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PDF LTC1148-3.3 Data sheet ( Hoja de datos )
| Número de pieza | LTC1148-3.3 | |
| Descripción | High Efficiency Synchronous Step-Down Switching Regulators | |
| Fabricantes | Linear Technology | |
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LTC1148
LTC1148-3.3/LTC1148-5
High Efficiency Synchronous
Step-Down Switching Regulators
FEATURES
s Ultrahigh Efficiency: Over 95% Possible
s Current-Mode Operation for Excellent Line and Load
Transient Response
s High Efficiency Maintained Over Three Decades of
Output Current
s Low 160µA Standby Current at Light Loads
s Logic Controlled Micropower Shutdown: IQ < 20µA
s Wide VIN Range: 3.5V* to 20V
s Short-Circuit Protection
s Very Low Dropout Operation: 100% Duty Cycle
s Synchronous FET Switching for High Efficiency
s Adaptive Nonoverlap Gate Drives
s Output Can Be Externally Held High in Shutdown
s Available in 14-Pin Narrow SO Package
APPLICATI S
s Notebook and Palmtop Computers
s Portable Instruments
s Battery-Operated Digital Devices
s Cellular Telephones
s DC Power Distribution Systems
s GPS Systems
DESCRIPTIO
The LTC®1148 series is a family of synchronous step-
down switching regulator controllers featuring automatic
Burst ModeTM operation to maintain high efficiencies at
low output currents. These devices drive external comple-
mentary power MOSFETs at switching frequencies up to
250kHz using a constant off-time current-mode architec-
ture providing constant ripple current in the inductor.
The operating current level is user-programmable via an
external current sense resistor. Wide input supply range
allows operation from 3.5V* to 18V (20V maximum).
Constant off-time architecture provides low dropout regu-
lation limited by only the RDS(ON) of the external MOSFET
and resistance of the inductor and current sense resistor.
The LTC1148 series combines synchronous switching for
maximum efficiency at high currents with an automatic low
current operating mode, called Burst Modeoperation, which
reduces switching losses. Standby power is reduced to only
2mW at VIN = 10V (at IOUT = 0). Load currents in Burst Mode
operation are typically 0mA to 300mA.
For operation up to 48V input, see the LTC1149 and
LTC1159 data sheets and Application Note 54.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
* LTC1148L and LTC1148L-3.3 only.
TYPICAL APPLICATI
+
1µF
0V = NORMAL
>1.5V = SHUTDOWN
RC
1k
CC
3300pF
CT
470pF
VIN (5.2V TO 18V)
VIN
P-DRIVE
LTC1148HV-5
SHUTDOWN
SENSE +
ITH
CT
SENSE –
N-DRIVE
SGND PGND
1000pF
P-CHANNEL
Si4431DY
N-CHANNEL
Si4412DY
+ CIN
100µF
L*
62µH
RSENSE**
0.05Ω
VOUT
5V/2A
D1
MBRS140T3
+ COUT
390µF
LT1148 • TA01
*COILTRONICS CTX62-2-MP
**KRL SL-1-C1-0R050J
Figure 1. High Efficiency Step-Down Converter
LTC1148-5 Efficiency
100
VIN = 6V
95 VIN = 10V
90
85
80
0.02
0.2
LOAD CURRENT (A)
2
LTC1148 • TA01
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1148
LTC1148-3.3/LTC1148-5
Gate Charge Supply Current
28
24
20 QN + QP = 100nC
16
12
8 QN + QP = 50nC
4
0
20 80 140 200 260
OPERATING FREQUENCY (kHz)
LTC1148 • TPC07
Off Time vs VOUT
80
70
VSENSE– = VOUT
60
50
40
30
20
10
LTC1148-3.3
0
012
LTC1148-5
34
OUTPUT VOLTAGE (V)
5
LTC1148 • TPC08
Current Sense Threshold Voltage
175
MAXIMUM
150 THRESHOLD
125
100
75
50
MINIMUM
THRESHOLD
25
0
0 20 40 60 80 100
TEMPERATURE (°C)
LTC1148 • TPC09
PI FU CTIO S
P-DRIVE (Pin 1): High Current Drive for Top P-Channel
MOSFET. Voltage swing at this pin is from VIN to ground.
NC (Pin 2): No Connection. Can connect to power ground.
VIN (Pin 3): Main Supply Pin. Must be closely decoupled
to power ground Pin 12.
CT (Pin 4): External capacitor CT from Pin 4 to ground sets
the operating frequency. The actual frequency is also
dependent upon the input voltage.
INTVCC (Pin 5): Internal Supply Voltage, Nominally 3.3V.
Can be decoupled to signal ground. Do not externally load
this pin.
ITH (Pin 6): Gain Amplifier Decoupling Point. The current
comparator threshold increases with the Pin 6 voltage.
SENSE – (Pin 7): Connects to internal resistive divider
which sets the output voltage in LTC1148-3.3 and
LTC1148-5 versions. Pin 7 is also the (–) input for the
current comparator.
SENSE + (Pin 8): The (+) Input to the Current Comparator.
A built-in offset between Pins 7 and 8 in conjunction with
RSENSE sets the current trip threshold.
VFB (Pin 9): For the LTC1148 adjustable version, Pin 9
serves as the feedback pin from an external resistive
divider used to set the output voltage. On LTC1148-3.3
and LTC1148-5 versions this pin is not used.
SHUTDOWN (Pin 10): When grounded, the LTC1148
series operates normally. Pulling Pin 10 high holds both
MOSFETs off and puts the LTC1148 series in micropower
shutdown mode. Requires CMOS logic signal with tR,
tF < 1µs, should not be left floating.
SGND (Pin 11): Small-Signal Ground. Must be routed
separately from other grounds to the (–) terminal of COUT.
PGND (Pin 12): Driver Power Ground. Connects to source
of N-channel MOSFET and the (–) terminal of CIN.
NC (Pin 13): No Connection. Can connect to power ground.
N-DRIVE (Pin 14): High Current Drive for Bottom
N-Channel MOSFET. Voltage swing at Pin 14 is from
ground to VIN.
5
5 Page 
UU W U
APPLICATIO S I FOR ATIO
In surface mount applications multiple capacitors may
have to be paralleled to meet the capacitance, ESR, or
RMS current handling requirements of the application.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the
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 series of surface mount tantalums,
available in case heights ranging from 2mm to 4mm. For
example, if 200µF/10V is called for in an application
requiring 3mm height, two AVX 100µF/10V (P/N TPSD
107K010) could be used. Consult the manufacturer for
other specific recommendations.
At low supply voltages, a minimum capacitance at COUT
is needed to prevent an abnormal low frequency oper-
ating mode (see Figure 4). When COUT is made too
small, the output ripple at low frequencies will be large
enough to trip the voltage comparator. This causes
Burst Modeoperation to be activated when the LTC1148
series would normally be in continuous operation. The
effect is most pronounced with low values of RSENSE
and can be improved by operating at higher frequencies
with lower values of L. The output remains in regulation
at all times.
1000
800
600
400
L = 50µH
RSENSE = 0.02Ω
L = 25µH
RSENSE = 0.02Ω
200
0
0
L = 50µH
RSENSE = 0.05Ω
1 2345
(VIN – VOUT) VOLTAGE (V)
LTC1148 • F04
Figure 4. Minimum Value of COUT
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
LTC1148
LTC1148-3.3/LTC1148-5
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, VOUT shifts by an
amount equal to ∆ILOAD • ESR, where ESR is the effective
series resistance of COUT. ∆ILOAD also begins to charge
or discharge COUT until the regulator loop adapts to the
current change and returns VOUT to its steady state
value. During this recovery time VOUT can be monitored
for overshoot or ringing which would indicate a stability
problem. The Pin 6 external components shown in the
Figure 1 circuit will prove adequate compensation for
most applications.
A second, more severe transient is caused by switching in
loads with large (>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 would require a 250µs rise time,
limiting the charging current to about 200mA.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc., are the individual losses as a percent-
age of input power. (For high efficiency circuits only small
errors are incurred by expressing losses as a percentage
of output power).
Although all dissipative elements in the circuit produce
losses, three main sources usually account for most of the
losses in LTC1148 series circuits: 1) LTC1148 DC bias
current, 2) MOSFET gate charge current, and 3) I2R
losses.
1. The DC supply current is the current which flows into
VIN Pin 3 less the gate charge current. For VIN = 10V the
11
11 Page | ||
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| PDF Descargar | [ Datasheet LTC1148-3.3.PDF ] | |
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