LTC3544
Quad Synchronous
Step-Down Regulator: 2.25MHz,
300mA, 200mA, 200mA, 100mA
FEATURES
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High Efficiency: Up to 95%
Four Independent Regulators Provide Up to 300mA,
200mA, 200mA and 100mA Output Current
2.25V to 5.5V Input Voltage Range
2.25MHz Constant-Frequency Operation
No Schottky Diodes Required
Extremely Low Channel-to-Channel Transient
Crosstalk
Low Ripple (20mVP-P) Burst Mode Operation:
IQ = 70μA (All Channels On)
0.8V Reference Allows Low Output Voltages
Shutdown Mode Draws 1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator
can deliver enough current to prevent this problem if the
load switch resistance is low and it is driven quickly. The
only solution is to limit the rise time of the switch drive
so that the load rise time is limited to approximately (25
• CLOAD). Thus, a 10μF capacitor charging to 3.3V would
require a 250μs rise time, limiting the charging current
to about 130mA.
3544fa
12
LTC3544
APPLICATIONS INFORMATION
PC Board Layout Checklist
3. Keep C8 and C9 as close to the part as possible.
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3544. These items are also illustrated graphically
in Figures 3 and 4. Check the following in your layout:
4. Keep the switching nodes (SWx) away from the sensitive VFBx nodes.
5. Keep the ground connected plates of the input and
output capacitors as close as possible.
1. The power traces, consisting of the PGND trace, the
GNDA trace, the SW traces, the PVIN trace and the VCC
trace should be kept short, direct and wide.
6. Care should be taken to provide enough space between
unshielded inductors in order to minimize any transformer coupling.
2. Does each of the VFBx pins connect directly to the
respective feedback resistors? The resistive dividers
must be connected between the (+) plate of the corresponding output filter capacitor (e.g., C13) and GNDA.
If the circuit being powered is at such a distance from
the part where voltage drops along circuit traces are
large, consider a Kelvin connection from the powered
circuit back to the resistive dividers.
VCC
2.25V TO 5.5V
L4
Design Example
As a design example, consider using the LTC3544 as a
portable application with a Li-Ion battery. The battery
provides VIN ranging from 2.8V to 4.2V. The demand at
2.5V is 250mA necessitating the use of the 300mA output
for this requirement.
GNDA
C8
VOUT1
R15
C15
R16
RUN100
C13
SW1000
VCC
GNDA
VFB100
C1
VFB200A
RUN100
VFB300
VFB100
C4
GND
VFB200B
VFB300
L1
L4
LTC3544
RUN200A
L2
VOUT3
R5
RUN200A
SW200A
SW200A
SW200B
C6
SW200B
C4
RUN300
RUN200B
PGND
PVIN
RUN300
C9
SW300
C9
L2
L1
VFB200A
R8
R2
PGND
VOUT2
C3
C2 C3
C1
PVIN
2.25V TO 5.5V
C12
L3
VOUT4
R6
L3
C10
VCC
RUN200B
SW300
PGND
R3
VFB200B
C10
3544 F04
3544 F03
R11
Figure 3. LTC3544 Layout Diagram
Figure 4. LTC3544 Suggested Layout
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13
LTC3544
APPLICATIONS INFORMATION
Beginning with this channel, first calculate the inductor
value for about 35% ripple current (100mA in this example)
at maximum VIN. Using a form of Equation 1:
L4 =
200k are a good compromise between efficiency and immunity to any adverse effects of PCB parasitic capacitance
on the feedback pins. Choosing 10μA with 0.8V feedback
voltage makes R7 = 80k. A close standard 1% resistor is
76.8k. Using:
2.5V
⎛ 2.5V ⎞
1–
= 4.5µH
2.25MHz • 100mA ⎜⎝ 4.2V ⎟⎠
⎛V
⎞
R8 = ⎜ OUT – 1⎟ • R7 = 163.2k
⎝ 0.8
⎠
For the inductor, use the closest standard value of 4.7μH.
A 4.7μF capacitor should be sufficient for the output capacitor. A larger output capacitor will attenuate the load
transient response, but increase the settling time. A value
for CIN = 4.7μF should suffice as the source impedance of
a Li-Ion battery is very low.
The closest standard 1% resistor is 162k. An optional
20pF feedback capacitor may be used to improve transient
response. The component values for the other channels
are chosen in a similar fashion.
The feedback resistors program the output voltage.
Minimizing the current in these resistors will maximize
efficiency at very light loads, but totals on the order of
Figure 5 shows the complete schematic for this example,
along with the efficiency curve and transient response for
the 300mA channel.
VSUPPLY
3.6V
C10
4.7μF
C9
4.7μF
15
L2
4.7μH
VOUT2
1.5V
C2
4.7μF
VOUT3
0.8V
R3
93.1k
16
4
C6
20pF
1
7
PVIN
SW200B
RUN100
SW100
VFB200B
R4
107k
VFB100
12
13
L1
10μH
C5
20pF
11
3
5
R6
100k
2
RUN200A
RUN300
SW200A
SW300
VFB200A
VFB300
GNDA
14
VOUT1
1.2V
R1
59k
C1
4.7μF
R2
118k
LTC3544
L3
4.7μH
C3
4.7μF
RUN200B
VCC
9
8
10
PGND
L4
4.7μH
C8
20pF
VOUT2
2.5V
R7
162k
C4
10μF
R8
76.8k
6
3544 F05a
Figure 5. Design Example
Efficiency vs Output Current—300mA Channel,
All Other Channels Off
Transient Response
100
90
VOUT300
50mV/DIV
AC COUPLED
80
EFFICIENCY (%)
70
60
IL
250mA/DIV
50
40
ILOAD
250mA/DIV
30
20
10
VOUT = 2.5V
TA = 25°C
0
0.0001
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
0.001
0.01
0.1
LOAD CURRENT (A)
1
VIN = 3.6V
20μs/DIV
VOUT = 2.5V
TA = 25°C
LOAD STEP = 300μA TO 300mA
3544B F05c
3544B F05b
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14
LTC3544
PACKAGE DESCRIPTION
UD Package
16-Lead Plastic QFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1691)
0.70 ±0.05
3.50 ± 0.05
1.45 ± 0.05
2.10 ± 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ± 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH R = 0.20 TYP
OR 0.25 × 45° CHAMFER
R = 0.115
TYP
0.75 ± 0.05
15
16
PIN 1
TOP MARK
(NOTE 6)
0.40 ± 0.10
1
1.45 ± 0.10
(4-SIDES)
2
(UD16) QFN 0904
0.200 REF
0.00 – 0.05
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
0.25 ± 0.05
0.50 BSC
3544fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3544
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ThinSOT and VLDO are trademarks of Linear Technology Corporation
3544fa
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