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PDF LTC3406B Data sheet ( Hoja de datos )
| Número de pieza | LTC3406B | |
| Descripción | 600mA Synchronous Step-Down Regulator in ThinSOT | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ High Efficiency: Up to 96%
■ 600mA Output Current at VIN = 3V
■ 2.5V to 5.5V Input Voltage Range
■ 1.5MHz Constant Frequency Operation
■ No Schottky Diode Required
■ Low Dropout Operation: 100% Duty Cycle
■ Low Quiescent Current: 300µA
■ 0.6V Reference Allows Low Output Voltages
■ Shutdown Mode Draws < 1µA Supply Current
■ Current Mode Operation for Excellent Line and
Load Transient Response
■ Overtemperature Protected
■ Low Profile (1mm) ThinSOTTM Package
U
APPLICATIO S
■ Cellular Telephones
■ Personal Information Appliances
■ Wireless and DSL Modems
■ Digital Still Cameras
■ MP3 Players
■ Portable Instruments
LTC3406B
1.5MHz, 600mA
Synchronous Step-Down
Regulator in ThinSOT
DESCRIPTIO
The LTC®3406B is a high efficiency monolithic synchro-
nous buck regulator using a constant frequency, current
mode architecture. The device is available in an adjustable
version and fixed output voltages of 1.5V and 1.8V. Supply
current with no load is 300µA and drops to <1µA in
shutdown. The 2.5V to 5.5V input voltage range makes the
LTC3406B ideally suited for single Li-Ion battery-powered
applications. 100% duty cycle provides low dropout op-
eration, extending battery life in portable systems. PWM
pulse skipping mode operation provides very low output
ripple voltage for noise sensitive applications.
Switching frequency is internally set at 1.5MHz, allowing
the use of small surface mount inductors and capacitors.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.6V feed-
back reference voltage. The LTC3406B is available in a low
profile (1mm) ThinSOT package. Refer to LTC3406 for
applications that require Burst Mode® operation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
Protected by U.S. Patents, including 6580258, 5481178.
TYPICAL APPLICATIO
VIN
2.7V
TO 5.5V
CIN**
4.7µF
CER
4
VIN
3 2.2µH*
SW
LTC3406B-1.8
15
RUN VOUT
GND
2
COUT†
10µF
CER
VOUT
1.8V
600mA
3406B F01a
*MURATA LQH32CN2R2M33
**TAIYO YUDEN JMK212BJ475MG
†TAIYO YUDEN JMK316BJ106ML
Figure 1a. High Efficiency Step-Down Converter
100
VOUT = 1.8V
90
80
70
VIN = 3.6V
60
50
VIN = 2.7V
40
30
20 VIN = 4.2V
10
0.1
1 10 100
OUTPUT CURRENT (mA)
1000
3406B F01b
Figure 1b. Efficiency vs Load Current
3406bfa
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values)
LTC3406B
Start-Up from Shutdown
Load Step
RUN
5V/DIV
VOUT
1V/DIV
IL
500mA/DIV
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
40µs/DIV
3406B G16
VOUT = 1.8V
ILOAD = 600mA (LOAD: 3Ω RESISTOR)
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 0mA TO 600mA
Load Step
Load Step
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
ILOAD
500mA/DIV
3406B G17
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
Load Step
3406B G18
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 100mA TO 600mA
3406B G19
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 200mA TO 600mA
3406B G20
PI FU CTIO S
RUN (Pin 1): Run Control Input. Forcing this pin above
1.5V enables the part. Forcing this pin below 0.3V shuts
down the device. In shutdown, all functions are disabled
drawing <1µA supply current. Do not leave RUN floating.
GND (Pin 2): Ground Pin.
SW (Pin 3): Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchro-
nous power MOSFET switches.
VIN (Pin 4): Main Supply Pin. Must be closely decoupled
to GND, Pin 2, with a 2.2µF or greater ceramic capacitor.
VFB (Pin 5) (LTC3406B): Feedback Pin. Receives the
feedback voltage from an external resistive divider across
the output.
VOUT (Pin 5) (LTC3406B-1.5/LTC3406B-1.8): Output Volt-
age Feedback Pin. An internal resistive divider divides the
output voltage down for comparison to the internal refer-
ence voltage.
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5 Page 
LTC3406B
APPLICATIO S I FOR ATIO
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3406B in dropout at an
input voltage of 2.7V, a load current of 600mA and an
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.52Ω. There-
fore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 187.2mW
For the SOT-23 package, the θJA is 250°C/ W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.1872)(250) = 116.8°C
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (RDS(ON)).
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 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 moni-
tored for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
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
with COUT, causing a rapid drop in VOUT. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25 • CLOAD).
Thus, a 10µF capacitor charging to 3.3V would require a
250µs rise time, limiting the charging current to about
130mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3406B. These items are also illustrated graphically in
Figures 5 and 6. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the VIN trace should be kept short, direct and
wide.
2. Does the VFB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be con-
nected between the (+) plate of COUT and ground.
3. Does the (+) plate of CIN connect to VIN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive
VFB node.
5. Keep the (–) plates of CIN and COUT as close as possible.
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3406B.PDF ] | |
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