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PDF LTC3407A Data sheet ( Hoja de datos )
| Número de pieza | LTC3407A | |
| Descripción | Step-Down DC/DC Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ High Efficiency: Up to 96%
■ Low Ripple (35mVPK-PK) Burst Mode Operation;
IQ = 40µA
■ 1.5MHz Constant Frequency Operation
■ No Schottky Diodes Required
■ Low RDS(ON) Internal Switches: 0.35Ω
■ Current Mode Operation for Excellent Line
and Load Transient Response
■ Short-Circuit Protected
■ Low Dropout Operation: 100% Duty Cycle
■ Ultralow Shutdown Current: IQ < 1µA
■ Output Voltages from 5V down to 0.6V
■ Power-On Reset Output
■ Externally Synchronizable Oscillator
■ Optional External Soft-Start
■ Small Thermally Enhanced MSOP and 3mm × 3mm
DFN Packages
U
APPLICATIO S
■ PDAs/Palmtop PCs
■ Digital Cameras
■ Cellular Phones
■ Wireless and DSL Modems
LTC3407A
Dual Synchronous 600mA,
1.5MHz Step-Down
DC/DC Regulator
DESCRIPTIO
The LTC®3407A is a dual, constant frequency, synchro-
nous step-down DC/DC converter. Intended for low power
applications, it operates from a 2.5V to 5.5V input voltage
range and has a constant 1.5MHz switching frequency,
enabling the use of tiny, low cost capacitors and inductors
1mm or less in height. Each output voltage is adjustable
from 0.6V to 5V. Internal synchronous 0.35Ω, 1A power
switches provide high efficiency without the need for
external Schottky diodes.
A user selectable mode input is provided to allow the user
to trade-off ripple noise for low power efficiency. Burst
Mode® operation provides the highest efficiency at light
loads, while Pulse Skip Mode provides the lowest ripple
noise at light loads.
To further maximize battery life, the P-channel MOSFETs
are turned on continuously in dropout (100% duty cycle),
and both channels draw a total quiescent current of only
40µA. In shutdown, the device draws <1µA.
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners. Protected by
U.S. Patents including 5481178, 6580258, 6304066, 6127815, 6498466, 6611131.
TYPICAL APPLICATIO
1mm High 2.5V/1.8V at 600mA Step-Down Regulators
VIN = 2.5V
TO 5.5V
10µF
VOUT2 = 2.5V
AT 600mA
2.2µH
22pF
RUN/SS2 VIN
MODE/SYNC
RUN/SS1
POR
LTC3407A
SW2 SW1
100k
RESET
2.2µH
22pF
VOUT1 = 1.8V
AT 600mA
10µF
887k
280k
VFB2
GND
VFB1
887k
442k
10µF
3407A TA01
LTC3407A Efficiency/Power Loss Curve
100
90
80
70
60
50
40
30
20
10
0
1
1
2.5V
1.8V
0.1
0.01
0.001
VIN = 3.3V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
0.0001
10 100 1000
LOAD CURRENT (mA)
3407A TA02
3407af
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TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
100
3.3V
90
2.7V
80
4.2V
70
60
50
40
30
20
10
VOUT = 1.2V Burst Mode OPERATION
0
1 10 100
LOAD CURRENT (mA)
1000
3407A G13
Efficiency vs Load Current
100
90
2.7V
80
70 4.2V
3.3V
60
50
40
30
20
10
VOUT = 1.5V Burst Mode OPERATION
0
1 10 100
LOAD CURRENT (mA)
1000
3407A G14
LTC3407A
Line Regulation
0.5
0.4
VOUT = 1.8V
IOUT = 200mA
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
2
34
VIN (V)
56
3407A G15
PI FU CTIO S
VFB1 (Pin 1): Output Feedback. Receives the feedback
voltage from the external resistive divider across the
output. Nominal voltage for this pin is 0.6V.
RUN/SS1 (Pin 2): Regulator 1 Enable and Soft-Start Input.
Forcing this pin to VIN enables regulator 1, while forcing it
to GND causes regulator 1 to shut down. Connect external
RC-network with desired time-constant to enable soft-
start feature. This pin must be driven; do not float.
VIN (Pin 3): Main Power Supply. Must be closely decoupled
to GND.
SW1 (Pin 4): Regulator 1 Switch Node Connection to the
Inductor. This pin swings from VIN to GND.
GND (Pin 5): Main Ground. Connect to the (–) terminal of
COUT, and (–) terminal of CIN.
MODE/SYNC (Pin 6): Combination Mode Selection and
Oscillator Synchronization. This pin controls the operation
of the device. When tied to VIN or GND, Burst Mode
operation or pulse skipping mode is selected, respec-
tively. Do not float this pin. The oscillation frequency can
be synchronized to an external oscillator applied to this pin
and pulse skipping mode is automatically selected.
SW2 (Pin 7): Regulator 2 Switch Node Connection to the
Inductor. This pin swings from VIN to GND.
POR (Pin 8): Power-On Reset. This common-drain logic
output is pulled to GND when the output voltage is not
within ±8.5% of regulation and goes high after 216 clock
cycles when both channels are within regulation.
RUN/SS2 (Pin 9): Regulator 2 Enable and Soft-Start Input.
Forcing this pin to VIN enables regulator 2, while forcing it
to GND causes regulator 2 to shut down. Connect external
RC-Network with desired time-constant to enable soft-
start feature. This pin must be driven; do not float.
VFB2 (Pin 10): Output Feedback. Receives the feedback
voltage from the external resistive divider across the
output. Nominal voltage for this pin is 0.6V.
Exposed Pad (GND) (Pin 11): Power Ground. Connect to
the (–) terminal of COUT, and (–) terminal of CIN. Must be
soldered to electrical ground on PCB.
3407af
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LTC3407A
APPLICATIO S I FOR ATIO
During this recovery time, VOUT can be monitored for
overshoot or ringing that would indicate a stability prob-
lem.
The initial output voltage step may not be within the
bandwidth of the feedback loop, so the standard second-
order overshoot/DC ratio cannot be used to determine
phase margin. In addition, a feed-forward capacitor can be
added to improve the high frequency response, as shown
in Figure 1. Capacitors C1 and C2 provide phase lead by
creating high frequency zeros with R2 and R4 respec-
tively, which improve the phase margin.
The output voltage settling behavior is related to the
stability of the closed-loop system and will demonstrate
the actual overall supply performance. For a detailed
explanation of optimizing the compensation components,
including a review of control loop theory, refer to Applica-
tion Note 76.
In some applications, a more severe transient can be
caused by switching in loads with large (>1µF) input
capacitors. The discharged input capacitors are effectively
put in parallel with COUT, causing a rapid drop in VOUT. No
regulator can deliver enough current to prevent this prob-
lem, if the switch connecting the load has low resistance
and is driven quickly. The solution is to limit the turn-on
speed of the load switch driver. A Hot SwapTM controller is
designed specifically for this purpose and usually incorpo-
rates current limiting, short-circuit protection, and soft-
starting.
Soft-Start
The RUN/SS pins provide a means to separately run or
shut down the two regulators. In addition, they can option-
ally be used to externally control the rate at which each
regulator starts up and shuts down. Pulling the RUN/SS1
pin below 1V shuts down regulator 1 on the LTC3407A.
Forcing this pin to VIN enables regulator 1. In order to
control the rate at which each regulator turns on and off,
connect a resistor and capacitor to the RUN/SS pins as
shown in Figure 1. The soft-start duration can be calcu-
lated by using the following formula:
Hot Swap is a registered trademark of Linear Technology Corporation.
tSS
=
⎛
RSSCSSIn⎝⎜
VIN − 1
VIN − 1.6
⎞
⎠⎟
(s)
For approximately a 1ms ramp time, use RSS = 4.7MΩ and
CSS = 680pF at VIN = 3.3V.
Efficiency Considerations
The percent 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. 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, 4 main sources usually account for most of the
losses in LTC3407A circuits: 1)VIN quiescent current, 2)
switching losses, 3) I2R losses, 4) other losses.
1) The VIN current is the DC supply current given in the
Electrical Characteristics which excludes MOSFET driver
and control currents. VIN current results in a small (<0.1%)
loss that increases with VIN, even at no load.
2) The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current results
from switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from VIN to
ground. The resulting dQ/dt is a current out of VIN that is
typically much larger than the DC bias current. In continu-
ous mode, IGATECHG = fO(QT + QB), where QT and QB are the
gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to VIN
and thus their effects will be more pronounced at higher
supply voltages.
3) I2R losses are calculated from the DC resistances of the
internal switches, RSW, and external inductor, RL. In
continuous mode, the average output current flows through
inductor L, but is “chopped” between the internal top and
bottom switches. Thus, the series resistance looking into
3407af
11
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| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3407A.PDF ] | |
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