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PDF LTC1736 Data sheet ( Hoja de datos )
| Número de pieza | LTC1736 | |
| Descripción | 5-Bit Adjustable High Efficiency Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1736
5-Bit Adjustable
High Efficiency Synchronous
Step-Down Switching Regulator
FEATURES
s Dual N-Channel MOSFET Synchronous Drive
s Synchronizable/Programmable Fixed Frequency
s Wide VIN Range: 3.5V to 36V Operation
s 5-Bit Digital-to-Analog VOUT Selection:
0.925V to 2.00V Range with 50mV/25mV Steps
s OPTI-LOOPTM Compensation Minimizes COUT
s ±1% Output Voltage Accuracy
s Power Good Output Voltage Monitor
s Active Voltage Positioning Compatible
s Output Overvoltage Crowbar Protection
s Internal Current Foldback
s Latched Short-Circuit Shutdown Timer
with Defeat Option
s Forced Continuous Control Pin
s Optional Programmable Soft-Start
s Remote Output Voltage Sense
s Available in 24-Lead SSOP Package
U
APPLICATIO S
s Notebook and Palmtop Computers, PDAs
s Power Supply for Mobile Pentium® II and
Pentium III Processors
s Low Voltage Power Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
OPTI-LOOP and Burst Mode are trademarks of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation.
DESCRIPTIO
The LTC®1736 is a synchronous step-down switching
regulator controller optimized for CPU power. The output
voltage is programmed by a 5-bit digital-to-analog con-
verter (DAC) that adjusts the output voltage from 0.925V
to 2.00V according to Intel mobile VID specifications. The
0.8V reference is compatible with future microprocessor
generations.
The operating frequency (synchronizable up to 500kHz) is
set by an external capacitor allowing maximum flexibility
in optimizing efficiency. The output voltage is monitored by
a power good window comparator that indicates when the
output is within 7.5% of its programmed value.
Protection features include: internal foldback current lim-
iting, output overvoltage crowbar and optional short-cir-
cuit shutdown. Soft-start is provided by an external capaci-
tor that can be used to properly sequence supplies. The
operating current level is user-programmable via an exter-
nal current sense resistor. Wide input supply range allows
operation from 3.5V to 30V (36V maximum).
Pin defeatable Burst ModeTM operation provides high effi-
ciency at low load currents. OPTI-LOOP compensation
allows the transient response to be optimized over a wide
range of output capacitance and ESR values.
TYPICAL APPLICATIO
COSC
47pF
CSS
0.1µF
RC
CC1
330pF
33k
CC2
47pF
47pF
COSC
VIN
RUN/SS
TG
SW
ITH LTC1736 VIDVCC
PGOOD
VID4
VID3
VID2
VID1
VID0
SGND
INTVCC
BOOST
BG
PGND
VOSENSE SENSE – SENSE +
1000pF
VIN
5V TO 24V
M1
FDS6680A
DB
CMDSH-3 CB
0.22µF
+
4.7µF
M2
FDS6680A
×2
CIN
22µF/50V
×2
CERAMIC
L1
1.2µH
RSENSE
0.004Ω
D1
MBRS340T3
VOUT
1.35V TO 1.60V
12A
+ COUT
180µF/4V
×4
COUT: PANASONIC EEFUEOG181R
CIN: MARCON THCR70EIH226ZT
L1: PANASONIC ETQP6RZIR20HFA
RSENSE: IRC LRF2010-01-R004J
Figure 1. High Efficiency Step-Down Converter
1736 F01
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Current Sense Threshold
vs Normalized Output Voltage
(Foldback)
80
70
60
50
40
30
20
10
0
0 25 50 75 100
NORMALIZED OUTPUT VOLTAGE (%)
1736 G10
Maximum Current Sense Threshold
vs ITH Voltage
90
80
70
60
50
40
30
20
10
0
–10
–20
–30
0 0.5 1 1.5 2 2.5
VITH (V)
1736 G13
RUN/SS Pin Current
vs Temperature
0
VRUN/SS = 0V
–1
–2
–3
–4
–5
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1736 G16
Maximum Current Sense Threshold
vs VRUN/SS
80
VSENSE(CM) = 1.6V
60
40
20
0
01 2 34 5 6
VRUN/SS (V)
1736 G11
Maximum Current Sense Threshold
vs Temperature
80
VSENSE(CM) = 1.6V
75
70
65
60
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1736 G18
FCB Pin Current vs Temperature
0
VFCB = 0.85V
–0.2
–0.4
–0.6
–0.8
–1.0
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1736 G17
LTC1736
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
80
76
72
68
64
60
0 0.5 1 1.5 2
COMMON MODE VOLTAGE (V)
1736 G12
VITH vs VRUN/SS
2.5
VOSENSE = 0.7V
2.0
1.5
1.0
0.5
0
012 34
VRUN/SS (V)
56
1736 G15
Output Current vs Duty Cycle
100
IOUT/IMAX
(SYNCHRONIZED)
80 IOUT/IMAX
(FREE RUN)
60
40
20
fSYNC = fO
0
0 20
40 60
DUTY CYCLE (%)
80 100
1736 G14
5
5 Page 
LTC1736
APPLICATIO S I FOR ATIO
COSC(pF)
=
1.61(107)
Frequency
–
11
A graph for selecting COSC versus frequency is given in
Figure 2. The maximum recommended switching fre-
quency is 550kHz .
The internal oscillator runs at its nominal frequency (fO)
when the FCB pin is pulled high to INTVCC or connected to
ground. Clocking the FCB pin above and below 0.8V will
cause the internal oscillator to lock to an external clock
signal with a frequency between 0.9fO and 1.3fO. The clock
high level must exceed 1.3V for at least 0.3µs, and the
clock low level must be less than 0.3V for at least 0.3µs.
The top MOSFET turn-on will synchronize with the rising
edge of the external clock.
Attempting to synchronize to too high an external fre-
quency (above 1.3fO) can result in inadequate slope com-
pensation and possible loop instability at high duty cycles.
If this condition exists simply lower the value of COSC so
fEXT = fO according to Figure 2.
100.0
87.5
75.0
62.5
50.0
37.5
25.0
12.5
0
0 100 200 300 400 500 600
OPERATING FREQUENCY (kHz)
1736 F02
Figure 2. Timing Capacitor Value
When synchronized to an external clock, Burst Mode op-
eration is disabled but the inductor current is not allowed
to reverse. The 25% minimum inductor current clamp
present in Burst Mode operation is removed, providing
constant frequency discontinuous operation over the wid-
est possible output current range. In this mode the
synchronous MOSFET is forced on once every 10 clock
cycles to recharge the bootstrap capacitor. This minimizes
audible noise while maintaining reasonably high efficiency.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate-charge losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current ∆IL decreases with higher induc-
tance or frequency and increases with higher VIN or VOUT:
∆IL
=
1
(f)(L)
VOUT
1–
VOUT
VIN
Accepting larger values of ∆IL allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆IL = 0.3 to 0.4(IMAX). Remember,
the maximum ∆IL occurs at the maximum input voltage.
The inductor value also has an effect on low current
operation. The transition to low current operation begins
when the inductor current reaches zero while the bottom
MOSFET is on. Burst Mode operation begins when the
average inductor current required results in a peak current
below 25% of the current limit determined by RSENSE.
Lower inductor values (higher ∆IL) will cause this to occur
at higher load currents, which can cause a dip in efficiency
in the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron
cores, forcing the use of more expensive ferrite,
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC1736.PDF ] | |
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