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PDF LT1513C Data sheet ( Hoja de datos )
| Número de pieza | LT1513C | |
| Descripción | SEPIC Constant- or Programmable-Current/ Constant-Voltage Battery Charger | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1513/LT1513-2
SEPIC Constant- or
Programmable-Current/
Constant-Voltage Battery Charger
FEATURES
s Charger Input Voltage May Be Higher, Equal to or
Lower Than Battery Voltage
s Charges Any Number of Cells Up to 20V
s 1% Voltage Accuracy for Rechargeable Lithium
Batteries
s 100mV Current Sense Voltage for High Efficiency
(LT1513)
s 0mV Current Sense Voltage for Easy Current
Programming (LT1513-2)
s Battery Can Be Directly Grounded
s 500kHz Switching Frequency Minimizes
Inductor Size
s Charging Current Easily Programmable or Shut Down
U
APPLICATIONS
s Charging of NiCd, NiMH, Lead-Acid or Lithium
Rechargeable Cells
s Precision Current Limited Power Supply
s Constant-Voltage/Constant-Current Supply
s Transducer Excitation
s Universal Input CCFL Driver
DESCRIPTION
The LT®1513 is a 500kHz current mode switching regula-
tor specially configured to create a constant- or program-
mable-current/constant-voltage battery charger. In addition
to the usual voltage feedback node, it has a current sense
feedback circuit for accurately controlling output current
of a flyback or SEPIC (Single-Ended Primary Inductance
Converter) topology charger. These topologies allow the
current sense circuit to be ground referred and completely
separated from the battery itself, simplifying battery switch-
ing and system grounding problems. In addition, these
topologies allow charging even when the input voltage is
lower than the battery voltage. The LT1513 can also drive
a CCFL Royer converter with high efficiency in floating or
grounded mode.
Maximum switch current on the LT1513 is 3A. This allows
battery charging currents up to 2A for a single lithium-ion
cell. Accuracy of 1% in constant-voltage mode is perfect
for lithium battery applications. Charging current can be
easily programmed for all battery types.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
WALL
ADAPTER
INPUT + C3
22µF
25V
• L1A*
7
C2**
4.7µF
VIN VSW 5
D1†
CHARGE
SYNC
AND/OR
SHUTDOWN SHUTDOWN
LT1513
6
S/S
GND VC
4 TAB 1
2
VFB
IFB R4
3 39Ω
R5
270Ω
C5
0.1µF
C4
0.22µF
L1B*
•
R1
R2
R3
0.08Ω
1.25A
+ C1
22µF
25V
×2
* L1A, L1B ARE TWO 10µH WINDINGS ON A
COMMON CORE: COILTRONICS CTX10-4
LT1513 • TA01
** CERAMIC MARCON THCR40EIE475Z OR TOKIN 1E475ZY5U-C304
† MBRD340 OR MBRS340T3. MBRD340 HAS 5µA TYPICAL
LEAKAGE, MBRS340T3 50µA TYPICAL
Figure 1. SEPIC Charger with 1.25A Output Current
Maximum Charging Current
2.4
2.2 SINGLE Li-Ion CELL
2.0 (4.1V)
1.8 DOUBLE Li-Ion
1.6 CELL (8.2V)
1.4
12V
1.2
16V
1.0
20V
0.8
0.6
0.4
0
BATTERY
VOLTAGE
5 10 15 20 25 30
INPUT VOLTAGE (V)
INDUCTOR = 10µH
ACTUAL PROGRAMMED CHARGING CURRENT WILL BE
INDEPENDENT OF INPUT VOLTAGE IF IT DOES NOT
EXCEED VALUES SHOWN
LT1513 • TA02
1
1 page  
LT1513/LT1513-2
PIN FUNCTIONS
VC (Pin 1): The compensation pin is primarily used for
frequency compensation, but it can also be used for soft
starting and current limiting. It is the output of the error
amplifier and the input of the current comparator. Peak
switch current increases from 0A to 3.6A as the VC voltage
varies from 1V to 1.9V. Current out of the VC pin is about
200µA when the pin is externally clamped below the
internal 1.9V clamp level. Loop frequency compensation
is performed with a capacitor or series RC network from
the VC pin directly to the ground pin (avoid ground loops).
FB (Pin 2): The feedback pin is used for positive output
voltage sensing. The R1/R2 voltage divider connected to
FB defines Li-Ion float voltage at full charge, or acts as a
voltage limiter for NiCd or NiMH applications. FB is the
inverting input to the voltage error amplifier. Input bias
current is typically 300nA, so divider current is normally
set to 100µA to swamp out any output voltage errors due
to bias current. The noninverting input of this amplifier is
tied internally to a 1.245V reference. The grounded end of
the output voltage divider should be connected directly to
the LT1513 ground pin (avoid ground loops).
IFB (Pin 3): The current feedback pin is used to sense
charging current. It is the input to a current sense amplifier
that controls charging current when the battery voltage is
below a programmed limit. During constant-current
operation, the LT1513 IFB pin regulates at – 100mV. Input
resistance of this pin is 5kΩ, so filter resistance (R4,
Figure 1) should be less than 50Ω. The 39Ω, 0.22µF filter
shown in Figure 1 is used to convert the pulsating current
in the sense resistor to a smooth DC current feedback
signal. The LT1513-2 IFB pin regulates at 0mV to provide
programmable current limit. The current through R5,
Figure 5, is balanced by the current through R4, program-
ming the maximum voltage across R3.
GND (Pin 4): The ground pin is common to both control
circuitry and switch current. VC, FB and S/S signals must
be Kelvin and connected as close as possible to this pin.
The TAB of the R package should also be connected to the
power ground.
VSW (Pin 5): The switch pin is the collector of the power
switch, carrying up to 3A of current with fast rise and fall
times. Keep the traces on this pin as short as possible to
minimize radiation and voltage spikes. In particular, the
path in Figure 1 which includes SW to C2, D1, C1 and
around to the LT1513 ground pin should be as short as
possible to minimize voltage spikes at switch turn-off.
S/S (Pin 6): This pin can be used for shutdown and/or
synchronization. It is logic level compatible, but can be
tied to VIN if desired. It defaults to a high ON state when
floated. A logic low state will shut down the charger to a
micropower state. Driving the S/S pin with a continuous
logic signal of 600kHz to 800kHz will synchronize switch-
ing frequency to the external signal. Shutdown is avoided
in this mode with an internal timer.
VIN (Pin 7): The input supply pin should be bypassed with
a low ESR capacitor located right next to the IC chip. The
grounded end of the capacitor must be connected directly
to the ground plane to which the TAB is connected.
TAB: The TAB on the surface mount R package is electri-
cally connected to the ground pin, but a low inductance
connection must be made to both the TAB and the pin for
proper circuit operation. See suggested PC layout in
Figure 4.
5
5 Page 
LT1513/LT1513-2
APPLICATIONS INFORMATION
(from the Electrical Characteristics). Amplifier output resis-
tance is modeled with a 330k resistor. The power stage
(modulator section) of the LT1513 is modeled as a transcon-
ductance whose value is 4(VIN)/(VIN + VBAT). This is a very
simplified model of the actual power stage, but it is sufficient
when the unity-gain frequency of the loop is low compared
to the switching frequency. The output filter capacitor model
includes its ESR (RCAP). A series resistance (RBAT) is also
assigned to the battery model.
Analysis of this loop normally shows an extremely stable
system for all conditions, even with 0Ω for R5. The one
condition which can cause reduced phase margin is with a
very large battery resistance (> 5Ω), or with the battery
replaced with a resistive load. The addition of R5 gives good
phase margin even under these unusual conditions. R5
should not be increased above 330Ω without checking for
two possible problems. The first is instability in the constant
current region (see Constant-Current Mode Loop Stability),
and the second is subharmonic switching where switch duty
cycle varies from cycle to cycle. This duty cycle instability is
caused by excess switching frequency ripple voltage on the
VC pin. Normally this ripple is very low because of the
filtering effect of C5, but large values of R5 can allow high
ripple on the VC pin. Normal loop analysis does not show this
problem, and indeed small signal loop stability can be
excellent even in the presence of subharmonic switching.
The primary issue with subharmonics is the presence of EMI
at frequencies below 500kHz.
Constant-Current Mode Loop Stability
The LT1513 is normally very stable when operating in con-
stant-current mode (see Figure 7), but there are certain con-
ditions which may create instabilities. The combination of
higher value current sense resistors (low programmed charg-
ing current), higher input voltages, and the addition of a loop
compensation resistor (R5) on the VC pin may create an un-
stable current mode loop. (A resistor is sometimes added in
series with C5 to improve loop phase margin when the loop
is operating in voltage mode.) Instability results
because loop gain is too high in the 50kHz to 150kHz region
where excess phase occurs in the current sensing amplifier
and the modulator. The IFBA amplifier (gain of –12.5) has a
pole at approximately 150kHz. The modulator section con-
sisting of the current comparator, the power switch and the
magnetics, has a pole at approximately 50kHz when the
coupled inductor value is 10µH. Higher inductance will reduce
the pole frequency proportionally. The design procedure pre-
sented here is to roll off the loop to unity-gain at a frequency
of 25kHz or lower to avoid these excess phase regions.
FB
VC
R5
330Ω
C5
0.1µF
RP**
1M
V1
CP
3pF
MODULATOR SECTION
IP
=
4(V1)(VIN)
VIN + VBAT
EA
RG
330k
gm
1500µmho
RA
100k
IFBA
1.245V
CA
10pF
VOLTAGE
GAIN = 12
IP
IFB
R4
24Ω
C4
0.22µF
R3
0.1Ω
THIS IS A SIMPLIFIED AC MODEL FOR THE LT1513 IN
CONSTANT-CURRENT MODE. RESISTOR AND CAPACITOR
NUMBERS CORRESPOND TO THOSE USED IN FIGURE 1.
RP AND CP MODEL THE PHASE DELAY IN THE PowerPath.
C3 IS 3pF FOR A 10µH INDUCTOR. IT SHOULD BE SCALED
PROPORTIONALLY FOR OTHER INDUCTOR VALUES (6pF
FOR 20µH). THE PowerPath IS A TRANSCONDUCTANCE
WHOSE GAIN IS A FUNCTION OF INPUT AND BATTERY
VOLTAGE AS SHOWN.
THE CURRENT AMPLIFIER HAS A FIXED VOLTAGE GAIN OF 12.
ITS PHASE DELAY IS MODELED WITH RA AND CA.
THE ERROR AMPLIFIER HAS A TRANSCONDUCTANCE OF
1500µmho AND AN INTERNAL OUTPUT SHUNT RESISTANCE OF
330k.
AS SHOWN, THIS LOOP HAS A UNITY-GAIN FREQUENCY OF
ABOUT 27kHz. R5 IS NOT USED IN ALL APPLICATIONS, BUT IT
GIVES BETTER PHASE MARGIN IN CONSTANT VOLTAGE MODE.
1513 F07
Figure 7. Constant-Current Small-Signal Model
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1513C.PDF ] | |
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