DN268 - Tiny Buck Regulator Accepts Inputs from 3.6V to 25V and Eliminates Heat Sink

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Tiny Buck Regulator Accepts Inputs from 3.6V to 25V
and Eliminates Heat Sink – Design Note 268
Jeff Witt
An external resistor divider programs the output voltage to any value above the part’s 1.25V reference. The
shutdown mode reduces the supply current to 1μA and
disconnects the load from the input supply.
An internal 3.4V undervoltage lockout prevents switching at low input supply. The LT1616 will also withstand
a shorted output. A fast current limit protects the circuit
in overload and limits output power; when the output
voltage is pulled to ground by a hard short, the LT1616
reduces its operating frequency to limit dissipation and
peak switch current.
5
VIN
D2
1
BOOST
C3
L1
0.01μF 10μH
LT1616
OFF ON
4
SHDN
GND
2
6
SW
FB
R1
16.5k
3
C1
1μF
VOUT
3.3V
300mA: VIN = 4.5V TO 25V
400mA: VIN = 4.7V TO 25V
D1
C2
10μF
R2
10k
C1: TAIYO-YUDEN TMK316BJ105
(408) 573-4150
C2: TAIYO-YUDEN JMK316BJ106ML
D1: ON SEMICONDUCTOR MBR0530 (602) 244-6600
D2: 1N4148
L1: SUMIDA CR43-100
(847) 956-0667
DN269 F01
FOR 5V OUT, CHANGE R1 TO 30.1k AND L1 TO 15μH
Figure 1. The LT1616 Application Accepts an Input from
4.5V to 25V and Produces an Output of 3.3V at Up to
400mA. The Circuit is Easily Modified for 5V Output
100
VOUT = 3.3V
90
VIN = 12V
VIN = 5V
80
EFFICIENCY (%)
Complete Switcher in ThinSOT Results in
Compact Solution
Several features of the LT1616 enable this combination of small size and large voltage range. The high
(1.4MHz) switching frequency allows the use of small
inductors and capacitors. The current mode control
circuit with its internal loop compensation eliminates
additional components and handles a wide variety of
output capacitors, including ceramic capacitors. The
internal NPN power switch drops just 200mV at 300mA.
VIN
4.5V TO 25V
VIN = 20V
70
60
50
40
30
0
100
200
300
400
LOAD CURRENT (mA)
500
DN269 F02a
Figure 2a. Efficiency of Figure 1’s Circuit, Output = 3.3V
100
VOUT = 5V
VIN = 8V
90
VIN = 12V
80
EFFICIENCY (%)
Introduction
The LT®1616 is a complete fixed-frequency step-down
switching regulator in a ThinSOT™ (1mm thick SOT-23)
package. It meets the needs of circuit designers who
require a large input voltage range or the smallest
solution possible. The LT1616 accepts an input from
3.6V to 25V, produces a low voltage output at 400mA
and occupies less than 0.15in2 of board space. With
this wide input range, the LT1616 can regulate a large
variety of power sources, from 4-cell alkaline batteries to lead-acid automobile batteries, from 5V logic
supplies to unregulated AC adapters. The LT1616 is
an ideal replacement for bulky (and potentially hot)
TO-220 linear regulators.
VIN = 24V
70
60
50
40
30
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
0
100
200
300
400
LOAD CURRENT (mA)
500
DN269 F02b
Figure 2b. Efficiency of Figure 1’s Circuit, Output = 5V
09/01/268_conv
The LT1616 Produces 3.3V at 400mA
Figure 1 shows a typical application of the LT1616. This
circuit generates 3.3V at 300mA from an input of 4.5V
to 25V. From a slightly more restricted input range of
4.7V to 25V, it will supply 400mA to the load. Figure 2
shows the circuit’s operating efficiency at several input
voltages (it also shows the efficiency for a 5V output).
This wide input range allows you to generate a local
3.3V logic supply from just about any source available.
Ceramic Capacitors are Best
The LT1616’s ability to work with ceramic capacitors
is a significant advantage. Where achieving low output
ripple from a switching regulator is concerned, low
equivalent series resistance (ESR) is the most important
characteristic of a capacitor. For a given package size or
capacitance value, a ceramic capacitor will have lower
ESR than other bulk, low ESR capacitor types (including
tantalum, aluminum and organic electrolytics). With
its high switching frequency, the LT1616 requires less
than 10μF of capacitance at the output. At this value,
ceramics are both smaller and lower in cost than the
competing low ESR capacitors.
To summarize, using ceramics results in low noise
outputs and a small circuit size. Figure 3 shows the
good transient response of the circuit in Figure 1. The
output recovers from a load current step in less than
30μs, without ringing. Because the time scale of 50μs
per division is much longer than the LT1616’s switching
period, the output ripple at the switching frequency is
not directly visible. The ripple appears as a slight broadening of the upper trace and amounts to just 5mVP-P.
Smaller than a TO-220
The small package size and high operating frequency of
the LT1616 results in a very small circuit size. In most
applications, the LT1616 circuit will occupy less space
than a linear regulator performing the same task and
will dissipate much less power. For example, an LT1616
circuit converting 12V to 3.3V at 300mA dissipates only
250mW. A linear regulator will dissipate 2.6W, requiring a TO-220 style package and either moving air or a
heat sink to get rid of the heat. Figure 4 compares the
size of the LT1616 solution with a TO-220 package. The
circuit on the left is designed for a maximum input of
16V and an output of 350mA. The circuit on the right
is designed for a maximum input of 25V (requiring a
physically larger input capacitor) and uses a larger
inductor to keep the efficiency high at its maximum
load current of 400mA. Both circuits are low profile,
with a maximum height of 2.2mm for the lower cost
circuit on the left and 2mm for the circuit on the right.
Figure 4. Tired of the Heat and Bulk of Linear
Regulators? Switch! The Entire LT1616 Circuit
Occupies Less Space Than a TO-220
2.5V Output
Figure 5 shows a 2.5V output circuit using the LT1616.
The input range is limited on the low end by the undervoltage lockout (3.6V max) and on the high end by the
voltage rating of the capacitors used and the maximum
voltage rating of the BOOST pin.
VOUT
20mV/DIV
D2
250mA
ILOAD
VIN
3.6V TO 16V
5
100mA
VIN
BOOST
1
C3
0.01μF
LT1616
OFF ON
50μs/DIV
4
GND
DN268 F03
Figure 3. The LT1616 Gets Along Fine with Ceramic
Capacitors, Resulting in Good Transient Response and
Low Output Ripple (~5mVP-P). The Upper Trace Shows
Output Voltage During a Stepped Load Current
(Circuit of Figure 1 with VIN = 10V)
SHDN
2
C1
1μF
16V
SW
FB
3
6
R1
10k
R2
10k
L1
4.7μH
VOUT
2.5V
350mA
D1
C2
4.7μF
6.3V
C1: TAIYO-YUDEN EMK212BJ105MG (408) 573-4150
C2: TAIYO-YUDEN JMK212BJ475ML
D1: ON SEMICONDUCTOR MBR0520 (602) 244-6600
D2: 1N4148 OR EQUIVALENT
L1: MURATA LQH3C4R7M24
(814) 237-1431
GND
DN269 F05
Figure 5. This Circuit Produces 2.5V at 350mA
from an Input Range of 3.6V to 16V
Data Sheet Download
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