OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LTC3552EDHC-1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LTC3552-1
16
35521fa
APPLICATIO S I FOR ATIO
WUU
U
charge storage and very low ESR at the switching fre-
quency. Other losses include diode conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
Thermal Considerations
The battery charger’s thermal regulation feature and the
switching regulator’s high effi ciency make it unlikely that
the LTC3552-1 will dissipate enough power to exceed
its maximum junction temperature. However, in applica-
tions where the LTC3552-1 is running at high ambient
temperature with low supply voltage and high duty cycles,
the power dissipated may result in excessive junction
temperatures. To prevent the LTC3552-1 from exceeding
the maximum junction temperature, the user will need to
do some thermal analysis. The goal of the thermal analysis
is to determine whether the power dissipated will raise
the junction temperature above the maximum rating. The
temperature rise is given by:
T
RISE
= P
D
• θ
JA
where P
D
is the power dissipated and θ
JA
is the ther-
mal resistance from the junction of the die to the
ambient temperature. The junction temperature, T
J
, is
given by:
T
J
= T
RISE
+ T
AMBIENT
As an example, consider the case when the battery char-
ger is idle, and both regulators are operating at an input
voltage of 2.7V with a load current of 400mA and 800mA
and an ambient temperature of 70°C. From the Typical
Performance Characteristics graph of Switch Resistance,
the R
DS(ON)
resistance of the main switch is
0.425Ω.
Therefore, power dissipated by each regulator is:
P
D
= I
2
• R
DS(ON)
= 272mW and 68mW
The DHC16 package junction-to-ambient thermal resis-
tance, θ
JA
, is 40°C/W. Therefore, the junction temperature
of the regulator operating in a 70°C ambient temperature
is approximately:
T
J
= (0.272 + 0.068) • 40 + 70 = 83.6°C
which is below the absolute maximum junction tempera-
ture of 125°C.
The majority of the LTC3552-1 power dissipation comes
from the battery charger. Fortunately, the LTC3552-1 au-
tomatically reduces the charge current during high power
conditions using a patented thermal regulation circuit. Thus,
it is not necessary to design for worst-case power dissipa-
tion scenarios. The conditions that cause the LTC3552-1
to reduce charge current through thermal feedback can
be approximated by considering the power dissipated in
the IC. The approximate ambient temperature at which the
thermal feedback begins to protect the IC is:
T
A
= 120°C – P
D
θ
JA
T
A
= 120°C – (P
D(CHARGER)
+ P
D(REGULATOR)
) • θ
JA
Most of the charger’s power dissipation is generated from
the internal charger MOSFET. Thus, the power dissipation
is calculated to be:
P
D(CHARGER)
= (V
IN
– V
BAT
) • I
BAT
V
IN
is the charger supply voltage, V
BAT
is the battery volt-
age and I
BAT
is the charge current.
Example: An LTC3552-1 operating from a 5V supply
is programmed to supply 800mA full-scale current to
a discharged Li-Ion battery with a voltage of 3.3V. For
simplicity, assume the regulators are disabled and dis-
sipate no power.
The charger power dissipation is calculated to be:
P
D(CHARGER)
= (5V – 3.3V) • 800mA = 1.36W
Thus, the ambient temperature at which the LTC3552-1
charger begins to reduce the charge current is approxi-
mately:
T
A
= 120°C – 1.36W • 40°C/W
T
A
= 120°C – 54.4°C
T
A
= 65.6°C
The LTC3552-1 can be used above 65°C ambient but the
charge current will be reduced from the programmed
800mA. The approximate current at a given ambient
temperature can be approximated by:
I
CT
VV
BAT
A
IN BAT JA
=
°120 –
(– •
)
θ
Using the previous example with an ambient temperature
of 70°C (and no heat dissipation from the regulator), the
charge current will be reduced to approximately:
LTC3552-1
17
35521fa
APPLICATIO S I FOR ATIO
WUU
U
I
CC
VV CW
C
CA
I
BAT
=
°°
°
=
°
°
120 70
53340
50
68
–
(–.)• / /
BBAT
mA= 735
The previous analysis can be repeated to take into account
the power dissipation of the regulator by:
I
CT T
VV
BAT
A RISE REGULATOR
IN BAT J
=
°−120 –
(– •
()
)
θ
AA
However, the regulator typically dissipates signifi cantly less
heat than the charger (even in worst-case situations), the
calculations here should work well as an approximation.
Moreover, when thermal feedback reduces the charge
current, the voltage at the PROG pin is also reduced pro-
portionally. It is important to remember that LTC3552-1
applications do not need to be designed for worst-case
thermal conditions since the IC will automatically reduce
charge current when the junction temperature reaches
approximately 120°C.
In order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on
the backside of the LTC3552-1 package is soldered to
the PC board ground. Failure to make thermal contact
between the exposed pad on the backside of the package
and the copper board will result in thermal resistances far
greater than 40°C/W. As an example, a correctly soldered
LTC3552-1 can deliver over 800mA to a battery from a
5V supply at room temperature. Without a good backside
thermal connection, this number will drop considerably.
Battery Charger Stability Considerations
The constant-voltage mode feedback loop is stable with-out
an output capacitor, provided a battery is connected to the
charger output. With no battery present, an output capacitor
on the BAT pin is recommended to reduce ripple voltage.
When using high value, low ESR ceramic capacitors, it
is recommended to add a 1Ω resistor in series with the
capacitor. No series resistor is needed if tantalum capaci-
tors are used. In constant-current mode, the PROG pin is
in the feedback loop, not the battery. The constant-current
mode stability is affected by the impedance at the PROG
pin. With no additional capacitance on the PROG
pin, the charger is stable with program resistor values as
high as 20k; however, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin is loaded with a capacitance,
C
PROG
, the following equation can be used to calculate
the maximum resistance value for R
PROG
:
R
C
PROG
PROG
≤
1
210
5
π ••
Average, rather than instantaneous charge current may
be of interest to the user. For example, when the switch-
ing regulators operating in Burst Mode
®
are connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
fi lter can be used on the PROG pin to measure the average
battery current, as shown in Figure 3. A 10k resistor has
been added between the PROG pin and the fi lter capacitor
to ensure stability.
Figure 3. Isolating Capacitive Load on PROG Pin and Filtering
LTC3552-1
GND
PROG
R
PROG
10k
C
FILTER
35521 F03
CHARGE
CURRENT
MONITOR
CIRCUITRY
V
IN
Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multilayer
ceramic capacitors. Because of the self-resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions such as connecting the charger input to a live
power source. Adding a 1.5Ω resistor in series with an X5R
ceramic capacitor will minimize start-up voltage transients.
For more information, see Application Note 88.
Reverse Polarity Input Voltage Protection
In some applications, protection from reverse polarity
voltage on V
IN
is desired. If the supply voltage is
high enough, a series blocking diode can be used. In
Burst Mode is a registered trademark of Linear Technology Corporation.
LTC3552-1
18
35521fa
APPLICATIO S I FOR ATIO
WUU
U
Design Example
As a design example, assume the LTC3552-1 is used in
a single lithium-ion battery-powered cellular phone ap-
plication.
Starting with the charger, choosing R
PROG
to be 1.24k
programs the charger for 806mA. A good rule of thumb
for I
TERMINATE
is one tenth the full charge current, so R
ITERM
is picked to be 1.24k (I
TERMINATE
= 80mA).
For the switching regulators powered from the battery, V
CC
can range from 4.2V to about 2.7V. The load requires a
maximum of 800mA in active mode and 2mA in standby
mode. Regulator 1 output voltage is 1.8V. Since the load
still needs power in standby, Burst Mode operation is used
for good low load effi ciency. First, calculate the inductor
value for about 30% ripple current at maximum V
CC
:
L
V
MHz mA
V
V
H=−
⎛
⎝
⎜
⎞
⎠
⎟
=µ
18
2 25 240
1
18
42
19
.
.•
.
.
.
Choosing a vendor’s closest inductor value of 2.2µH,
results in a maximum ripple current of:
∆I
V
MHz H
V
V
mA
L
=
µ
−
⎛
⎝
⎜
⎞
⎠
⎟
=
18
225 22
1
18
42
208
.
.•.
.
.
For cost reasons, a ceramic capacitor will be used. C
OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C
mA
MHz V
F
OUT
=
()
=µ25
800
225 5 25
71.
.•%•.
.
A good standard value is 10µF. Since the impedance of a
Li-Ion battery is very low, C
IN
is typically 10µF. Following
the same procedure, for V
OUT2
= 1.575V, inductor value
can be derived as 4.7µH, and output capacitor is 4.7µF.
Figure 2 shows the complete schematic for this design
example.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3552-1. These items are also illustrated graphically
in the layout diagram of Figure 5. Check the following in
your layout:
1. Does the capacitor C
IN
connect to the power V
CC
and
GND (exposed pad) as closely as possible? This ca-
pacitor provides the AC current to the internal power
MOSFETs and their drivers.
2. The feedback signals V
OUT
should be routed away from
noisy components and traces, such as the SW line, and
its trace should be minimized.
3. Are the C
OUT
and L1 closely connected? The (–) plate of
C
OUT
returns current to GND and the (–) plate of C
IN
.
4. Keep sensitive components away from the SW pins.
The input capacitor C
IN
should be routed away from
the SW traces and the inductors.
5. A ground plane is preferred, but if not available, keep
the signal and power grounds segregated with small
signal components returning to the GND pin at one
point and should not share the high current path of C
IN
or C
OUT
.
6. Flood all unused areas on all layers with copper. Flooding
with copper will reduce the temperature rise of power
components. These copper areas should be connected
to V
CC
or GND.
Figure 5. Layout Diagram
Figure 4. Low Loss Input Reverse Polarity Protection
3552-1 F05
L1L2
BOLD LINES INDICATE
HIGH CURRENT PATHS
LTC3552-1
GND
V
IN
SW2
V
OUT2
V
FB2
V
CC
SW1
V
OUT1
V
FB1
V
OUT2
V
OUT1
C
IN
C
S
C
OUT1
C
OUT2
C
FF2
C
FF1
LTC3552-1
V
IN
V
IN
35521 F04
DRAIN-BULK
DIODE OF FET
other cases, where the voltage drop must be kept low, a
P-channel MOSFET can be used (as shown in Figure 4).
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTC3552EDHC-1#PBF

Mfr. #:
Buy LTC3552EDHC-1#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management Standalone Linear Li-Ion Battery Charger & Dual Synch Buck Converters
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet