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PDF LTC1878 Data sheet ( Hoja de datos )
| Número de pieza | LTC1878 | |
| Descripción | High Efficiency Monolithic Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1878
High Efficiency
Monolithic Synchronous
Step-Down Regulator
FEATURES
s High Efficiency: Up to 95%
s Very Low Quiescent Current: Only 10µA
During Operation
s 600mA Output Current at VIN = 3.3V
s 2.65V to 6V Input Voltage Range
s 550kHz Constant Frequency Operation
s Synchronizable from 400kHz to 700kHz
s Selectable Burst ModeTM Operation or
Pulse Skipping Mode
s No Schottky Diode Required
s Low Dropout Operation: 100% Duty Cycle
s 0.8V Reference Allows Low Output Voltages
s Shutdown Mode Draws < 1µA Supply Current
s ±2% Output Voltage Accuracy
s Current Mode Control for Excellent Line and
Load Transient Response
s Overcurrent and Overtemperature Protected
s Available in 8-Lead MSOP Package
U
APPLICATIO S
s Cellular Telephones
s Wireless Modems
s Personal Information Appliances
s Portable Instruments
s Distributed Power Systems
s Battery-Powered Equipment
DESCRIPTIO
The LTC®1878 is a high efficiency monolithic synchro-
nous buck regulator using a constant frequency, current
mode architecture. Supply current during operation is
only 10µA and drops to < 1µA in shutdown. The 2.65V to
6V input voltage range makes the LTC1878 ideally suited
for single Li-Ion battery-powered applications. 100% duty
cycle provides low dropout operation, extending battery
life in portable systems.
Switching frequency is internally set at 550kHz, allowing
the use of small surface mount inductors and capacitors.
For noise sensitive applications the LTC1878 can be
externally synchronized from 400kHz to 700kHz. Burst
Mode operation is inhibited during synchronization or
when the SYNC/MODE pin is pulled low, preventing low
frequency ripple from interfering with audio circuitry.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.8V feed-
back reference voltage. The LTC1878 is available in a
space saving 8-lead MSOP package.
For higher input voltage (11V abs max) applications, refer
to the LTC1877 data sheet.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
High Efficiency Step-Down Converter
VIN
2.65V
TO 6V 22µF**
CER
220pF
7
SYNC
5
SW
6
VIN
1 LTC1878
RUN
23
ITH GND VFB
4
10µH*
20pF
887k
280k
*TOKO D62CB A920CY-100M
**TAIYO-YUDEN CERAMIC JMK325BJ226MM
***SANYO POSCAP 6TPA47M
†VOUT CONNECTED TO VIN FOR 2.65V < VIN < 3.3V
VOUT†
3.3V
+
47µF***
1878 TA01
Efficiency vs Output Load Current
100
95 VIN = 3.6V
90
VIN = 4.2V
85
VIN = 6V
80
75
70
0.1
Burst Mode OPERATION
VOUT = 3.3V
L = 10µH
1 10 100
OUTPUT CURRENT (mA)
1000
1878 TA02
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1878
Switch Leakage vs Temperature
2.5
VIN = 7V
RUN = 0V
2.0
1.5
1.0 MAIN
SWITCH
SYNCHRONOUS
0.5 SWITCH
0
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1878 G13
Pulse Skipping Mode Operation
Switch Leakage vs Input Voltage
1.2
RUN = 0V
1.0
SYNCHRONOUS
SWITCH
0.8
0.6
0.4
MAIN
SWITCH
0.2
0
0 1234 56 78
INPUT VOLTAGE (V)
1878 G20
Start-Up from Shutdown
Burst Mode Operation
SW
5V/DIV
VOUT
50mV/DIV
AC
COUPLED
IL
200mA/DIV
VIN = 4.2V
VOUT = 1.5V
L = 10µH
10µs/DIV
CIN = 22µF
COUT = 47µF
ILOAD = 50mA
Load Step Response
1878 G14
SW
5V/DIV
VOUT
20mV/DIV
AC
COUPLED
IL
200mA/DIV
VIN = 4.2V
VOUT = 1.5V
L = 10µH
1µs/DIV
CIN = 22µF
COUT = 47µF
ILOAD = 50mA
RUN
2V/DIV
VOUT
1V/DIV
IL
500mA/DIV
1878 G15
VIN = 3.6V
VOUT = 1.5V
L = 10µH
40µs/DIV
CIN = 22µF
COUT = 47µF
ILOAD = 500mA
Load Step Response
VOUT
50mV/DIV
AC
COUPLED
IL
500mA/DIV
1878 G16
ITH
1V/DIV
VIN = 3.6V
VOUT = 1.5V
L = 10µH
40µs/DIV
CIN = 22µF
COUT = 47µF
ILOAD = 200mA TO 500mA 1878 G17
PULSE SKIPPING MODE
Load Step Response
VOUT
100mV/DIV
AC
COUPLED
IL
500mA/DIV
VOUT
100mV/DIV
AC
COUPLED
IL
500mA/DIV
ITH
1V/DIV
VIN = 3.6V
VOUT = 1.5V
L = 10µH
40µs/DIV
CIN = 22µF
COUT = 47µF
ILOAD = 50mA TO 500mA
PULSE SKIPPING MODE
1878 G18
ITH
1V/DIV
VIN = 3.6V
VOUT = 1.5V
L = 10µH
40µs/DIV
CIN = 22µF
COUT = 47µF
ILOAD = 50mA TO 500mA
Burst Mode OPERATION
1878 G19
5
5 Page 
LTC1878
APPLICATIO S I FOR ATIO
external and internal frequencies are the same but exhibit
a phase difference, the current sources turn on for an
amount of time corresponding to the phase difference.
Thus the voltage on the PLL LPF pin is adjusted until the
phase and frequency of the external and internal oscilla-
tors are identical. At this stable operating point the phase
comparator output is high impedance and the filter
capacitor CLP holds the voltage.
The loop filter components CLP and RLP smooth out the
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
component’s CLP and RLP determine how fast the loop
acquires lock. Typically RLP = 10k and CLP is 2200pF to
0.01µF. When not synchronized to an external clock, the
internal connection to the VCO is disconnected. This
disallows setting the internal oscillator frequency by a DC
voltage on the VPLL LPF pin.
Efficiency Considerations
The 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. 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, two main sources usually account for most of the
losses in LTC1878 circuits: VIN quiescent current and I2R
losses. The VIN quiescent current loss dominates the
efficiency loss at very low load currents whereas the I2R
loss dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve at
very low load currents can be misleading since the actual
power lost is of no consequence as illustrated in Figure 6.
1. The VIN quiescent current is due to two components:
the DC bias current as given in the electrical character-
istics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
1
VIN = 4.2V
L = 10µH
0.1 VOUT = 1.5V
VOUT = 2.5V
VOUT = 3.3V
0.01 Burst Mode OPERATION
0.001
0.0001
0.00001
0.1
1 10 100
LOAD CURRENT (mA)
1000
1878 F06
Figure 6. Power Lost vs Load Current
internal power MOSFET switches. 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 VINthat is typically larger than
the DC bias current. In continuous mode, IGATECHG =
f(QT + QB) where QT and QB are the gate charges of the
internal top and bottom switches. Both the DC bias and
gate charge losses are proportional to VIN and thus
their effects will be more pronounced at higher supply
voltages.
2. I2R losses are calculated from the resistances of the
internal switches, RSW, and external inductor RL. In
continuous mode the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET RDS(ON) and the duty cycle
(DC) as follows:
RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)
The RDS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Charateristics
curves. Thus, to obtain I2R losses, simply add RSW to
RL and multiply the result by the square of the average
output current.
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for less
than 2% total additional loss.
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC1878.PDF ] | |
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