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A New, High Efficiency Monolithic Buck Converter
Design Note 105
San-Hwa Chee
The LTC ®1265 is a 14-pin SOIC step-down converter
capable of operating at frequencies up to 700kHz. High
frequency operation permits the use of small inductors
for size sensitive applications. The LTC1265 has an
internal 0.3Ω (at a supply voltage of 10V) P-channel
power MOSFET switch, which is capable of supplying
up to 1.2A of output current. With no load, the converter requires only 160μA of quiescent current; this
decreases to a mere 5μA in shutdown conditions. In
dropout mode, the internal P-channel power MOSFET
switch is turned on continuously (at DC), thereby maximizing battery life. The part is protected from output
shorts by its built-in current limiting. In addition to the
features already mentioned, the LTC1265 incorporates
a low-battery detector.
The LTC1265 is a current mode DC/DC converter with
Burst Mode® operation. The current mode architecture
gives the LTC1265 excellent load and line regulation.
Burst Mode operation results in high efficiency with
both high and low load currents. The LTC1265 comes
in three versions: LTC1265-5 (5V output), LTC1265-3.3
(3.3V output) and LTC1265 (adjustable). All versions
operate down to an input voltage of 3.5V and up to an
absolute maximum of 13V.
Efficiency
Figure 1 shows a typical LTC1265-5 application circuit.
The efficiency curves for two different input voltages
are shown in Figure 2. Note that the efficiency for a 6V
input exceeds 90% over a load range from less than
10mA to 850mA. This makes the LTC1265 attractive
for all battery-operated products and efficiency sensitive applications.
High Frequency Operation
Although the LTC1265 is capable of operation at frequencies up to 700kHz, the highest efficiency is achieved
at an operating frequency of about 200kHz. As the
frequency increases, losses due to the gate charge of
the P-channel power MOSFET increase. In space sensitive applications, high frequency operation allows the
use of smaller components at the cost of four to five
efficiency points.
Constant Off-Time Architecture
The LTC1265 uses a constant off-time, current mode
architecture. This results in a power supply that has
very high efficiency over a wide load current range, fast
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
VIN
5.4V TO 12V
CIN††
68μF
20V
PWR VIN PWR VIN
2
10
SW
VIN
0.1μF
5
1k
6
7
14
LTC1265-5
SHDN
CT
130pF
3900pF
VOUT
5V/1A
13
PGND
SGND
12
L1*
33μH
100
RSENSE**
0.1Ω
VIN = 6V
+
D1
MBRS130LT3
11
CT
ITH
SENSE–
NC
SENSE+
9
8
1000pF
* COILTRONICS CTX33-4
** KRL SL-C1-OR100J
†
AVX TPSE227K010
††
AVX TPSE686K020
COILTRONICS 407-241-7876
KRL/BANTRY 603-668-3210
DN105 • F01
Figure 1. High Efficiency 5V/1A Step-Down Converter
06/95/105_conv
COUT†
220μF
10V
95
VIN = 9V
EFFICIENCY (%)
+
1
90
85
80
75
70
0.01
L = 33μH
VOUT = 5V
RSENSE = 0.1Ω
CT = 130pF
0.10
LOAD CURRENT (A)
1.00
DN105 • F02
Figure 2. Efficiency vs Load Current
transient response and very low dropout characteristics.
The off-time is set by an external timing capacitor CT
and is constant whenever the output is in regulation.
When the output is not in regulation, the off-time is
inversely proportional to the output voltage. By using a
constant off-time scheme, the inductor’s ripple current
is predictable and well controlled under all operating
conditions, making the selection of the inductor much
easier. The inductor’s peak-to-peak ripple current is
inversely proportional to the inductance in continuous
mode. If a lower ripple current is desired, a larger inductor can be used for a given value of timing capacitor.
100% Duty Cycle in Dropout Mode
When the input voltage decreases, the switching
frequency decreases. With the off-time constant, the
on-time is increased to maintain the same peak-to-peak
ripple current in the inductor. When the input-to-output
voltage differential drops below 2.0V, the off-time is
reduced. This prevents the operating frequency from
dropping below 20kHz as the regulator approaches the
dropout region. As the input voltage drops further, the
P-channel switch is turned on for 100% of the cycle. The
dropout voltage is governed by the switch resistance,
load current and current sense resistor.
Good Start-Up and Transient Behavior
The LTC1265 exhibits excellent start-up behavior when
it is initially powered on or recovering from a short circuit. This is achieved by making the off-time inversely
proportional to the output voltage while the output is
still in the process of reaching its regulated value. When
the output is shorted to ground, the off-time is extended
long enough to prevent inductor current runaway. When
the short is removed, the output capacitor begins to
charge and the off-time gradually decreases.
In addition, the LTC1265 has excellent load transient
response. When the load current drops suddenly, the
feedback loop responds quickly by turning off the
internal P-channel switch. Sudden increases in output
current will be met initially by the output capacitor,
causing the output voltage to drop slightly. Tight
control of the inductor’s current, as mentioned above,
means that output voltage overshoot and undershoot
are virtually eliminated.
2.5mm Typical Height 5V-to-3.3V Regulator
Figure 3 shows the schematic for a very thin 5V-to-3.3V
converter. For the LTC1265 to be able to source 500mA
output current and yet meet the height requirement, a
small value inductor must be used. The circuit operates
at a high frequency (typically 500kHz), increasing the
gate charge losses.
Conclusion
The LTC1265, with its low dropout and high efficiency,
is ideal for battery-operated products and efficiency
sensitive applications. In addition, its ability to operate
at high frequencies allows the use of small inductors
for size sensitive applications.
VIN
5V
1
13
0.1μF
+
PWR VIN PWR VIN
2
SW
VIN
14
4
3
5
6
3900pF
1k
7
LBIN
PGND
LBOUT
SGND
CT
SHDN
ITH
NC
SENSE–
* SUMIDA CLS62180
** KRL SL-C1-OR200J
†
AVX TAJB155K010
††
AVX TAJB225K06
SENSE+
VOUT
3.3V/500mA
0.2Ω**
L1*
18μH
+
D1
MBRS0520LT1
LTC1265-3.3
CT
51pF
CIN†
15μF
10V
2w
12
11
10
COUT††
22μF
0.3V
2w
SHUTDOWN
9
8
1000pF
SUMIDA 708-956-0660
KRL/BANTRY 603-668-3210
DN105 • F03
Figure 3. 2.5mm High 5V-to-3.3V Converter (500mA Output Current)
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn105f_conv LT/GP 0695 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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© LINEAR TECHNOLOGY CORPORATION 1995