DN183 - The LT1370: A 500kHz, 6A Monolithic Boost Converter

advertisement
A 500kHz, 6A Monolithic Boost Converter – Design Note 183
Karl Edwards
Circuit Description
The LT1370 is a current mode switcher. This means that
switch duty cycle is directly controlled by the switch
current rather than by the output voltage. This technique has several advantages: immediate response to
input voltage variations, greatly simplified closed-loop
frequency compensation, and pulse-by-pulse current
limiting, which provides maximum switch protection.
An internal low dropout regulator provides a 2.3V supply
to all control circuitry. This low dropout design allows
the input voltage to vary from 2.7V to 30V with virtually
no change in device performance. An internal 500kHz
oscillator is the basic clock for all timing. A bandgap
provides the reference for the feedback error amplifier.
As with the LT1371, error amplifier circuitry allows the
LT1370 to directly regulate negative output voltages.
The NFB pin regulates at –2.48V, while the amplifier’s
output internally drives the FB pin to 1.245V. The error
amplifier is a current output (gm) type, so its output
voltage, present on the VC pin, can be externally clamped
to lower the current limit. A capacitor-coupled external
clamp provides soft start.
The S/S pin has two functions: synchronization and
shutdown. The internal oscillator can be synchronized
07/98/183_conv
to a higher frequency by applying a TTL square wave
to this pin. This allows the part to be synchronized to
a system clock. If the S/S pin is held low, the LT1370
will enter shutdown mode. In this mode, all internal
circuitry is disabled, reducing supply current to 12μA.
An internal pull-up ensures start-up when the S/S pin
is left open circuit.
5V to 12V Boost Converter
Figure 1 shows a typical 5V to 12V boost application.
The high 6A switch rating permits the circuit to deliver up to 24W. Figure 2 shows the overall converter
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.
5V
L1*
D1
MBRD835L
VIN
ON
OFF
S/S
VOUT†
12V
VSW
R1
53.6k
1%
FB
+
LT1370
+
C1**
22μF
25V
VC
GND
R2
6.19k
1%
C2
0.047μF
R3
2k
C4**
22μF
25V
w2
C3
0.0047μF
DN183 F01
†MAX I
*COILTRONICS
OUT
(561) 241-7876
L1
IOUT
UP2-4R7 (4.7μH)
4.7μH 1.8A
UP4-100 (10μH)
10μH 2.0A
**AVX TPSD226M025R0200
Figure 1. 5V to 12V Boost Converter
95
VIN = 5V
90
85
EFFICIENCY (%)
Complementing and expanding on the current LT®1371/
LT1372 family of 500kHz switchers, Linear Technology
introduces the LT1370, a 6A boost converter. A high
efficiency switch is included on the die, along with all
the oscillator, control and protection circuitry necessary
for a complete switching regulator. This part combines
the convenience and low parts count of a monolithic
solution with the switching capabilities of a discrete
power device and controller. At 0.065Ω on-resistance,
42V maximum switch voltage and 500kHz switching
frequency, the LT1370 can be used in a wide range of
output voltage and current applications. Only a few
surface mount components are needed to complete a
small, high efficiency DC/DC converter. LT1370 features
include current mode operation, external synchronization and low current shutdown mode (12μA typical).
80
75
70
65
60
0
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
DN183 F02
Figure 2. 12V Output Efficiency
efficiency. Notice that peak efficiency is 90%; efficiency
stays above 86% at the circuit’s maximum 2A output
current. The inductor needs to be chosen carefully to
meet peak current values. The output capacitor can
see high ripple currents—often, as in this application,
higher than the ripple rating of a single capacitor. This
requires the use of two surface mount tantalums in
parallel; both capacitors should be of the same value
and manufacturer. The input capacitor does not have to
endure such high ripple currents and a single capacitor
will normally suffice. The catch diode, D1, must be rated
for the output voltage and average output current. The
compensation capacitor, C2, normally forms a pole in
the 2Hz to 20Hz range, with a series resistor, R3, to
add a zero at 1kHz to 5kHz. The S/S pin in this example
is driven by a logical on/off signal, a low input forcing
the LT1370 into its 12μA shutdown mode.
Positive to Negative Converter
The negative feedback (NFB) pin, enables negative output regulators to be designed with direct feedback. In
the circuit shown in Figure 3, a 2.7V to 13V input, –5V
output converter, the output is monitored by the NFB pin
and a simple divider network. No complex level shifting
or unusual grounding techniques are required. The S/S
pin is used to synchronize the switching frequency to
a 600kHz external clock signal.
VIN
2.7V TO 13V
+
C1
100μF
VIN
VSW
S/S
600kHz
LT1370
T1*
2
D2
P6KE-15A
D3
1N4148 1 t
t4
GND
C2
0.047μF
R1
2k
C3
0.0047μF
C4
100μF
w2
3
D1
MBRD835L
NFB
VC
+
*BH ELECTRONICS
501-0726
†MAX I
OUT
IOUT VIN
1.75A 3V
2.25A 5V
3.0A 9V
R2
2.49k
1%
–VOUT†
–5V
R3
2.49k
1%
DN183 F03
Figure 3. Positive to Negative Converter
with Direct Feedback
The switch clamp diodes, D2 and D3, prevent the leakage spike from the transformer, T1, from exceeding the
switch’s absolute maximum voltage rating. The Zener
voltage of D2 must be higher than the output voltage,
but low enough that the sum of input voltage and clamp
voltage does not exceed the switch-voltage rating.
Data Sheet Download
www.linear.com
Linear Technology Corporation
5V SEPIC Converter
Figure 4 is an example of a SEPIC converter. The SEPIC
topology has the advantage of an input voltage range
that extends both above and below the output voltage. In
Figure 4, the batteries can be at a charge level from 9V
to below 4V while maintaining a fixed 5V output. Also,
there is no direct path from input to output. When the
S/S pin is grounded, forcing the LT1370 into shutdown,
there is no leakage into the output. In shutdown, battery current is reduced to 12μA, the input current of
the LT1370. The magnetic coupling of inductors L1A
and L1B is not critical for operation, but generally they
are wound on the same core. C2 couples the inductors
together and eliminates the need for a switch snubber
network.
VIN**
4V TO 9V
L1A*
6.8μH
VIN
OFF
ON
t
VSW
S/S
C1
33μF
20V
FB
GND
t
VC
+
L1B*
6.8μH
R1
2k
C4
0.047μF
VOUT†
5V
R2
18.7k
1%
C2
4.7μF
LT1370
+
D1
MBRD835L
R3
6.19k
1%
C5
0.0047μF
C3
100μF
10V
w2
DN183 F04
C1 = AVX TPSD 336M020R0200
C2 = TOKIN 1E475ZY5U-C304
C3 = AVX TPSD107M010R0100
* BH ELECTRONICS 501-0726
(612) 894-9590
INPUT VOLTAGE MAY BE GREATER OR
**
LESS THAN OUTPUT VOLTAGE
†MAX I
OUT
IOUT VIN
1.8A 4V
2A 5V
2.6A 7V
2.9A 9V
Figure 4. Two Li-Ion Cells to 5V SEPIC Converter
Conclusion
With its low resistance switch, 6A operating current and
500kHz operation, the LT1370 is ideal for small, low
parts count, high current applications. Its high switching
frequency removes the need for large bulky magnetics
and capacitors. Compared to a separate control device
and power switch, the LT1370’s monolithic approach
simplifies the design effort, allows operation at lower
input voltages and reduces the board space required
to implement a complete DC/DC converter.
For applications help,
call (408) 432-1900
dn183f_conv LT/TP 0798 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1998
Similar pages