DN300 - Monolithic Synchronous Step-Down Regulators Pack 600mA Current Rating in a ThinSOT Package

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Monolithic Synchronous Step-Down Regulators Pack 600mA
Current Rating in a ThinSOT Package – Design Note 300
Jaime Tseng
Introduction
The new LTC ®3406, LTC3406-1.5, LTC3406-1.8,
LTC3406B, LTC3406B-1.5 and LTC3406B-1.8 are the
industry’s first monolithic synchronous step-down
regulators capable of supplying 600mA of output current in a 1mm profile ThinSOT™ package. These devices
are designed to save space and increase efficiency for
battery-powered portable devices. The LTC3406 series
uses Burst Mode ® operation to increase efficiency at
light loads, consuming only 20μA of supply current at
no load. For noise-sensitive applications, the LTC3406B
series disables Burst Mode operation and operates in a
pulse skipping mode under light loads. Both consume
less than 1μA quiescent current in shutdown.
Space Saving
Everything about the LTC3406/LTC3406B series is
designed to make power supplies tiny and efficient. An
entire regulator can fit into a 5 × 7mm board space. These
devices are high efficiency monolithic synchronous
buck regulators using a constant frequency, current
mode architecture. Their on-chip power MOSFETs
provide up to 600mA of continuous output current.
Their internal synchronous switches increase efficiency
and eliminate the need for an external Schottky diode.
Internal loop compensation eliminates additional external components.
VIN
2.7V
TO 5.5V
4
CIN**
4.7μF
CER
VIN
SW
3
2.2μH*
COUT†
10μF
CER
LTC3406B-1.8
1
RUN
VOUT
5
DN300 F01
GND
2
*MURATA LQH3C2R2M24
**TAIYO YUDEN JMK212BJ475MG
†
TAIYO YUDEN JMK316BJ106ML
Figure 1. 1.8V/600mA Step-Down
Regulator Using All Ceramic Capacitors
11/02/300_conv
Versatile
These devices have a versatile 2.5V to 5.5V input
voltage range, which makes them ideal for single cell
Li-Ion or 3-cell NiCd and NiMH applications. The 100%
duty cycle capability for low dropout allows maximum
energy to be extracted from the battery. In dropout,
the output voltage is determined by the input voltage
minus the voltage drop across the internal P-channel
MOSFET and the inductor resistance. The fixed voltage output versions—the LTC3406-1.5/LTC3406B-1.5
and LTC3406-1.8/LTC3406B-1.8, are fixed to 1.5V and
1.8V, respectively—require no external voltage divider
for feedback, further saving space and improving efficiency. The adjustable voltage output versions—the
LTC3406 and LTC3406B—allow the output voltage to
be externally programmed with two resistors to any
value above the 0.6V internal reference voltage.
Fault Protection
The LTC3406 and LTC3406B protect against output
overvoltage, output short-circuit and power overdissipation conditions. When an overvoltage condition at
the output (>6.25% above nominal) is sensed, the top
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
VOUT
1.8V
600mA
VOUT
100mV/DIV
AC COUPLED
INDUCTOR
CURRENT
500mA/DIV
LOAD CURRENT
500mA/DIV
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
DN300 F02
Figure 2. LTC3406-1.8 Transient Response to a 50mA to
600mA Load Step (Burst Mode Operation at Light Load)
MOSFET is turned off until the fault is removed. When
the output is shorted to ground, the frequency of the
oscillator slows to 210kHz to prevent inductor-current
runaway. The frequency returns to 1.5MHz when VFB
is allowed to rise to 0.6V. When there is a power overdissipation condition and the junction temperature
reaches approximately 160°C, the thermal protection
circuit turns off the power MOSFETs allowing the part to
cool. Normal operation resumes when the temperature
drops to 150°C.
Efficient Burst Mode Operation (LTC3406 Series)
In Burst Mode operation, the internal power MOSFETs
operate intermittently based on load demand (see
Figure 2). Short burst cycles of normal switching are
followed by longer idle periods where the load current is
supplied by the output capacitor. During the idle period,
the power MOSFETs and any unneeded circuitry are
turned off, reducing the quiescent current to 20μA. At
no load, the output capacitor discharges slowly through
the feedback resistors resulting in very low frequency
burst cycles that add only a few microamperes to the
supply current.
Pulse Skipping Mode (LTC3406B Series)
for Low Noise
Pulse skipping mode lowers output ripple, thus reducing possible interference with audio circuitry. In
pulse skipping mode, constant-frequency operation is
maintained at lower load currents to lower the output
voltage ripple. If the load current is low enough, cycle
skipping eventually occurs to maintain regulation.
Efficiency in pulse skipping mode is lower than Burst
Mode operation at light loads, but comparable to Burst
Mode operation when the output load exceeds 50mA.
1.8V/600mA Step-Down Regulator Using All
Ceramic Capacitors
Figure 1 shows an application of the LTC3406/
LTC3406B-1.8 using all ceramic capacitors. This particular design supplies a 600mA load at 1.8V with an
input supply between 2.5V and 5.5V. Ceramic capacitors
have the advantages of small size and low equivalent
series resistance (ESR), making possible very low ripple
voltages at both the input and output. For a given package size or capacitance value, ceramic capacitors have
lower ESR than other bulk, low ESR capacitor types
(including tantalum capacitors, aluminum and organic
electrolytics). Because the LTC3406/LTC3406B’s control loop does not depend on the output capacitor’s
ESR for stable operation, ceramic capacitors can be
used to achieve very low output ripple and small circuit
size. Figures 2 and 3 show the transient response to
a 50mA to 600mA load step for the LTC3406-1.8 and
LTC3406B-1.8, respectively.
Efficiency Considerations
Figure 4 shows the efficiency curves for the LTC3406-1.8
(Burst Mode operation enabled) at various supply voltages. Burst Mode operation significantly lowers the
quiescent current, resulting in high efficiencies even
with extremely light loads.
Figure 5 shows the efficiency curves for the LTC3406B-1.8
(pulse skipping mode enabled) at various supply
voltages. Pulse skipping mode maintains constantfrequency operation at lower load currents. This necessarily increases the gate charge losses and switching
losses, which impact efficiency at light loads. Efficiency
is still comparable to Burst Mode operation at higher
loads.
100
95
90
VIN = 2.7V
90
80
VOUT
100mV/DIV
AC COUPLED
INDUCTOR
CURRENT
500mA/DIV
EFFICIENCY (%)
EFFICIENCY (%)
85
VIN = 3.6V
80
VIN = 4.2V
75
70
LOAD CURRENT
500mA/DIV
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
DN300 F03
Figure 3. LTC3406B-1.8 Transient
Response to a 50mA to 600mA
Load Step
Data Sheet Download
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Linear Technology Corporation
70
60
VIN = 3.6V
VIN = 2.7V
50
40
30
VIN = 4.2V
65
20
60
0.1
10
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
1
100
10
OUTPUT CURRENT (mA)
DN300 F04
Figure 4. Efficiency vs Load
Current for LTC3406-1.8
1000
DN300 F05
Figure 5. Efficiency vs Load
Current for LTC3406B-1.8
For applications help,
call (408) 432-1900
dn300f_conv LT/TP 1102 316.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2002
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