1
32
125
5
6
UNITS
V
mA
mA
mA
mA
mA
mA
V
mV
mV
mV
q
q
1.3
q
q
q
0
0
–5
–6
q
LT1620/LT1621
IN = 16.8V, V
CC
= 5V, V
IOUT
= 2V, T
A
= 25°C unless otherwise noted.
SYMBOL PARAMETER
Current Sense Amplifier
Input Offset - Measured at
×10
Output
V
OSAVG
(V
AVG
)
V
OSAVG2
V
SENSE
I
B(IN
+
, IN
–
)
Input Offset - Measured at
×
20 Output
(V
AVG2
)
No-Load Output Offset
Input Bias Current (Sink)
Input Bias Current (Source)
Transconductance Amplifier
Amplifier Transconductance
g
m
q
ELECTRICAL CHARACTERISTICS
+
CONDITIONS
V
CC
≤
V
CM
≤
32V
35mV
≤
V
ID
≤
125mV
V
CM
= 0V, V
ID
= 80mV
V
CC
≤
V
CM
≤
32V
0V
≤
V
ID
≤
35mV
0V
≤
V
CM
≤
32V, V
ID
= 0V, Referenced to V
CC
V
CC
≤
V
CM
≤
32V (Note 2)
V
CM
= 0V (Note 2)
q
MIN
–3
–4
– 10
–3
–4
– 0.1
200
185
TYP
MAX
3
4
15
3
4
UNITS
mV
mV
mV
mV
mV
mV
µA
µA
mA
mA
µmho
µmho
dB
V
V
V
V
nA
mV
mV
V
mV
nA
V
V
q
q
q
q
q
–3
270
4.0
400
430
5.25
5.50
4000
4800
0.15
0.30
0.65
V
CC
7
8
V
CC
– 0.15
A
V
V
OLIOUT
Amplifier Voltage Gain
I
OUT
Saturation Limit (Sink)
1V
≤
V
IOUT
≤
3V
I
IOUT
= 50µA
I
IOUT
= 200µA
I
IOUT
= 1mA
Measured at PROG Pin
I
IOUT
= 130µA
3000
2200
60
3500
80
0.05
0.10
0.35
20
q
q
q
q
V
PROG
I
BPROG
V
OSPROG
PROG Input Range
Input Bias Current
Input Offset Voltage
(V
AVG
– V
PROG
)
End-of-Cycle Comparator
V
PROG2
PROG2 Input Range
V
HYST
Input Hysteresis
I
BPROG2
Input Bias Current
V
OLMODE
Output Logic Low Output (Sink)
V
CC
– 1.25
–7
–8
V
CC
– 2.5
15
20
0.1
0.5
q
q
Measured at AVG2 Pin
Measured at PROG2 Pin
I
MODE
= 0.5mA
I
MODE
= 10mA
q
q
0.5
1.2
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
Absolute Maximum Ratings are those values beyond which the
life of a device may be impaired.
Note 2:
Input bias currents are disabled when V
CC
is removed, even
with common mode voltage present at IN
+
, IN
–
.
PI FU CTIO S
V
CC
:
5V
±10%
Power Supply Input.
IN
+
:
Sense Amplifier Positive Input. Typically connected
to inductor side of current sense resistor. Common mode
voltage range is 0V to 32V.
IN
–
:
Sense Amplifier Negative Input. Typically connected
to load side of current sense resistor. Common mode
voltage range is 0V to 32V.
SENSE:
Sense Amplifier A
V
= – 1 Output. Used as level-
shifted output for PWM controller current sense input. The
sense output is designed to have an inherent offset to
ensure continuity around zero inductor current. Typical out-
put is –3mV with differential input voltage (IN
+
– IN
–
) = 0.
AVG:
Sense Amplifier A
V
= –10 Output and
Transconductance Amplifier Positive Input. Used as inte-
gration node for average current control. Integration time
constant is calculated using 2.5kΩ typical output imped-
ance.
PROG:
Transconductance Amplifier Negative Input. Pro-
gram node for average current delivered to load during
current mode operation. Average current delivered to load
imposes voltage differential at current sense amplifier
U
U
U
3
LT1620/LT1621
PI FU CTIO S
input (across external sense resistor) equal to (V
CC
–
V
PROG
)/10. Input voltage range is V
CC
to (V
CC
– 1.25V).
AVG2:
Sense Amplifier A
V
= – 20 Output and Comparator
Positive Input. Used as integration node for end-of-cycle
determination flag. Integration time constant is calculated
using 5kΩ typical output impedance.
PROG2:
Comparator Negative Input. Program node for
end-of-cycle determination typically used during voltage
mode operation. The comparator threshold is reached
when the current sense amplifier differential input voltage
equals (V
CC
– V
PROG2
)/20. Input voltage range is (V
CC
–
0.15V) to (V
CC
– 2.5V).
GND:
Ground Reference.
MODE:
Comparator Open Collector Output. Output is logic
low when magnitude of current sense amplifier differential
input voltage is less than (V
CC
– V
PROG2
)/20.
I
OUT
:
Transconductance Amplifier Output. In typical appli-
cation, I
OUT
sinks current from current-setting node on
companion PWM controller IC, facilitating current mode
loop control.
FUNCTIONAL BLOCK DIAGRA
V
CC
500Ω
+
–
+
–
IN
+
CURRENT
SENSE
RESISTOR
+
SENSE
AMPLIFIER
V
ID
IN
–
–
PROG
PROG2*
*AVAILABLE IN THE LT1620GN ONLY
OPERATION
(Refer to the Functional Block Diagram)
Current Sense Amplifier
The current sense amplifier is a multiple output voltage
amplifier with an operational input common mode range
from 0V to 32V. The amplifier generates scaled output
voltages at the SENSE, AVG and AVG2 (available in
LT1620GN) pins. These output signal voltages are refer-
enced to the V
CC
supply by pulling signal current through
internal V
CC
referred resistors.
4
W
U
U
U
U
U
U
5V
(×1 GAIN) SENSE
2.5k
(×10 GAIN)
5k
AVG
INTV
CC
SENSE
+
SENSE
–
(×20 GAIN) AVG2*
PWM
CONTROLLER
+
g
m
I
OUT
I
TH
–
+
–
GND
LT1620/21 • FBD
MODE*
END-OF-CYCLE
(ACTIVE LOW)
The first output (SENSE) is a unity gain, level-shifted repre-
sentation of the input signal (IN
+
– IN
–
). In typical PWM/
charger type applications, this output is used to drive the
current sense amplifier of the mated PWM controller IC.
The other two outputs (AVG and AVG2) are internally
connected to a transconductance amplifier and compara-
tor, respectively. The AVG output yields a gain of 10, and
the AVG2 output provides a gain of 20. These pins are
LT1620/LT1621
OPERATION
used as integration nodes to facilitate averaging of the
current sense amplifier signal. (Note: filter capacitors on
these pins should bypass to the V
CC
supply.) Integration
of these signals enables direct sensing and control of DC
load current, eliminating the inclusion of ripple current in
load determination.
Transconductance Amplifier
The transconductance amplifier converts the difference
between the current programming input voltage (V
PROG
)
and the average current sense output (V
AVG
) into a current
at the amplifier output pin (I
OUT
). The amplifier output is
unidirectional and only sinks current. The amplifier is
designed to operate at a typical output current of 130µA
APPLICATIONS INFORMATION
In Figure 2, an LT1620MS8 is coupled with an LTC1435
switching regulator in a high performance lithium-ion
battery charger application. The LTC1435 switching regu-
lator delivers extremely low dropout as it is capable of
approximately 99% duty cycle operation. No additional
power supply voltage is required for the LT1620 in this
application; it is powered directly from a 5V local supply
generated by the LTC1435. The DC charge current control
and high common mode current sense range of the
LT1620 combine with the low dropout capabilities of the
LTC1435 to make a 4-cell Li-Ion battery charger with over
96% efficiency, and only 0.5V input-to-output drop at 3A
charging current. Refer to the LTC1435 data sheet (available
from the LTC factory) for additional information on IC func-
tionality, performance and associated component selection.
This LT1620/LTC1435 battery charger is designed to yield
a 16.8V float voltage with a battery charge current of 3.2A.
The V
IN
supply can range from 17.3V to 28V (limited by the
switch MOSFETs). The charger provides a constant 3.2A
charge current until the battery voltage reaches the pro-
grammed float voltage. Once the float voltage is achieved,
a precision voltage regulation loop takes control, allowing
the charge current to fall as required to complete the
battery charge cycle.
R
SENSE
Selection
The LT1620 will operate throughout a current program-
ming voltage (V
PROG
) range of 0V to – 1.25V (relative to
V
CC
), however, optimum accuracy will be obtained with a
current setting program voltage of – 0.8V, corresponding
to 80mV differential voltage across the current sense
amplifier inputs. Given the desired current requirement,
selection of the load current sense resistor R
SENSE
is
possible. For the desired 3.2A charge current;
R
SENSE
= 80mV/3.2A or 0.025Ω
At the programmed 3.2A charge current, the sense resis-
tor will dissipate (0.08V)(3.20A) = 0.256W, and must be
rated accordingly.
Current Sense
The current sense inputs are connected on either side of
the sense resistor with IN
+
at the more positive potential,
given average charging current flow. The sense resistor to
IN
+
, IN
–
input paths should be connected using twisted
pair or minimum PC trace spacing for noise immunity.
Keep lead lengths short and away from noise sources for
best performance.
U
W
U
U
U
(Refer to the Functional Block Diagram)
with V
AVG
= V
PROG
. In typical PWM/charger type applica-
tions, the I
OUT
current is used to servo the current control
loop on the mated PWM controller IC to maintain a
programmed load current.
Comparator
The comparator circuit (available only in the LT1620GN)
may be used as an end-of-cycle sensor in a Li-Ion battery
charging system. The comparator detects when the charg-
ing current has fallen to a small value (typically 20% of the
maximum charging current). The comparator drives an open
collector output (MODE) that pulls low when the V
AVG2
voltage is more positive than V
PROG2
(output current below
the programmed threshold).
5
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