5V High Current Step-Down Switchers - Design Note 59
Ron Vinsant and Milton Wilcox
volt • second balance is assured by the duty cycle limit
of 50% inherent in the LT1241. The output inductor (L1)
is made of Magnetics Kool-Mu material and is only 0.7
inches in diameter.
Low Cost High Efficiency (80%), High Power
Density DC/DC Converter
The LT®1241 current mode PWM control IC can be used
to make a simple high frequency step-down converter.
This converter also has low manufacturing costs due
to simple magnetic components. This circuit exhibits
a wide input range of 30V to 60V while maintaining its
12A 5V output. It has short-circuit protection and uses
minimal PC board area due to its 300kHz switching
frequency.
Short-circuit protection is provided through bootstrap operation of the LT1241. If the output is shorted
the LT1241 limits its pulse width to ≤250ns. Because
there is not enough current supplied to make the aux
winding on the output inductor 15V, the LT1241 stops
operation. It will then try to start by C11 charging through
R4. If the output is still shorted it will stop again. Thus
in a short, the circuit starts and stops, protecting itself
from overload.
Figure 1 shows the LT1241 being used to drive the
switching transistor Q1 through a ferrite pulse transformer T2. This transformer is built on a high μ material resulting in an 11 turn bifilar wound toroid that
is only 0.15 inches in diameter and can be surface
mounted. T1 acts as a current sense transformer whose
+
INPUT
7o7
C5
'
7
-9'
D4
R9
36Ω .63
5
4
5 3
4
563/40/
,00-.6
N"
5 CORE
8
2
+
$
C4
' '
25V
5
#3
#(9"
COILTRONICS
$59
8
T2
'&&%#"$,
C6
'
3
L
4
8
4
C2
'
R2
L
'#
COMP OUTPUT
V3&'
RT/CT
ISENSE
GND
5
R5
L
4
'&&%#"$,
+
C7
'
7
-9'
+
C3
'
6
+
3
R3
8Ω
D2
.#3$5-
%
BAT85
$
Q'
/05&46/-&4405)&38*4&41&$*'*&%
"--3&4*45"/$&4"3&*/0).48
"--$"1"$*5"/$&4"3&*/.*$30'"3"%47
.#3$5-.06/5*/(5"#*45*&%505)&%&7*$&4$"5)0%&4*/5&3/"--:
5.&5(-"4.11"'$03&
Figure 1
06/92/59_conv
7
"
$
'
7
-9'
VCC
-5
6T
D6
.63
7
2
7
"50"
-
3
Ā
2
3
D5
/"
2
*3'
8
–
R4
L
–
7
'30.-
5
5
+
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.
C8
'
7
-9'
R8
L
RETURN
= POWER GND
= SIGNAL GND
%/t'
Synchronous Switching Eliminates Heat Sinks in
a 50W DC/DC Converter
The new LT1158 half-bridge N-channel power MOSFET
driver makes an ideal synchronous switch driver to
improve the efficiency of step-down (buck) switching
regulators. The diode losses in a conventional stepdown regulator become increasingly significant as VIN
is increased. By replacing the high current Schottky
diode with a synchronously-switched power MOSFET,
efficiencies well over 90% can be realized (see Figure 2).
(2) 28mΩ power MOSFETs for each switch, reducing
individual device dissipation to 0.7W worst case. This
eliminates the need for heat sinks for operation up to
10A at a temperature of 50°C ambient. The inductor
and current shunt losses for the Figure 3 circuit are
1.2W and 0.7W respectively at 10A.
An additional loss potentially larger than those already
mentioned results from the gate charge being delivered
to multiple large MOSFETs at the switching frequency.
This driver loss can only be controlled by running the
oscillator at as low a frequency as practical — in the case
of the Figure 3 circuit, 25kHz. A very low ESR (<20mΩ)
output capacitor is used to limit output ripple to less
than 50mVp-p with 2.5Ap-p ripple current.
In the Figure 3 circuit an LT3525 provides a voltage mode
PWM to drive the LT1158 input pin. The LT1158 drives
100
VIN = 12V
The LT1158 also provides current limit for DC/DC converter applications. When the voltage across RS exceeds
110mV, the LT1158 fault pin conducts, and assumes
control of the PWM duty cycle. This provides true current mode short-circuit protection with soft recovery.
The Figure 3 regulator current limit is set at 15A which
raises the dissipation in each bottom MOSFET to 1.7W
during a short. Therefore 30°C/W heat sinking must
be added for the bottom side MOSFETs if continuous
short-circuit operation is required. Care should also
be taken when routing the sense+ and sense– leads
to prevent coupling from the inductor.
EFFICIENCY (%)
90
VIN = 24V
80
70
60
0
2
6
4
8
10
OUTPUT CURRENT (A)
%/t'
Figure 2. Operating Efficiency for Figure 3 Circuit
'
L
'
77*/
L
L
NI
INV
VREF
+
'
COMP
V+
VC
V+
+
'
BIAS
A
T FB
-5
IN
/
O'
RS
7mΩ
7"
OUT
SEN+
+
SEN–
COUT
'
*
B DR
FAULT
+
-
)
L
SRC
L
/
40'545
CIN
'
(2) IRFZ44
B
RT
+
(2) IRFZ44
*
T DR
'
CT
%*4$)"3(&
'
BST
BST
DR
V+
LT3525
'
/
.#3
B FB
'
27k
Ā
CINCOUT6/*5&%$)&.*$0/-9'7#.9-RS = 7*4)":6-530/*93$4
* = PARALLELED MOSFETS MAY REQUIRE INDIVIDUAL GATE DECOUPLING
3&4*45034$0/46-5."/6'"$563&343&$0..&/%"5*0/4
-."(/&5*$4.11"$03&563/4("8*3&
%/t'
Figure 3. High Efficiency 50W DC/DC Converter
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn59f_conv BA/GP 0492 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1992