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PDF LTC3406A Data sheet ( Hoja de datos )
| Número de pieza | LTC3406A | |
| Descripción | 600mA Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ High Efficiency: Up to 96%
■ Very Low Quiescent Current: Only 20µA
■ Low Output Ripple Voltage During Burst Mode®
Operation
■ 600mA Output Current
■ 2.5V to 5.5V Input Voltage Range
■ 1.5MHz Constant Frequency Operation
■ No Schottky Diode Required
■ Low Dropout Operation: 100% Duty Cycle
■ ±2% 0.6V Reference
■ Shutdown Mode Draw ≤1µA Supply Current
■ Internal Soft-Start Limits Inrush Current
■ Current Mode Operation for Excellent Line and
Load Transient Response
■ Overtemperature Protected
■ Low Profile (1mm) ThinSOTTM Package
APPLICATIONS
■ Cellular Telephones
■ Wireless and DSL Modems
■ Digital Still Cameras
■ Media Players
■ Portable Instruments
■ Point of Load Regulation
TYPICAL APPLICATION
2.2μH
VIN VIN SW
4.7μF
22pF
CER LTC3406A
RUN VFB
GND
619k
309k
3406A TA01
VOUT
1.8V
600mA
10μF
CER
LTC3406A
1.5MHz, 600mA
Synchronous Step-Down
Regulator in ThinSOT
DESCRIPTION
The LTC®3406A is a high efficiency monolithic synchro-
nous buck regulator using a constant frequency, current
mode architecture. Supply current during operation is only
20μA, dropping to ≤1μA in shutdown. The 2.5V to 5.5V
input voltage range makes the LTC3406A ideally suited
for single Li-Ion battery-powered applications. 100% duty
cycle provides low dropout operation, extending battery
runtime portable systems. Automatic Burst Mode opera-
tion increases efficiency at light loads, further extending
battery runtime.
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
feedback reference voltage. The LTC3406A is available in
a low profile (1mm) ThinSOT package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
ThinSOT is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5481178, 6580258.
Efficiency vs Load Current
100
90
80
70
60
50
40
30
20 VIN = 2.7V
10 VIN = 3.6V
0 VOUT = 1.8V
VIN = 4.2V
0.1 1 10 100 1000
OUTPUT CURRENT (mA)
3406A TA01b
3406af
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LTC3406A
TYPICAL PERFORMANCE CHARACTERISTICS
(From Front Page Figure Except for the Resistive Divider Resistor Values)
Start-Up from Shutdown
RUN
2V/DIV
VOUT
1V/DIV
ILOAD
500mA/DIV
Load Step
VOUT
200mV/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
Load Step
VOUT
200mV/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
500μs/DIV
VOUT = 1.8V
ILOAD = 600mA (3Ω RES)
3406A G16
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 0mA TO 600mA
3406A G17
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
3406A G18
Load Step
VOUT
200mV/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 100mA TO 600mA
Load Step
VOUT
200mV/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
3406A G19
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 200mA TO 600mA
Discontinuous Operation
SW
2V/DIV
VOUT
20mV/DIV
AC COUPLED
IL
200mA/DIV
3406A G20
VIN = 3.6V
VOUT = 1.8V
ILOAD = 50mA
500ns/DIV
3406A G21
PIN FUNCTIONS
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 synchronous
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): Feedback Pin. Receives the feedback voltage
from an external resistive divider across the output.
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LTC3406A
APPLICATIONS INFORMATION
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3406A in dropout at
an input voltage of 2.7V, a load current of 600mA and
an ambient temperature of 70°C. From the typical per-
formance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.27Ω. There-
fore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 97.2mW
For the SOT-23 package, the θJA is 250°C/ W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.0972)(250) = 94.3°C
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction temperature
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 (ΔILOAD • ESR), where ESR is the effective series
resistance of COUT. ΔILOAD also begins to charge or dis-
charge 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. 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 paral-
lel 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
LTC3406A. These items are also illustrated graphically in
Figures 3 and 4. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace, the VOUT 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
connected between the (+) plate of COUT and ground.
3. Does 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, and the IC ground,
as close as possible.
3406af
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3406A.PDF ] | |
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