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PDF LTC1877EMS8 Data sheet ( Hoja de datos )
| Número de pieza | LTC1877EMS8 | |
| Descripción | High Efficiency Monolithic Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s High Efficiency: Up to 95%
s Very Low Quiescent Current: Only 10µA
During Operation
s 600mA Output Current at VIN = 5V
s 2.65V to 10V Input Voltage Range
s 550kHz Constant Frequency Operation
s Synchronizable from 400kHz to 700kHz
s Selectable Burst ModeTM Operation/
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-LeaUd MSOP Package
APPLICATIONS
s Cellular Telephones
s Wireless Modems
s Personal Information Appliances
s Portable Instruments
s Distributed Power Systems
s Battery-Powered Equipment
Final Electrical Specifications
LTC1877
High Efficiency
Monolithic Synchronous
Step-Down Regulator
DESCRIPTION
May 2000
The LTC®1877 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
10V input voltage range makes the LTC1877 ideally suited
for both single and dual Li-Ion battery-powered applica-
tions. 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 LTC1877 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 LTC1877 is available in a
space saving 8-lead MSOP package. Lower input voltage
applications (less than 7V abs max) should refer to the
LTC1878 data sheet.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
VIN
2.65V
TO 10V
High Efficiency Step-Down Converter
10µF**
CER
220pF
7
SYNC
5
SW
6
VIN
1 LTC1877
RUN
2
ITH
GND
VFB 3
4
10µH*
20pF
887k
280k
VOUT†
3.3V
+
47µF***
*TOKO D62CB A920CY-100M
**TAIYO-YUDEN CERAMIC LMK325BJ106MN
***SANYO POSCAP 6TPA47M
†VOUT CONNECTED TO VIN FOR 2.65V < VIN < 3.3V
1877 TA01
Efficiency vs Output Current
100
95 VIN = 3.6V
90
85 VIN = 10V
80 VIN = 7.2V
75 VIN = 5V
70
65
60
55
50
0.1
VOUT = 3.3V
L = 10µH
Burst Mode OPERATION
1.0 10 100
OUTPUT CURRENT (mA)
1000
1877 • TA02
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC1877
Switch Leakage vs Temperature
1400
1200
VIN = 10V
RUN = 0V
1000
800
MAIN SWITCH
600
400 SYNCHRONOUS
SWITCH
200
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1877 • G12
Switch Leakage vs Input Voltage
16
RUN = 0V
14
12
Burst Mode Operation
SW
5V/DIV
10
8
SYNCHRONOUS
SWITCH
6
VOUT
20mV/DIV
AC COUPLED
4 IL
200mA/DIV
2
MAIN SWITCH
0
0 2 4 6 8 10
INPUT VOLTAGE (V)
1877 • G13
VIN = 5V
VOUT = 1.5V
CIN = 10µF
10µs/DIV
L = 10µH
COUT = 47µF
ILOAD = 50mA
Start-Up from Shutdown
Load Step Response
1877 • G14
RUN
5V/DIV
VOUT
1V/DIV
IL
500mA/DIV
VIN = 5V
VOUT = 1.5V
L = 10µH
50µs/DIV
CIN = 10µF
COUT = 47µF
ILOAD = 500mA
1877 • G15
Load Step Response
VOUT
50mV/DIV
AC COUPLED
IL
500mA/DIV
ITH
1V/DIV
VIN = 5V
VOUT = 1.5V
L = 10µH
40µs/DIV
CIN = 10µF
COUT = 47µF
ILOAD = 50mA TO 500mA
PULSE SKIPPING MODE
1877 • G16
Load Step Response
VOUT
50mV/DIV
AC COUPLED
IL
500mA/DIV
10µs/DIV
VOUT
50mV/DIV
AC COUPLED
IL
500mA/DIV
ITH
1V/DIV
VIN = 5V
VOUT = 1.5V
L = 10µH
40µs/DIV
CIN = 10µF
COUT = 47µF
ILOAD = 50mA TO 500mA
Burst Mode OPERATION
1877 • G17
ITH
1V/DIV
VIN = 5V
VOUT = 1.5V
L = 10µH
20µs/DIV
CIN = 10µF
COUT = 47µF
ILOAD = 200mA TO 500mA
PULSE SKIPPING MODE
1877 • G18
5
5 Page 
LTC1877
APPLICATIONS INFORMATION
frequency is less than 550kHz, current is sunk continu-
ously, pulling down the PLL LPF pin. If the 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 oscillators are
identical. At this stable operating point the phase com-
parator 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 LTC1877 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
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.
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
1877 F06
Figure 6. Power Lost vs Load Current
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC1877EMS8.PDF ] | |
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