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PDF LTC1624IS Data sheet ( Hoja de datos )
| Número de pieza | LTC1624IS | |
| Descripción | High Efficiency SO-8 N-Channel Switching Regulator Controller | |
| Fabricantes | Linear Technology | |
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FEATURES
s N-Channel MOSFET Drive
s Implements Boost, Step-Down, SEPIC
and Inverting Regulators
s Wide VIN Range: 3.5V to 36V Operation
s Wide VOUT Range: 1.19V to 30V in Step-Down
Configuration
s ±1% 1.19V Reference
s Low Dropout Operation: 95% Duty Cycle
s 200kHz Fixed Frequency
s Low Standby Current
s Very High Efficiency
s Remote Output Voltage Sense
s Logic-Controlled Micropower Shutdown
s Internal Diode for Bootstrapped Gate Drive
s Current Mode Operation for Excellent Line and
Load Transient Response
s Available in an 8-Lead SO Package
U
APPLICATIONS
s Notebook and Palmtop Computers, PDAs
s Cellular Telephones and Wireless Modems
s Battery-Operated Digital Devices
s DC Power Distribution Systems
s Battery Chargers
LTC1624
High Efficiency SO-8
N-Channel Switching
Regulator Controller
DESCRIPTION
The LTC®1624 is a current mode switching regulator
controller that drives an external N-channel power MOSFET
using a fixed frequency architecture. It can be operated in
all standard switching configurations including boost,
step-down, inverting and SEPIC. Burst ModeTM operation
provides high efficiency at low load currents. A maximum
high duty cycle limit of 95% provides low dropout operation
which extends operating time in battery-operated systems.
The operating frequency is internally set to 200kHz, allowing
small inductor values and minimizing PC board space. The
operating current level is user-programmable via an external
current sense resistor. Wide input supply range allows
operation from 3.5V to 36V (absolute maximum).
A multifunction pin (ITH/RUN) allows external
compensation for optimum load step response plus
shutdown. Soft start can also be implemented with the
ITH/RUN pin to properly sequence supplies.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
CC
470pF
RC
6.8k
SENSE–
VIN
ITH/RUN BOOST
LTC1624
VFB
100pF
TG
GND SW
VIN
4.8V TO 28V
1000pF
M1
Si4412DY
RSENSE
0.05Ω
+ CIN
22µF
35V
×2
CB
0.1µF
D1
MBRS340T3
L1
10µH
VOUT
3.3V
2A
R2
35.7k
+ COUT
100µF
R1 10V
20k × 2
Figure 1. High Efficiency Step-Down Converter
1624 F01
1
1 page  
LTC1624
PIN FUNCTIONS
TG (Pin 6): High Current Gate Drive for Top N-Channel
MOSFET. This is the output of a floating driver with a
voltage swing equal to INTVCC superimposed on the
switch node voltage SW.
BOOST (Pin 7): Supply to Topside Floating Driver. The
bootstrap capacitor CB is returned to this pin. Voltage
swing at this pin is from INTVCC to VIN + INTVCC in step-
down applications. In non step-down topologies the volt-
age at this pin is constant and equal to INTVCC if SW = 0V.
VIN (Pin 8): Main Supply Pin and the (+) Input to the
Current Comparator. Must be closely decoupled to ground.
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC1624 uses a constant frequency, current mode
architecture. During normal operation, the top MOSFET is
turned on each cycle when the oscillator sets the RS latch
and turned off when the main current comparator I1 resets
the RS latch. The peak inductor current at which I1 resets
the RS latch is controlled by the voltage on the ITH/RUN
pin, which is the output of error amplifier EA. The VFB pin,
described in the pin functions, allows EA to receive an
output feedback voltage from an external resistive divider.
When the load current increases, it causes a slight
decrease in VFB relative to the 1.19V reference, which in
turn causes the ITH/RUN voltage to increase until the
average inductor current matches the new load current.
While the top MOSFET is off, the internal bottom MOSFET
is turned on for approximately 300ns to 400ns to recharge
the bootstrap capacitor CB.
The top MOSFET driver is biased from the floating boot-
strap capacitor CB that is recharged during each off cycle.
The dropout detector counts the number of oscillator
cycles that the top MOSFET remains on and periodically
forces a brief off period to allow CB to recharge.
The main control loop is shut down by pulling the ITH/RUN
pin below its 1.19V clamp voltage. Releasing ITH/RUN
allows an internal 2.5µA current source to charge com-
pensation capacitor CC. When the ITH/RUN pin voltage
reaches 0.8V the main control loop is enabled with the ITH/
RUN voltage pulled up by the error amp. Soft start can be
implemented by ramping the voltage on the ITH/RUN pin
from 1.19V to its 2.4V maximum (see Applications Infor-
mation section).
Comparator OV guards against transient output over-
shoots >7.5% by turning off the top MOSFET and keeping
it off until the fault is removed.
Low Current Operation
The LTC1624 is capable of Burst Mode operation in which
the external MOSFET operates intermittently based on
load demand. The transition to low current operation
begins when comparator B detects when the ITH/RUN
voltage is below 1.5V. If the voltage across RSENSE does
not exceed the offset of I2 (approximately 20mV) for one
full cycle, then on following cycles the top and internal
bottom drives are disabled. This continues until the ITH
voltage exceeds 1.5V, which causes drive to be returned to
the TG pin on the next cycle.
INTVCC Power/Boost Supply
Power for the top and internal bottom MOSFET drivers is
derived from VIN. An internal regulator supplies INTVCC
power. To power the top driver in step-down applications
an internal high voltage diode recharges the bootstrap
capacitor CB during each off cycle from the INTVCC supply.
A small internal N-channel MOSFET pulls the switch node
(SW) to ground each cycle after the top MOSFET has
turned off ensuring the bootstrap capacitor is kept fully
charged.
5
5 Page 
LTC1624
APPLICATIONS INFORMATION
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 percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1624 circuits:
1. LTC1624 VIN current
2. I2R losses
3. Topside MOSFET transition losses
4. Voltage drop of the Schottky diode
1. The VIN current is the sum of the DC supply current IQ,
given in the Electrical Characteristics table, and the
MOSFET driver and control currents. The MOSFET
driver current results from switching the gate
capacitance of the power MOSFET. Each time a MOSFET
gate is switched from low to high to low again, a packet
of charge dQ moves from INTVCC to ground. The
resulting dQ/dt is a current out of VIN which is typically
much larger than the control circuit current. In
continuous mode, IGATECHG = f (QT + QB), where QT and
QB are the gate charges of the topside and internal
bottom side MOSFETs.
By powering BOOST from an output-derived source
(Figure 10 application), the additional VIN current
resulting from the topside driver will be scaled by a
factor of (Duty Cycle)/(Efficiency). For example, in a
20V to 5V application, 5mA of INTVCC current results in
approximately 1.5mA of VIN current. This reduces the
midcurrent loss from 5% or more (if the driver was
powered directly from VIN) to only a few percent.
2. I2R losses are predicted from the DC resistances of the
MOSFET, inductor and current shunt. In continuous
mode the average output current flows through L but is
“chopped” between the topside main MOSFET/current
shunt and the Schottky diode. The resistances of the
topside MOSFET and RSENSE multiplied by the duty
cycle can simply be summed with the resistance of L to
obtain I2R losses. (Power is dissipated in the sense
resistor only when the topside MOSFET is on. The I2R
loss is thus reduced by the duty cycle.) For example, at
50% DC, if RDS(ON) = 0.05Ω, RL = 0.15Ω and RSENSE =
0.05Ω, then the effective total resistance is 0.2Ω. This
results in losses ranging from 2% to 8% for VOUT = 5V
as the output current increases from 0.5A to 2A. I2R
losses cause the efficiency to drop at high output
currents.
3. Transition losses apply only to the topside MOSFET(s),
and only when operating at high input voltages (typically
20V or greater). Transition losses can be estimated
from:
Transition Loss = 2.5(VIN)1.85 (IMAX)(CRSS)(f)
4. The Schottky diode is a major source of power loss at
high currents and gets worse at high input voltages.
The diode loss is calculated by multiplying the forward
voltage drop times the diode duty cycle multiplied by
the load current. For example, assuming a duty cycle of
50% with a Schottky diode forward voltage drop of
0.5V, the loss is a relatively constant 5%.
As expected, the I2R losses and Schottky diode loss
dominate at high load currents. Other losses including
CIN and COUT ESR dissipative losses and inductor core
losses generally account for less than 2% total additional
loss.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, VOUT immediately 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 which generates a feedback
error signal. The regulator loop then acts to return VOUT to
its steady-state value. During this recovery time VOUT can
be monitored for overshoot or ringing that would indicate
a stability problem. The ITH external components shown in
the Figure 1 circuit will provide 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
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC1624IS.PDF ] | |
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