LTC1771E [Linear]
Low Quiescent Current High Efficiency Step-Down DC/DC Controller; 低静态电流高效率降压型DC / DC控制器型号: | LTC1771E |
厂家: | Linear |
描述: | Low Quiescent Current High Efficiency Step-Down DC/DC Controller |
文件: | 总16页 (文件大小:208K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
Final Electrical Specifications
LTC1771
Low Quiescent Current
High Efficiency Step-Down
DC/DC Controller
February 2000
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FEATURES
DESCRIPTIO
The LTC®1771 is a high efficiency current mode step-
down DC/DC controller that draws as little as 10µA DC
supply current to regulate the output at no load while
maintaining high efficiency for loads up to several amps.
■
Very Low Standby Current: 10µA
■
Available in Space-Saving 8-Lead MSOP Package
■
High Output Currents
■
Wide VIN Range: 2.8V to 20V Operation
■
VOUT Range: 1.23V to 18V
TheLTC1771drivesanexternalP-channelpowerMOSFET
using a current mode, constant off-time architecture. An
external sense resistor is used to program the operating
current level. Current mode control provides short-circuit
protection, excellent transient response and controlled
start-up behavior. Burst Mode operation enables the
LTC1771 to maintain high efficiency down to extremely
low currents. Shutdown mode further reduces the supply
current to a mere 2µA. For low noise applications, Burst
Mode operation can be easily disabled with the MODE pin.
■
High Efficiency: Over 93% Possible
■
±2% Output Accuracy
■
Very Low Dropout Operation: 100% Duty Cycle
■
Current Mode Operation for Excellent Line and
Load Transient Response
Defeatable Burst ModeTM Operation
■
■
Short-Circuit Protected
Optional Programmable Soft-Start
■
■
Micropower Shutdown: IQ = 2µA
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Wide input supply range of 2.8V to 18V (20V maximum)
and 100% duty cycle operation for low dropout make the
LTC1771 ideal for a wide variety of battery-powered appli-
cations where maximizing battery life is important.
APPLICATIO S
■
Cellular Telephones and Wireless Modems
■
1- to 4-Cell Lithium-Ion-Powered Applications
■
Portable Instruments
The LTC1771’s availability in both 8-lead MSOP and SO
packages provides for a minimum area solution.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
■
Battery-Powered Equipment
■
Battery Chargers
■
Scanners
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TYPICAL APPLICATIO
V
IN
4.5V TO 18V
LTC1771 Efficiency
100
10µF
25V
CER
R
SENSE
0.05Ω
V
IN
= 5V
90
80
70
60
50
40
V
= 10V
IN
M1
V
SENSE
PGATE
IN
RUN/SS
Si6447DQ
L1
V
IN
= 15V
15µH
V
3.3V
2A
OUT
C
SS
0.01µF
LTC1771
I
TH
R
C
V
V
MODE
UPS5817
+
FB
IN
C
OUT
10k
GND
150µF
R2
1.64M
1%
C
C
6.3V
22OpF
R1
V
= 3.3V
SENSE
OUT
1M
1%
R
= 0.05Ω
5pF
0.1
1
10
100
1000
10000
1771 F01
LOAD CURRENT (mA)
1771 F01b
Figure 1. High Efficiency Step-Down Converter
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
1
LTC1771
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ABSOLUTE AXI U RATI GS (Note 1)
Input Supply Voltage (VIN)........................ –0.3V to 20V
Peak Driver Output Current < 10µs (PGATE) ............. 1A
RUN/SS Voltage ........................... –0.3V to (VIN + 0.3V)
MODE Voltage .......................................... –0.3V to 20V
ITH, VFB Voltage .......................................... –0.3V to 5V
SENSE Voltage (VIN > 12V)...(VIN – 12V) to (VIN + 0.3V)
SENSE Voltage (VIN ≤ 12V) .......... –0.3V to (VIN + 0.3V)
Junction Temperature (Note 2)............................ 125°C
Operating Temperature Range (Note 3)
LTC1771E......................................... –40°C to 85°C
LTC1771I ......................................... –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
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PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
ORDER PART
TOP VIEW
NUMBER
TOP VIEW
RUN/SS
1
2
3
4
8
7
6
5
MODE
SENSE
RUN/SS 1
8 MODE
7 SENSE
LTC1771ES8
LTC1771IS8
LTC1771EMS8
I
TH
I
V
2
3
TH
FB
6 V
IN
V
V
FB
IN
5 PGATE
GND 4
GND
PGATE
MS8 PACKAGE
8-LEAD PLASTIC MSOP
S8 PART MARKING
MS8 PART MARKING
LTKD
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 150°C/ W
1771
1771I
TJMAX = 125°C, θJA = 110°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are T = 25°C.
A
VIN = 10V, VRUN = open unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
(Note 5)
MIN
TYP
1.230
1
MAX
1.255
10
UNITS
V
V
Feedback Voltage
●
●
1.205
FB
I
I
Feedback Current
(Note 5)
nA
FB
SUPPLY
No-Load Supply Current
Reference Voltage Line Regulation
Output Voltage Load Regulation
V
IN
V
IN
= 10V, I
= 0 (Note 6)
LOAD
10
µA
∆V
∆V
= 5V to 15V (Note 5)
●
●
0.003
0.25
0.03
1
%/V
%
LINEREG
I
= 0.5V to 2V, Burst Disabled (Note 5)
LOADREG
TH
I
Input DC Supply Current
Active Mode (PGATE = 0V)
Sleep Mode (Note 6)
Shutdown
(Note 4)
Q
V
IN
V
IN
V
IN
V
IN
= 2.8V to 18V
= 2.8V to 18V, V = 1.5V
= 2.8V to 18V, V
= 2.8V to 18V, V = 0V
150
9
2
235
15
6
µA
µA
µA
µA
FB
= 0V
RUN
Short Circuit
175
275
FB
∆V
∆V
∆V
Maximum Current Sense Threshold
Minimum Current Sense Threshold
Sleep Current Sense Threshold
Switch Off Time
V
V
= V
= V
– 20mV
●
●
110
0.5
140
–25
50
180
mV
mV
mV
SENSE(MAX)
SENSE(MIN)
SENSE(SLEEP)
FB
FB
REF
REF
+ 10mV, Burst Disabled
I
= 1V
TH
t
V
FB
V
FB
at Regulated Value
= 0V
3.5
70
µs
µs
OFF
V
Mode Pin Threshold
V
MODE
Rising
1.3
2
V
MODE
2
LTC1771
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are T = 25°C.
A
VIN = 10V, VRUN = open unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
0.5
TYP
1.0
1
MAX
UNITS
V
V
RUN/SS Pin Threshold
Source Current
V
V
Rising
RUN/SS
●
2
3
RUN/SS
RUN
I
= 0V, V = 2.8V to 18V
0.3
µA
RUN
IN
PGATE t , t
PGATE Transition Time (Note 7)
Rise Time
Fall Time
r
f
C
C
= 2000pF
= 2000pF
80
90
ns
ns
LOAD
LOAD
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 2: T is calculated from the ambient temperature T and power
Note 5: The LTC1771 is tested in a feedback loop that servos V to the
FB
J
A
dissipation P according to the following formulas:
balance point for the error amplifier (V = 1.23V).
D
ITH
LTC1771S8: T = T + (P )(110°C/W)
Note 6: No-load supply current consists of sleep mode current (9µA
typical) plus a small switching component necessary to overcome
Schottky diode leakage and feedback resistor current.
J
A
D
LTC1771MS8: T = T + (P )(150°C/W)
J
A
D
Note 3: The LTC1771E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LTC1771I is guaranteed and tested
over the –40°C to 85°C operating temperature range.
Note 7: t and t measured at 10% to 90% levels.
r
f
U
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PI FU CTIO S
PGATE (Pin 5): High Current Gate Driver for External
P-Channel MOSFET Switch. Voltage swing is from ground
to VIN.
RUN/SS (Pin 1): The voltage level on this pin controls
shutdown/run mode (ground = shutdown, open/high =
run). Connecting an external capacitor to this pin provides
soft-start.
VIN (Pin 6): Main Input Voltage Supply Pin.
ITH (Pin 2): Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 3V.
SENSE(Pin7):CurrentSenseInputforMonitoringSwitch
Current. Maximum switch current and Burst Mode
threshold is programmed with an external resistor be-
tween SENSE and VIN.
VFB (Pin 3): Feedback of Output Voltage for Comparison
to Internal 1.23V Reference. An external resistive divider
across the output is returned to this pin.
MODE (Pin 8): Burst Mode Enable/Disable Pin. Connect-
ing this pin to VIN (or above 2V) enables Burst Mode
operation, while connecting this pin to ground disables
Burst Mode operation. Do not leave floating.
GND (Pin 4): Ground Pin.
3
LTC1771
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FUNCTIONAL BLOCK DIAGRA
V
IN
V
IN
V
6
IN
+
1µA
C
READY
IN
C
SS
1.23V
REFERENCE
RUN/SS
1
22k
R
SENSE
MODE
(BURST ENABLE)
8
10% CURRENT
SOFT-START
–
+
1.23V
+
–
ON
EA
C
SENSE
7
ON
10% CURRENT
V
OUT
SLEEP
READY
*
250k
I
TH
2
2V
R
C
1V
BLANKING
C
C
V
GND
4
IN
–
B
+
SW
5
1V
MODE
ON TRIGGER
*
OPTIONAL FOR FOLDBACK
L
1-SHOT
CURRENT LIMITING
3.5µs
STRETCH
V
OUT
V
FB
+
3
C
OUT
1771 BD
4
LTC1771
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OPERATIO
(Refer to Functional Block Diagram)
Main Control Loop
Burst Mode operation is provided by clamping the mini-
mum ITH voltage at 1V which represents about 25% of
maximumloadcurrent. Iftheloadfallsbelowthislevel, i.e.
the ITH voltage tries to fall below 1V, the burst comparator
B switches state signaling the LTC1771 to enter sleep
mode.Duringthistime, EAisreducedto10%ofitsnormal
operating current and the external compensation capaci-
tor is disconnected and clamped to 1V so that the EA can
driveitsoutputwiththeloweravailablecurrent.Astheload
discharges the output capacitor, the internal ITH voltage
increases. When it exceeds 1V the burst comparator exits
sleep mode, reconnects the external compensation com-
ponentstotheerroramplifieroutput, andreturnsEAtofull
power along with the other necessary circuitry. This
scheme (patent pending) allows the EA to be reduced to
such a low operating current during sleep mode without
adding unacceptable delay to wake up the LTC1771 due to
the compensation capacitor on ITH required for stability in
normal operation.
The LTC1771 uses a constant off-time, current mode
step-down architecture. During normal operation, the
P-channel MOSFET is turned on at the beginning of each
cycle and turned off when the current comparator C
triggers the 1-shot timer. The external MOSFET switch
stays off for the 3.5µs 1-shot duration and then turns back
on again to begin a new cycle. The peak inductor current
at which C triggers the 1-shot is controlled by the voltage
on Pin 3 (ITH), the output of the error amplifier EA. An
external resistive divider connected between VOUT and
ground allows EA to receive an output feedback voltage
VFB. When the load current increases, it causes a slight
decrease in VFB relative to the 1.23V reference, which in
turn causes the ITH voltage to increase until the average
inductor current matches the new load current.
The main control loop is shut down by pulling Pin 1
(RUN/SS) low. Releasing RUN/SS allows an internal 1µA
current source to charge soft-start capacitor CSS. When
CSS reaches 1V, the main control loop is enabled with the
ITH voltage clamped at approximately 40% of its maxi-
mum value. As CSS continues to charge, ITH is gradually
released allowing normal operation to resume.
Burst Mode operation can be disabled by pulling the
MODE pin to ground. In this mode of operation, the burst
comparator B is disabled and the ITH voltage allowed to go
allthewayto0V.Theloadcannowbereducedtoabout1%
of maximum load before the loop skips cycles to maintain
regulation. This mode provides a low noise output spec-
trum, useful for reducing both audio and RF interference,
at the expense of reduced efficiency at light loads.
Burst Mode Operation
The LTC1771 provides outstanding low current efficiency
and ultralow no-load supply current by using Burst Mode
operation when the MODE pin is pulled above 2V. During
BurstModeoperation,shortburstcyclesofnormalswitch-
ing are followed by a longer idle period with the switch off
and the load current is supplied by the output capacitor.
During this idle period, only the minimum required cir-
cuitry—1.23V reference and error amp—are left on, and
thesupplycurrentisreducedto9µA.Atnoload,theoutput
capacitor is still discharged very slowly by leakage current
in the Schottky diode and feedback resistor current result-
ing in very low frequency burst cycles that add a few more
microamps to the supply current.
Off-Time
The off-time duration is 3.5µs when the feedback voltage
is close to the reference voltage; however, as the feedback
voltage drops, the off-time lengthens and reaches a maxi-
mum value of about 70µs when VFB is zero. This ensures
that the inductor current has enough time to decay when
the reverse voltage across the inductor is low such as
during short circuit, thus protecting the MOSFET and
inductor.
5
LTC1771
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APPLICATIO S I FOR ATIO
The basic LTC1771 application circuit is shown in Figure
1 on the first page. External component selection is driven
by the load requirement and begins with the selection of
where tOFF = 3.5µs. However, the ripple current at low
loads during Burst Mode operation is:
∆IL(BURST) ≈ 35% of IPEAK ≈ 0.05/RSENSE
R
SENSE. Once RSENSE is known, L can be chosen. Next, the
For best efficiency when Burst Mode operation is enabled,
choose:
MOSFET and D1 are selected. The inductor is chosen
based largely on the desired amount of ripple current and
for Burst Mode operation. Finally CIN is selected for its
ability to handle the required RMS input current and COUT
is chosen with low enough ESR to meet the output voltage
ripple and transient specifications.
∆IL(CONT) ≤ ∆IL(BURST)
so that the inductor current is continuous during the burst
periods. This sets a minimum inductor value of:
L
MIN = (70µH)(VOUT + VD)(RSENSE)
R
SENSE Selection
Whenburstisdisabled,ripplecurrentslessthan∆IL(BURST)
can be achieved by choosing L > LMIN. Lower ripple
current reduces output voltage ripple and core losses, but
too low of ripple current will adversely effect efficiency.
RSENSE is chosen based on the required output current.
The LTC1771 current comparator has a maximum thresh-
old of 140mV/RSENSE. The current comparator threshold
sets the peak inductor current, yielding a maximum aver-
age output current IMAX equal to the peak less half the
peak-to-peak ripple current ∆IL. For best performance
when Burst Mode operation is enabled, choose ∆IL equal
to 35% of peak current. Allowing a margin for variations in
theLTC1771andexternalcomponentsgivesthefollowing
Inductor Core Selection
Once the value of 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,
molypermalloy or Kool Mµ® cores. Actual core loss is
independent of core size for a fixed inductor value, but is
very dependent on inductance selected. As inductance
increases, core losses go down. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses will increase.
equation for choosing RSENSE
:
RSENSE = 100mV/IMAX
At higher supply voltages, the peak currents may be
slightly higher due to overshoot from current comparator
delay and can be predicted from the second term in the
following equation:
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent increase in voltage ripple.
Do not allow the core to saturate!
1/2
0.14
RSENSE
V – VOUT
IN
IPEAK
+ 0.5
L(µH)
Inductor Value Selection
Once RSENSE is known, the inductor value can be deter-
mined. The inductance value has a direct effect on ripple
current. The ripple current decreases with higher induc-
tance and increases with higher VOUT. The ripple current
during continuous mode operation is set by the off-time
and inductance to be:
Molypermalloy (from Magnetics, Inc.) is a very good, low
losscorematerialfortoroids,butitismoreexpensivethan
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mµ. Toroids are space efficient, especially
when you can use several layers of wire. Because they
generally lack a bobbin, mounting is more difficult. How-
ever, designs for surface mount are available that do not
increase the height significantly.
V
OUT + VD
∆IL(CONT) = tOFF
L
Kool Mµ is a registered trademark of Magnetics, Inc.
6
LTC1771
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APPLICATIO S I FOR ATIO
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diode conducts most of the time. As VIN approaches VOUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short-circuited. Under this condition, the diode must
safely handle IPEAK at close to 100% duty cycle.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1771. The main selection criteria for
the power MOSFET are the threshold voltage VGS(TH) and
the “on” resistance RDS(ON), reverse transfer capacitance
and total gate charge.
To maximize both low and high current efficiencies, a fast
switching diode with low forward drop and low reverse
leakage should be used. Low reverse leakage current is
critical to maximize low current efficiency since the leak-
age can potentially exceed the magnitude of the LTC1771
supply current. Low forward drop is critical for high
current efficiency since loss is proportional to forward
drop. The effect of reverse leakage and forward drop on
no-loadsupplycurrentandefficiencyforvariousSchottky
diodes is shown in Table 1. As can be seen, these are
conflicting parameters and the user must weigh the
importance of each spec in choosing the best diode for the
application.
Since the LTC1771 can operate down to input voltages as
low as 2.8V, a sublogic level threshold MOSFET (RDS(ON)
guaranteed at VGS = 2.5V) is required for applications that
workclosetothisvoltage.WhentheseMOSFETsareused,
makesurethattheinputsupplytotheLTC1771islessthan
the absolute maximum VGS rating (typically 12V), as the
MOSFET gate will see the full supply voltage.
The required RDS(ON) of the MOSFET is governed by its
allowable power dissipation. For applications that may
operate the LTC1771 in dropout, i.e. 100% duty cycle, at
its worst case the required RDS(ON) is given by:
Table 1. Effect of Catch Diode on Performance
PP
RDS(ON)
=
LEAKAGE
NO-LOAD
EFFICIENCY
2
DIODE
(V = 3.3V) V @ 1A SUPPLY CURRENT AT 10V/1A
R
F
I
(
1+ δ
(
P
)
)
OUT(MAX)
MBR0540
UPS5817
0.25µA
0.50V
10.4µA
11.8µA
12.2µA
12.2µA
14.0µA
20.0µA
86.3%
88.2%
88.4%
87.9%
89.4%
89.8%
where PP is the allowable power dissipation and δP is the
temperature dependency of RDS(ON). (1 + δP) is generally
given for a MOSFET in the form of a normalized RDS(ON) vs
temperature curve, but = 0.005/°C can be used as an
approximation for low voltage MOSFETs.
2.8µA
3.7µA
4.4µA
8.3µA
19.7µA
0.41V
0.36V
0.43V
0.32V
0.29V
MBR0520
MBRS120T3
MBRM120LT3
MBRS320
In applications where the maximum duty cycle is less than
100%andtheLTC1771isincontinuousmode,theRDS(ON)
is governed by:
CIN and COUT Selection
At higher load currents, when the inductor current is
continuous, the source current of the P-channel MOSFET
is a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
capacitor current is given by:
PP
RDS(ON)
=
2
DC IOUT 1+ δP
(
)
(
)
V
OUT + VD
DC =
V + VD
IN
1/2
]
where DC is the maximum operating duty cycle of the
LTC1771.
IMAX
V
V − V
(
)
[
OUT IN OUT
CIN required IRMS
=
VIN
Catch Diode Selection
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monlyusedfordesignbecauseevensignificantdeviations
donotoffermuchrelief.Notethatcapacitormanufacturer’s
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
7
LTC1771
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APPLICATIO S I FOR ATIO
ripplecurrentratingsareoftenbasedon2000hoursoflife.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Do not underspecify this component. An addi-
tional 0.1µF ceramic capacitor is also helpful on VIN for
high frequency decoupling.
electrolyticsanddrytantalumcapacitorsarebothavailable
in surface mount configurations. In case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the
AVX TPS, AVX TPSV and KEMET T510 series of surface
mount tantalums, available in case heights ranging from
2mm to 4mm. Other capacitor types include Sanyo
OS-CON, Sanyo POSCAP, Nichicon PL series and
Panasonic SP.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple (∆VOUT) in continuous mode is approxi-
mated by:
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
oftenusefultoanalyzeindividuallossestodeterminewhat
is limiting efficiency and which change would produce the
most improvement. Efficiency can be expressed as:
1
∆VOUT ≈IRIPPLE ESR +
8fCOUT
where f is the operating frequency, COUT is the output
capacitance and IRIPPLE is the ripple current in the
inductor. For output ripple less than 100mV, assure COUT
Efficiency = 100% – (L1 + L2 +L3 + ...)
required ESR is <2RSENSE
.
whereL1, L2, etc. aretheindividuallossesasapercentage
of input power.
ThefirstconditionrelatestotheripplecurrentintotheESR
of the output capacitance while the second term guaran-
tees that the output capacitance does not significantly
discharge during the operating frequency period due to
ripple current. The choice of using smaller output capaci-
tance increases the ripple voltage due to the discharging
term but can be compensated for by using capacitors of
very low ESR to maintain the ripple voltage at or below
50mV. The ITH pin OPTI-LOOPTM compensation compo-
nents can be optimized to provide stable, high perfor-
mance transient response regardless of the output
capacitors selected.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in the LTC1771 circuits: the LTC1771 DC bias
current, MOSFET gate charge current, I2R losses and
catch diode losses.
1. The DC bias current is 9µA at no load and increases
proportionally with load up to a constant 150µA during
continuous mode. This bias current is so small that this
loss is negligible at loads above a milliamp but at no
load accounts for nearly all of the loss.
2. The MOSFET gate charge current results from switch-
ing the gate capacitance of the power MOSFET switch.
Each time the gate is switched from high to low to high
again, a packet of charge dQ moves from VIN to ground.
The resulting dQ/dt is the current out of VIN which is
typically much larger than the DC bias current. In con-
tinuousmode,IGATECHG =fQPwhereQPisthegatecharge
of the internal switch. Both the DC bias and gate charge
losses are proportional to VIN and thus their effects will
be more pronounced at higher supply voltages.
Manufacturers such as Nichicon, United Chemicon and
Sanyoshouldbeconsideredforhighperformancethrough-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR for its
size of any aluminum electrolytic at a somewhat higher
price. Typically once the ESR requirement is satisfied, the
RMS current rating generally far exceeds the IRIPPLE(P-P)
requirement.
In surface mount applications multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum
3. I2Rlossesarepredictedfromtheinternalswitch,induc-
tor and current sense resistor. In continuous mode the
averageoutputcurrentflowsthroughLbutis“chopped”
OPTI-LOOP is a trademark of Linear Technology Corporation.
8
LTC1771
W U U
APPLICATIO S I FOR ATIO
U
between the P-channel MOSFET in series with RSENSE
andtheoutputdiode.TheMOSFETRDS(ON) plusRSENSE
multiplied by the duty cycle can be summed with the
resistance of L to obtain I2R losses.
A 5pF feedforward capacitor across R2 is recommended
tominimizeoutputvoltagerippleinBurstModeoperation.
Run/Soft-Start Function
The RUN/SS pin is a dual purpose pin that provides the
soft-startfunctionandameanstoshutdowntheLTC1771.
Soft-start reduces the input surge current from VIN by
gradually increasing the internal current limit. Power
supply sequencing can also be accomplished using
this pin.
4. Thecatchdiodelossisproportionaltotheforwarddrop
asthediodeconductscurrentduringtheoff-timeandis
more pronounced at high supply voltages where the
off-time is long. However, as discussed in the Catch
Diode section, diodes with lower forward drops often
have higher leakage currents, so although efficiency is
improved, the no-load supply current will increase. The
diode loss is calculated by multiplying the forward
voltage drop times the diode duty cycle multiplied by
the load current.
An internal 1µA current source charges up an external
capacitor CSS. When the voltage on the RUN/SS reaches
1V, the LTC1771 begins operating. As the voltage on the
RUN/SS continues to ramp from 1V to 2.2V, the internal
current limit is also ramped at a proportional linear rate.
The current limits begins near 40% maximum load at
Other losses including CIN and COUT ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
VRUN/SS = 1V and ends at maximum load at VRUN/SS
=
2.2V. The output current thus ramps up slowly, reducing
the starting surge current required from the input power
supply. If the RUN/SS has been pulled all the way to
ground, there will be a delay before the current limit starts
increasing and is given by:
Output Voltage Programming
Theoutputvoltageisprogrammedwithanexternaldivider
from VOUT to VFB (Pin 1) as shown in Figure 2. The
regulated voltage is determined by:
tDELAY ≈ CSS/ICHG
R2
VOUT = 1.23 1+
R1
where ICHG 1µA. Pulling the RUN/SS pin below 0.5V
puts the LTC1771 into a low quiescent current shutdown
(IQ < 2µA).
To minimize no-load supply current, resistor values in the
megohm range should be used. The increase in supply
current due to the feedback resistors can be calculated
from:
Foldback Current Limiting
As described in the Catch Diode Selection, the worst-case
dissipation for diode occurs with a short-circuit output,
when the diode conducts the current limit value almost
continuously. In most applications this will not cause
excessive heating, even for extended fault intervals. How-
ever, when heat sinking is at a premium or higher forward
voltage drop diodes are being used, foldback current
limiting should be added to reduce the current in propor-
tion to the severity of the fault.
VOUT VOUT
R1+ R2 VIN
∆IVIN
=
V
OUT
C
FF
R2
5pF
V
FB
R1
LTC1771
Foldback current limiting is implemented by adding two
diodes in series between the output and the ITH pin as
GND
shown in the Functional Diagram. In a hard short (VOUT
=
1771 F02
0V) the current will be reduced to approximately 25% of
the maximum output current.
Figure 2. LTC1771 Adjustable Configuaration
9
LTC1771
W U U
U
APPLICATIO S I FOR ATIO
Minimum On-Time Considerations
1. Is the Schottky diode closely connected to the drain of
the external MOSFET and the input cap ground?
Minimum on-time tON(MIN) is the smallest amount of time
that the LTC1771 is capable of turning the top MOSFET on
andoffagain.Itisdeterminedbyinternaltimingdelaysand
the amount of gate charge required to turn on the
P-channel MOSFET. Low duty cycle applications may
approach this minimum on-time limit and care should be
taken to ensure that:
2. Is the 0.1µF input decoupling capacitor closely con-
nected between VIN (Pin 6) and ground (Pin 4)? This
capacitor carries the high frequency peak currents.
3. Does the VFB pin connect directly to the feedback
resistors? The resistive divider R1 and R2 must be
connected between the (+) plate of COUT and signal
ground. Locate the feedback resistors right next to the
LTC1771. TheVFB lineshouldnotberoutedclosetoany
nodes with high slew rates.
V
OUT + VD
t
ON = tOFF
> tON(MIN)
VIN− VOUT
where tOFF = 3.5µs and tON(MIN) is generally about 0.5µs
4. Is the 1000pF decoupling capacitor for the current
sense resistor connected as close as possible to Pins 6
and 7? Ensure accurate current sensing with Kelvin
connections to the sense resistor.
for the LTC1771.
Ifthedutycyclefallsbelowwhatcanbeaccommodatedby
the minimum on-time, the LTC1771 will remain in Burst
Mode operation even at high load currents. The output
voltage will continue to be regulated, but the ripple current
and ripple voltage will increase.
5. Is the (+) plate of CIN closely connected to the sense
resistor ? This capacitor provides the AC current to the
MOSFET.
6. Are the signal and power grounds segregated? The
signal ground consists of the (–) plate of COUT, Pin 4 of
theLTC1771andtheresistivedivider.Thepowerground
consists of the Schottky diode anode and the (–) plate
of CIN which should have as short lead lengths as
possible.
Mode Pin
Burst Mode operation is disabled by pulling MODE (Pin 8)
below 0.5V. Disabling Burst Mode operation provides a
lownoiseoutputspectrum, usefulforreducingbothaudio
and RF interference. It does this by keeping the frequency
constant (for fixed VIN) down to much lower load current
(1% to 2% of IMAX) and reducing the amount of output
voltage and current ripple at light loads. When Burst Mode
operation is disabled, efficiency is reduced at light loads
and no load supply current increases to 175µA.
7. Keep the switching node (SW) and the gate node
(PGATE) away from sensitive small signal nodes, espe-
cially the voltage sensing feedback pin (VFB), and mini-
mize their PC trace area.
8. High impedance nodes such as ITH and VFB are very
sensitive to leakage paths on the PC board due to stray
flux, solder, epoxy, etc. Make sure PC board is clean.
Water-soluble solder flux can be especially leaky if not
cleaned properly. Leakage on ITH will manifest itself as
excessive output ripple during Burst Mode operation. If
the problem persists, adding a 10M resistor from Pin 2
to ground should eliminate the problem.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1771. These items are also illustrated graphically in
the layout diagram of Figure 3. Check the following in your
layout:
10
LTC1771
W U U
U
APPLICATIO S I FOR ATIO
Design Example
0.25W
P -Channel RDS(ON)
=
2
As a design example, assume VIN = 10V (nominal), VIN =
15V(MAX), VOUT = 3.3V, and IMAX = 2A. With this informa-
tion, wecaneasilycalculatealltheimportantcomponents.
3.3V + 0.5V
10V + 0.5V
2A 1.33
( ) (
)
= 0.130Ω
RSENSE = 100mV/2A = 0.05Ω
SincethegateoftheMOSFETwillseethefullinputvoltage,
a MOSFET must be selected whose VGS(MAX) > 15V. A
P-channel MOSFET that meets both the VGS(MAX) and
RDS(ON) requirement is the Si6447DQ.
To optimize low current efficiency, MODE pin is tied to VIN
to enable Burst Mode operation, thus the minimum induc-
tance necessary is:
LMIN = 70µH(3.3V + 0.5)(0.05Ω) = 13.3µH
15µH is chosen for the application.
The most stringent requirement for the Schottky diode
occurs when VOUT = 0V (i.e., short circuit) at maximum
VIN. In this case the worst-case dissipation rises to:
3.3V + 0.5V
∆IL = 3.5µs
= 0.89A
15µH
VIN
V + VD
IN
PD = ISC(AVG)
V
( )
D
For the feedback resistors, choose R1 = 1M to minimize
supply current. R2 can then be calculated to be:
With a 0.05Ω sense resistor ISC(AVG) = 2A will result,
increasing the 0.5V Schottky diode dissipation to 1W.
R2 = (VOUT/1.23 – 1) • R1 = 1.68M
Assume that the MOSFET dissipation is to be limited to
PP = 0.25W.
C
IN is chosen for a RMS current rating of at least 1A at
temperature. COUT is chosen with an ESR of 0.05Ωfor low
output ripple. The output voltage ripple due to ESR is
approximately:
If TA = 70°C and the thermal resistance of the MOSFET is
83°C/W, then the junction temperatures will be 91°C and
δP = 0.33. The required RDS(ON) for the MOSFET can now
be calculated:
V
ORIPPLE ≈ (RESR)(∆IL) = 0.05Ω (0.89AP-P) = 45mVP-P
C
SS
1
2
3
4
8
7
6
5
RUN/SS
MODE
MODE
SENSE
C
ITH
R
ITH
I
TH
LTC1771
R1
V
V
FB
IN
Q1
L
+
+
C
GND
PGATE
D1
IN
R2
C
C
FF
0.1µF
OUT
V
OUT
1771 F03
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 3. LTC1771 Layout Diagram
11
LTC1771
TYPICAL APPLICATIO S
U
3.3V to 2.5V/1A Regulator with Burst Mode Operation Enabled
0.01µF
1
2
3
4
8
7
6
5
RUN/SS
MODE
SENSE
220pF
10k
I
TH
1000pF
LTC1771
V
FB
V
IN
V
IN
3.3V
GND
PGATE
R
+
+
SENSE
C
TO 12V
IN
0.1Ω
22µF
Si3443DV
25V
22µH
1M
1%
1.02M
1%
V
2.5V
1A
OUT
C
OUT
5pF
UPS5817
150µF
6.3V
1771 TA01
5V/2A Regulator with Burst Mode Operation Disabled
0.01µF
1
8
7
6
5
RUN/SS
MODE
SENSE
220pF
10k
2
I
TH
1000pF
LTC1771
3
4
V
V
FB
IN
V
IN
5.5V
GND
PGATE
+
+
C
R
TO 18V
IN
SENSE
22µF
0.05Ω
Si6447DQ
25V
22µH
1M
1%
3.09M
1%
V
5V
2A
OUT
C
OUT
5pF
UPS5817
150µF
6.3V
1771 TA04
12
LTC1771
U
TYPICAL APPLICATIO S
Low Dropout Single Cell Lithium-Ion to 3V
0.01µF
1
2
3
4
8
MODE
MODE
RUN/SS
220pF
10k
7
I
SENSE
TH
1000pF
LTC1771
6
5
V
V
FB
IN
GND
PGATE
+
+
C
R
IN
Li-Ion
SENSE
22µF
0.05Ω
3.3V TO 4.2V
Si3443DV
UPS5817
25V
15µH
1M
1%
1.43M
1%
V
3V
2A
OUT
C
OUT
5pF
150µF
6.3V
1771 TA02
12V/1A Zeta Converter
0.01µF
1
2
3
4
8
RUN/SS
MODE
MODE
220pF
10k
7
6
5
I
SENSE
TH
1000pF
LTC1771
V
V
FB
IN
V
IN
5V
GND
PGATE
+
+
C
TO 18V
R
IN
SENSE
22µF
0.025Ω
Si6435DQ
25V
1M
1%
8.66M
1%
47µH
V
12V
1A
5pF
OUT
•
•
C
OUT
22µF
20V
UPS5817
150µF
47µH
20V
1771 TA05
13
LTC1771
U
TYPICAL APPLICATIONS
2.5V/1A Regulator with Foldback Current Limit
0.01µF
1
RUN/SS
8
MODE
MODE
SENSE
220pF
10k
2
7
6
5
I
TH
1000pF
LTC1771
3
4
V
V
FB
IN
V
IN
2.8V
GND
PGATE
+
C
R
TO 12V
IN
SENSE
22µF
0.1Ω
25V
1M
1%
1.02M
1%
4
5
3
6
2
1
8
I
TH
5pF
U1
1N4148
×2
22µH
V
2.5V
1A
OUT
7
U1: INTERNATIONAL RECTIFIER
FETKY TM IRF7422D2
+
C
OUT
150µF
6.3V
1771 TA06
4-NiCd Battery Charger
0.01µF
1
RUN/SS
8
MODE
MODE
SENSE
220pF
10k
2
7
I
TH
1000pF
6
LTC1771
3
4
V
V
FB
IN
V
IN
5
8V
GND
PGATE
R
+
SENSE
C
TO 18V
IN
0.1Ω
22µF
Si6447DQ
25V
UPS5817
47µH
1M
1%
4.69M
1%
V
OUT
4-NiCd
1A
+
C
OUT
5pF
UPS5817
100µF
10V
1771 TA07
14
LTC1771
U
PACKAGE DESCRIPTIO
Dimension in inches (millimeters) unless otherwise noted.
MS8 Package
8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.118 ± 0.004*
(3.00 ± 0.102)
8
7
6
5
0.118 ± 0.004**
(3.00 ± 0.102)
0.193 ± 0.006
(4.90 ± 0.15)
1
0.040 ± 0.006
2
3
4
0.034 ± 0.004
(0.86 ± 0.102)
(1.02 ± 0.15)
0.007
(0.18)
0° – 6° TYP
SEATING
PLANE
0.012
(0.30)
REF
0.021 ± 0.006
(0.53 ± 0.015)
0.006 ± 0.004
(0.15 ± 0.102)
0.0256
(0.65)
BSC
MSOP (MS8) 1098
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,
PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
7
5
8
6
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
3
4
2
0.010 – 0.020
(0.254 – 0.508)
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0°– 8° TYP
0.016 – 0.050
(0.406 – 1.270)
0.050
(1.270)
BSC
0.014 – 0.019
(0.355 – 0.483)
TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
SO8 1298
15
LTC1771
U
TYPICAL APPLICATIO
5V/1A Zeta Converter
V
(V)
2.8
3.3
5
7.5
10
12
I
(A)
IN
LOAD(MAX)
0.8
0.01µF
1.1
1
2
3
4
8
1.7
MODE
RUN/SS
MODE
220pF
2.3
10k
7
6
5
2.7
I
SENSE
LTC1771
TH
2.9
1000pF
V
V
FB
IN
V
IN
2.8V
GND
PGATE
+
C
TO 12V
R
IN
SENSE
22µF
0.025Ω
Si3443DV
25V
1M
1%
3.09M
1%
22µH
V
5V
1A
5pF
OUT
•
•
+
C
OUT
22µF
10V
UPS5817
150µF
22µH
6.3V
1771 TA03
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
100% DC, 3.5V ≤ V ≤ 16V
LTC1147 Series
High Efficiency Step-Down Switching Regulator Controllers
High Efficiency Step-Down and Inverting DC/DC Converters
1.2A High Efficiency Step-Down DC/DC Converter
Low Quiscent Current Step-Down Regulators
IN
LTC1174/LTC1174-3.3/LTC1174-5
LTC1265
Selectable I
= 300mA or 600mA
PEAK
Burst Mode Operation, Internal MOSFET
LTC1474/LTC1475
Monolithic, I = 10µA, 400mA, MS8
Q
LTC1574/LTC1574-3.3/LTC1574-5
High Efficiency Step-Down DC/DC Converters
with Internal Schottky Diode
LTC1174 with Internal Schottky Diode
LTC1622
Low Input Voltage Step-Down DC/DC Controller
Constant Frequency, 2V to 10V V , MS8
IN
LTC1624
High Efficiency SO-8 N-Channel Switching Regulator Controller
100mA, Low Noise, LDO Micropower Regulators in SOT-23
500mA, Low Noise, LDO Micropower Regulators
95% DC, 3.5V to 36V V
IN
LT®1761 Series
LT1763 Series
LTC1772
20µA Quiescent Current, 20µV
30µA Quiescent Current, 20µV
Noise
Noise
RMS
RMS
Constant Frequency Step-Down DC/DC Controller
SOT-23, 2.2V to 9.8V V
IN
1771i LT/TP 0200 4K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 2000
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