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PDF LTC1775 Data sheet ( Hoja de datos )
| Número de pieza | LTC1775 | |
| Descripción | High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC1775 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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FEATURES
s Highest Efficiency Current Mode Controller
s No Sense Resistor Required
s 300mV Maximum Current Sense Voltage
s Stable High Current Operation
s Dual N-Channel MOSFET Synchronous Drive
s Wide VIN Range: 4V to 36V
s Wide VOUT Range: 1.19V to VIN
s ±1% 1.19V Reference
s Programmable Fixed Frequency with Injection Lock
s Very Low Drop Out Operation: 99% Duty Cycle
s Forced Continuous Mode Control Pin
s Optional Programmable Soft Start
s Pin Selectable Output Voltage
s Foldback Current Limit
s Output Overvoltage Protection
s Logic Controlled Micropower Shutdown: IQ < 30µA
s Available in 16-Lead Narrow SSOP and SO Packages
U
APPLICATIO S
s Notebook Computers
s Automotive Electronics
s Battery Chargers
s Distributed Power Systems
LTC1775
High Power
No RSENSETM Current Mode
Synchronous Step-Down
Switching Regulator
DESCRIPTIO
The LTC®1775 is a synchronous step-down switching
regulator controller that drives external N-channel power
MOSFETs using few external components. Current mode
control with MOSFET VDS sensing eliminates the need for
a sense resistor and improves efficiency. Largely similar
to the LTC1625, the LTC1775 has twice the maximum
sense voltage for high current applications. The frequency
of a nominal 150kHz internal oscillator can be synchro-
nized to an external clock over a 1.5:1 frequency range.
Burst ModeTM operation at low load currents reduces
switching losses and low dropout operation extends oper-
ating time in battery-powered systems. A forced continu-
ous mode control pin can assist secondary winding
regulation by disabling Burst Mode operation when the
main output is lightly loaded.
Fault protection is provided by foldback current limiting
and an output overvoltage comparator. An external ca-
pacitor attached to the RUN/SS pin provides soft start
capability for supply sequencing. A wide supply range
allows operation from 4V (4.3V for LTC1775I) to 36V at the
input and 1.19V to VIN at the output.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No RSENSE and Burst Mode are trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
CC
2.2nF
CSS 0.1µF
LTC1775
SYNC
VIN
TK
RUN/SS TG
VPROG
SW
BOOST
RC
10k
ITH INTVCC
SGND
BG +
VOSENSE PGND
CB 0.33µF
DB
CMDSH-3
CVCC
4.7µF
+
M1
SUD50N03-10 L1
6µH
D1
MBRS340
M2
SUD50N03-10
CIN
22µF
VIN
5V TO
28V
30V
×4
VOUT
3.3V
+ COUT 10A
680µF
6.3V
1775 F01
Figure 1. High Efficiency Step-Down Converter
Efficiency vs Load Current
100
VIN = 10V
f = 150kHz
95 FCB = INTVCC
VOUT = 5V
90 VOUT = 3.3V
85
80
75
70
0.01
0.1 1
LOAD CURRENT (A)
10
1775 F01b
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1775
Maximum Current Sense Voltage
vs Duty Cycle
350
300
250
200
150
100
50
0
0 0.2 0.4 0.6 0.8 1.0
DUTY CYCLE
1775 • G10
FCB Pin Current vs Temperature
0
– 0.5
– 1.0
– 1.5
– 2.0
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1775 • G13
Transient Response
Maximum Current Sense Voltage
vs Temperature
320
Oscillator Frequency
vs Temperature
300
250
310 SYNC = 1.5V
200
SYNC = 0V
300 150
100
290
50
280
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1775 • G11
RUN/SS Pin Current vs
Temperature
0
0
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1775 • G12
Soft Start
RUN/SS
–1 5V/DIV
–2 VOUT
5V/DIV
–3
IL
–4 5A/DIV
–5
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1775 • G14
Transient Response
(Burst Mode Operation)
VIN = 20V
20ms/DIV
VOUT = 5V
RLOAD = 0.5Ω
FIGURE 1 CIRCUIT
Burst Mode Operation
1530 G16
VOUT
100mV
/DIV
IL
5A/DIV
VIN = 20V
100µs/DIV
VOUT = 5V
ILOAD = 2A TO 8A
FIGURE 1 CIRCUIT
VOUT
100mV
/DIV
1530 G18
IL
5A/DIV
VIN = 20V
200µs/DIV
VOUT = 5V
ILOAD = 100mA TO 2A
FIGURE 1 CIRCUIT
VOUT
50mV
/DIV
ITH
100mV
/DIV
1530 G17
IL
2A/DIV
VIN = 20V
20µs/DIV
VOUT = 5V
ILOAD = 100mA
FIGURE 1 CIRCUIT
1530 G15
5
5 Page 
LTC1775
APPLICATIO S I FOR ATIO
A reasonable starting point is to choose a ripple current
that is about 40% of IO(MAX). Note that the largest ripple
current occurs at the highest VIN. To guarantee that ripple
current does not exceed a specified maximum, the induc-
tor should be chosen according to:
L
≥
VOUT
(f)(∆IL(MAX))
1–
VOUT
VIN(MAX)
Burst Mode Operation Considerations
The choice of RDS(ON) and inductor value also determines
the load current at which the LTC1775 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
IBURST(PEAK)
=
6 0mV
RDS(ON)
The corresponding average current depends on the amount
of ripple current. Lower inductor values (higher ∆IL) will
reduce the load current at which Burst Mode operation
begins.
The output voltage ripple can increase during Burst Mode
operation if ∆IL is substantially less than IBURST. This will
primarily occur when the duty cycle is very close to unity
(VIN is close to VOUT) or if very large value inductors are
chosen. This is generally only a concern in applications
with VOUT ≥ 5V. At high duty cycles, a skipped cycle
causes the inductor current to quickly descend to zero.
However, it takes multiple cycles to ramp the current back
up to IBURST(PEAK). During this interval, the output capaci-
tor must supply the load current and enough charge may
be lost to cause significant droop in the output voltage. It
is a good idea to keep ∆IL comparable to IBURST(PEAK).
Otherwise, one might need to increase the output capaci-
tance in order to reduce the voltage ripple or else disable
Burst Mode operation by forcing continuous operation
with the FCB pin.
Fault Conditions: Current Limit and Output Shorts
The LTC1775 current comparator can accommodate a
maximum sense voltage of 300mV. This voltage and the
sense resistance determine the maximum allowed peak
inductor current. The corresponding output current limit
is:
( )( )ILIMIT =
300mV
RDS(ON) ρT
–
1
2
∆IL
The current limit value should be checked to ensure that
ILIMIT(MIN) > IO(MAX). The minimum value of current limit
generally occurs with the largest VIN at the highest ambi-
ent temperature, conditions which cause the highest power
dissipation in the top MOSFET. Note that it is important to
check for self-consistency between the assumed junction
temperature of the top MOSFET and the resulting value of
ILIMIT which heats the junction.
Caution should be used when setting the current limit
based upon RDS(ON) of the MOSFETs. The maximum
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for RDS(ON), but not a minimum. A
reasonable, but perhaps overly conservative, assumption
is that the minimum RDS(ON) lies the same amount below
the typical value as the maximum RDS(ON) lies above it.
Consult the MOSFET manufacturer for further guidelines.
The LTC1775 includes current foldback to help further
limit load current when the output is shorted to ground. If
the output falls by more than half, then the maximum
sense voltage is progressively lowered from 300mV to
about 80mV. Under short-circuit conditions with very low
duty cycle, the LTC1775 will begin skipping cycles in order
to limit the short-circuit current. In this situation the
bottom MOSFET RDS(ON) will control the inductor current
valley rather than the top MOSFET controlling the inductor
current peak. The short-circuit ripple current is deter-
mined by the minimum on-time tON(MIN) of the LTC1775
(approximately 0.5µs), the input voltage, and inductor
value:
∆IL(SC) = tON(MIN) VIN/L.
The resulting short-circuit current is:
( )( )ISC =
80mV
RDS(ON)(BOT)
ρT
+
1
2
∆ IL(SC)
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC1775.PDF ] | |
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