Aug 2002 Monolithic Synchronous Step-Down Regulators Pack 600mA Current Rating in a ThinSOT Package

DESIGN IDEAS
Monolithic Synchronous Step-Down
Regulators Pack 600mA
Current Rating in a ThinSOT Package
by Jaime Tseng
Introduction
LTC®3406,
The new
LTC3406-1.5,
LTC3406-1.8, LTC3406B, LTC3406B1.5 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 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
5mm×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.
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 Pchannel MOSFET and the inductor
resistance. The fixed voltage output
versions—available for 1.5V and
1.8V—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
VOUT
100mV/DIV
AC COUPLED
VIN
2.7V
TO 5.5V
4
CIN**
4.7µF
CER
VIN
SW
3
2.2µH*
VOUT
1.8V
600mA
LTC3406B-1.8
1
RUN
VOUT
5
COUT†
10µF
CER
GND
2
* MURATA LQH3C2R2M24
** TAIYO YUDEN JMK212BJ475MG
†
TAIYO YUDEN JMK316BJ106ML
(814) 237-1431
(408) 573-4150
Figure 1. 1.8V/600mA step-down regulator
using all ceramic capacitors
Linear Technology Magazine • August 2002
condition at the output (>6.25% above
nominal) is sensed, the top 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. 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
19µA. 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
IL
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
Figure 2. LTC3406-1.8 transient
response to a 50mA to 600mA load step
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
31
DESIGN IDEAS
100
95
VIN = 2.7V
90
VOUT
100mV/DIV
AC COUPLED
90
80
IL
500mA/DIV
ILOAD
500mA/DIV
EFFICIENCY (%)
EFFICIENCY (%)
85
VIN = 3.6V
80
VIN = 4.2V
75
70
60
VIN = 3.6V
50
VIN = 2.7V
40
70
30
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
Figure 3. LTC3406B-1.8 Transient
Response to a 50mA to 600mA
Load Step
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 for very low ripple
65
VIN = 4.2V
20
60
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
10
0.1
Figure 4. Efficiency vs Load
Current for LTC3406-1.8
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 LTC34061.8 and LTC3406B-1.8, respectively.
Authors can be contacted
at (408) 432-1900
1
100
10
OUTPUT CURRENT (mA)
1000
Figure 5. Efficiency vs Load
Current for LTC3406B-1.8
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 constant-frequency 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.
LTC3720, continued from page 20
optional short-circuit latch-off, a
Power Good indicator of output regulation and a current limit foldback for
overload protection. A selectable discontinuous conduction mode of
operation maintains high efficiency
at light loads, when the CPU is running at deep sleep mode, for example,
thereby improving battery life in portable applications.
Figure 1 shows the schematic diagram of a 20A VRM8.5 design for an
Intel processor operating at 1.2GHz.
Efficiency is greater than 80% over a
wide load range, as shown in Figure␣ 2.
With two 680µF Sanyo POSCAPs, the
output voltage deviation remains
within the VRM8.5 specification when
32
load current switches between CPU
leakage and full load, as shown in
Figure 3. The entire VRM design fits
into a 1.25"×1.5", double-sided PCB
area with an overall height below
0.35".
In summary, the LTC3720 is an
ideal device for low current CPU power
supplies. Its unique control architecture and its powerful gate drivers
facilitate the design of space-saving
VRMs that have a fast transient response. For CPUs that consume more
than 20A, the LTC1709-85 dualphase controller addresses the current
distribution and thermal management
issues associated with higher current
applications.
1.535V
VOUT
1.370V
20A
ILOAD
1A
25µs/DIV
Figure 3. With two POSCAPs at output, the
design in Figure 1 meets VRM 8.5 transient
requirements with significant margin.
Linear Technology Magazine • August 2002
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