DN71 - Regulator Circuit Generates Both 3.3V and 5V Outputs from 3.3V or 5V to Run Computers and RS232

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Regulator Circuit Generates Both 3.3V and 5V Outputs from
3.3V or 5V to Run Computers and RS232 – Design Note 71
David Dinsmore and Richard Markell
Many portable microprocessor-based systems use a
mix of 3.3V and 5V circuits. Some are still using only
5V and inevitably some systems will end up being
solely 3.3V based. If accessories are to be plugged
into, or connected to any of these systems, a voltage
conversion/power generation problem presents itself.
The circuit shown in Figure 1 addresses the situation
where either 5V or 3.3V power is available from the
bus, but the accessory needs both 5V and 3.3V power.
above the supply voltage and efficiently turns on and
off the appropriate MOSFETs.
IC2 also forms a flying capacitor buck/boost DC/DC
converter circuit. This topology is used so that no
transformers are necessary. Q1 is used to control this
section’s voltage (V1). When VIN is at 5V, Q1 is off, forcing this section to operate as a step-down converter.
It produces 3.3V which is sent to the 3.3V output of
the circuit through IC4B. In this state, 5V power is sent
directly through IC3A while IC3B and IC4A are off.
The circuit consists of two sections, one being a DC/DC
converter and the other being a pair of dual N-channel
MOSFETs and their associated high side drivers that
effectively form a DPDT switch.
When VIN is at 3.3V, IC1 turns on Q1 shorting out the
140k resistor and forcing the DC/DC converter into
step-up mode so that it generates 5V at V1 which is
sent to the 5V output through IC3B, while 3.3V power
is sent from input to output through IC4A. IC3A and
IC4B are off.
When first powered up, a comparator inside of the
LT®1111 (IC2) determines the state of the circuit. The
comparator’s output (IC2, Pin 6) is wired to the input
of the LTC1157 MOSFET driver (IC1). The LTC1157
internally generates a gate drive voltage which is 8.8V
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.
1
IC3A
2
3
IC3B
4
3.3V OR 5V
1M
1M
VS
IN1
IN2
G2
GND
2N7002
120μF/10V
*20μH
1
1M
+
6
180μF
6V
7
430k
ILIM
2
3
VIN SW1
AO
SW2
LT1111
IC2
SET
FB
GND
5
3
Si9956DY
V1
ZTX869-M1
174k
1%
8
47Ω
IC4A
2
5,6
IC4B
4
Q1
2N7002
*20μH
4 39Ω
105k
1%
140k
1%
+
100μF
6V
%/t'
*CTX20-3 COILTRONICS
COILTRONCIS (305) 781-8900
ZTX = ZETEX (516) 543-7100
Figure 1
06/93/71_conv
IC3
Si9956DY
1
MBRS12OT3
+
5V/150mA
5,6
7,8
G1
LTC1157
IC1
7,8
3.3V/150mA
IC4
No load quiescent current is about 500μA. By replacing the LT1111 with the lower frequency LT1173 this
could be reduced to 315μA, at the expense of a larger
inductor size.
Overall efficiency of the circuit exceeds 80% with VIN
= 3.3V and 86% with VIN = 5V. All components are
available in surface mount.
Mixed 3.3V and 5V RS232 Operation
Portable computers also require RS232 interfacing
circuitry for inter-computer and mouse interfacing
applications. Most portable computers now use a mix
of 3.3V and 5V logic. Linear Technology offers a wide
variety of interfacing circuits that can, not only work
with these voltages, but upgrade to single 3.3V supplies
when that is required.
Figure 2 shows the LT1330, a 3-driver/5-receiver, PC
compatible, RS232 interface running on both 3.3V and
5V supplies. The LT1330’s charge pump power is taken
from the 5V supplies maximizing the RS232 transmitters
load driving capability. The center trace of the photo
demonstrates the ability of the transmitters to drive
a 3000Ω/2500pF load at 120k Baud. The drive level
shown here are –6V to 7V when fully loaded.
The LT1330’s receivers are powered by the 3.3V supply
on Pin 14. This allows the logic levels to be compatible
with either TTL or 3.3V logic since the output logic levels
are typically 0.2V to 2.7V. Logic inputs to the transmitters respond to TTL levels, so they can be driven from
either 3.3V or 5V logic families.
w˜'
DRIVER 1 OUT
RX1 IN
%3*7&3065
39*/
RX3 IN
RX4 IN
DRIVER 3 OUT
RX5 IN (LOW-Q)
0/0''
3.3V
-5
3
4
5
6
7
8
9
19
11
18
17
13
16
14
15
LT1342
LT1330
LT1331
LTC1327
ESD
Protection
±10kV
±10kV
±10kV
±10kV
3V Logic
Interface
✓
✓
✓
✓
Power Supply
3V/5V
3V/5V
3V, 5V or
3V/5V
3V
Supply
Current in
SHUTDOWN
1μA
60μA
60μA
1μA
✓
✓
✓
✓
Receiver
Active in
SHUTDOWN
Driver Disable
✓
External
Capacitors
0.1, 0.2μF 0.1, 0.2, 1μF 0.1, 0.2μF
Rx Output
(Typ)
0.2V-2.7V
0.2V-2.7V
RS232 Tx
Compliant
✓
✓
RS232 Tx
Compatible
✓
✓
0.1μF
0.2V-2.7V
0V-3.3V
✓
✓
V–
+
+
TO
LINE
1
5V VCC
w˜'
+
V+
˜'
When the entire system can be operated on 3.3V, an
LT1331 may be directly substituted for the LT1330. The
LT1331 can be operated at 120k Baud with the only
limitation being transmitter output levels are –3.5V to
4V. While these levels are not RS232 compliant, they
can be used to interface with all known RS232/RS562
systems. In all cases the LT1331 operated at 3.3V would
provide a reliable communications link. The table below
shows the details of 3-driver/5-receiver RS232 transceivers for 3.3V and mixed 5V/3.3V systems.
˜'
RX OUT
VL = 3.3V
DRIVER 1 IN
RX1 OUT
%3*7&3*/
39065
DR OUT
RL = 3k
DL = 2500pF
TO LOGIC
RX3 OUT
RX4 OUT
DRIVER 3 IN
RX5 OUT (LOW-Q)
GND
DRIVER
DISABLE
INPUTS
120kbps
RING DETECT IN
˜$0/530--&303
˜130$&4403
DN71 F02b
SHUTDOWN
CONTROL OUT
%/t'B
Figure 2. LT1330 Mixed 5V/3V Operation
Data Sheet Download
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dn71f_conv LT/GP 0693 190K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 1993
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