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PDF LTC1625C Data sheet ( Hoja de datos )
| Número de pieza | LTC1625C | |
| Descripción | No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1625
No RSENSETM Current Mode
Synchronous Step-Down
Switching Regulator
FEATURES
s Highest Efficiency Current Mode Controller
s No Sense Resistor Required
s Stable High Current Operation
s Dual N-Channel MOSFET Synchronous Drive
s Wide VIN Range: 3.7V 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
APPLICATIONS
s Notebook and Palmtop Computers, PDAs
s Cellular Telephones and Wireless Modems
s Battery Chargers
s Distributed Power
DESCRIPTION
The LTC®1625 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. The frequency of
a nominal 150kHz internal oscillator can be synchronized
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 3.7V (3.9V for LTC1625I) 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
CSS
0.1µF
RC
10k
CC
2.2nF
SYNC
RUN/SS
VIN
TK
TG
LTC1625
ITH SW
BOOST
VPROG INTVCC
SGND
BG
VOSENSE PGND
CB 0.22µF
DB
CMDSH-3
+
CVCC 4.7µF
M1
Si4410DY L1
10µH
D1
MBRS140T3
M2
Si4410DY
+
CIN
10µF
VIN
5V TO
28V
30V
×2
VOUT
3.3V
+ COUT 4.5A
100µF
10V
×3
1625 F01
Figure 1. High Efficiency Step-Down Converter
Efficiency vs Load Current
100
VIN = 10V
VOUT = 5V
90
VOUT = 3.3V
80
70
60
0.01
0.1 1
LOAD CURRENT (A)
10
1625 TA01
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1625
Maximum Current Sense Voltage
vs Duty Cycle
200
150
100
50
0
0 0.2 0.4 0.5 0.8 1.0
DUTY CYCLE
1625 G10
FCB Pin Current vs Temperature
0
– 0.25
– 0.50
– 0.75
– 1.00
– 1.25
– 1.50
– 40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1625 G13
Transient Response
Maximum Current Sense Voltage
vs Temperature
160
Oscillator Frequency
vs Temperature
300
250
SYNC = 1.5V
155
200
SYNC = 0V
150 150
100
145
50
140
– 40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1625 G11
0
– 40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1625 G12
RUN/SS Pin Current
vs Temperature
0
Soft Start:
Load Current vs Time
INDUCTOR
– 1 CURRENT
2A/DIV
–2
–3
–4
–5
–40 –15
10 35 60 85
TEMPERATURE (°C)
110 135
1625 G14
RUN/SS
2V/DIV
20ms/DIV
VIN = 20V
VOUT = 5V
RLOAD = 1Ω
FIGURE 1 CIRCUIT
Transient Response
(Burst Mode Operation)
Burst Mode Operation
1625 F06
VOUT
50mV/DIV
VIN = 20V
200µs/DIV
VOUT = 5V
ILOAD = 1A TO 4A
FIGURE 1 CIRCUIT
VOUT
50mV/DIV
1625 F07
VIN = 20V
500µs/DIV
VOUT = 5V
ILOAD = 50mA TO 1A
FIGURE 1 CIRCUIT
VOUT
50mV/DIV
ITH
100mV/DIV
1625 F08
VIN = 20V
50µs/DIV
VOUT = 5V
ILOAD = 50mA
FIGURE 1 CIRCUIT
1625 F09
5
5 Page 
LTC1625
APPLICATIONS INFORMATION
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 LTC1625 current comparator can accommodate a
maximum sense voltage of 150mV. This voltage and the
sense resistance determine the maximum allowed peak
inductor current. The corresponding output current limit
is:
( )( )ILIMIT
=
150mV
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 LTC1625 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 150mV to
30mV. Under short-circuit conditions with very low duty
cycle, the LTC1625 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 trough
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 LTC1625
(approximately 0.5µs), the input voltage, and inductor
value:
∆IL(SC) = tON(MIN) VIN/L.
The resulting short-circuit current is:
( )( )ISC =
30mV
RDS(ON)(BOT)
ρT
+
1
2
∆ IL(SC)
Normally, the top and bottom MOSFETs will be of the same
type. A bottom MOSFET with lower RDS(ON) than the top
may be chosen if the resulting increase in short-circuit
current is tolerable. However, the bottom MOSFET should
never be chosen to have a higher nominal RDS(ON) than the
top MOSFET.
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, molypermalloy
or Kool Mµ® cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
the inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Kool Mµ is a registered trademark of Magnetics, Inc.
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC1625C.PDF ] | |
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