DN52 - DC-DC Converters for Portable Computers

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DC/DC Converters for Portable Computers – Design Note 52
Steve Pietkiewicz
Jim Williams
Portable computers require simple and efficient converters for 5V power and display driving. A regulated 5V
supply can be generated from two “AA” cells using the
circuit shown in Figure 1. U1, an LT®1073-5 micropower
DC/DC converter, is arranged as a step-up, or “boost”
converter. The 5V output, monitored by U1’s SENSE pin,
is internally divided down and compared to a 212mV
reference voltage inside the device. U1’s oscillator
turns on when the output drops below 5V, cycling the
switch on and off at a 19kHz rate. This action alternately
causes current to build up in L1, then dump into C1
through D1, increasing the output voltage. When the
output reaches 5V, the oscillator turns off. The gated
oscillator provides the mechanism to keep the output
at a constant 5V. R1 invokes the current limit feature of
the LT1073, limiting peak switch current to 1A. U1 limits
switch current by turning off the switch when the current
reaches the programmed limit set by R1. Switch “on”
time, therefore, decreases as VIN is increased. Switch
“off” time is not affected. This scheme keeps peak
switch current constant over the entire input voltage
range, allowing maximum energy transfer to occur at
low battery voltage without exceeding L1’s maximum
current rating at high battery voltage.
The circuit delivers 5V at 150mA from an input range
of 3.5V to 2.0V. Efficiency measures 80% at 3.0V,
decreasing to 70% at 2.0V for load currents in the 15mA
to 150mA range. Output ripple measures 170mVP-P and
no-load quiescent current is just 135μA.
A –24V LCD bias generator is shown in Figure 2. In this
circuit U1 is an LT1173 micropower DC/DC converter.
The 3V input is converted to +24V by U1’s switch, L1, D1,
and C1. The switch pin (SW1) then drives a charge pump
composed of C2, C3, D2, and D3 to generate –24V. Line
regulation is less than 0.2% from 3.3V to 2.0V inputs.
Load regulation, although it suffers somewhat since
the –24V output is not directly regulated, measures 2%
from a 1mA to 7mA load. The circuit will deliver 7mA
from a 2.0V input at 73% efficiency.
If greater output power is required, Figure 2’s circuit
can be driven from a 5V source. R1 should be changed
to 47Ω and C3 to 47μF. With a 5V input, 40mA is available at 75% efficiency. Shutdown is accomplished by
bringing the anode of D4 to a logic high, forcing the
feedback pin of U1 to go above the internal reference
voltage of 1.25V. Shutdown current is 110μA from the
input source and 36μA from the shutdown signal.
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.
D1
L1*
1N5818
100μH
R1
100
L1*
100μH
+
SW1
I LIM
3V
2 w AA
CELL
U1
LT1173
D1
1N5818
VIN
SENSE
GND
SW2
GND
*TOKO 262LYF-0092K
†
SANYO OS-CON 16SA100
+
5V OUTPUT
150mA
C1
100μF†
C2
4.7μF
C1
0.1μF
+
SW2
D3
1N5818
3V
2w AA
CELL
OPERATE SHUTDOWN
118k**
D2
1N5818
D4
1N4148
+
C3
22μF
-5%/t
Figure 1. Two “AA” Cell to 5V Step-Up Converter
Delivers 150mA
07/91/52_conv
+
FB
SW1
U1
LT1073-5
2.21M**
VIN
I LIM
R1
47Ω
*TOKO 262LYF-0092K
**1% METAL FILM
–24V OUTPUT
-5%/t
Figure 2. DC/DC Converter Generates –24V from 3V to 5V
Current generation portables require back lit LCD displays using cold cathode fluorescent lamps (CCFLs).
Figure 3 provides 78% efficiency with full control over
lamp brightness .82% efficiency is possible if the LT1072
is driven from a low voltage (e.g., 3V to 5V) source. Additional benefits include a 4.5V to 20V supply range and
low radiated power due to sine wave based operation.
lamp life is enhanced because current cannot increase
as the lamp ages. Detailed information on this circuit
appears in LTC Application Note 45, “Measurement and
Control Circuit Collection.”
Data Sheet Download
www.linear.com
Linear Technology Corporation
9
7
D1
1N4148
L1
L1 and the transistors comprise a current driven Royer
class converter which oscillates at a frequency primarily set by L1’s characteristics and the 0.02μF capacitor.
LT1072 driven L2 sets the magnitude of the Q1-Q2 tail
current, and hence L1’s drive level. The 1N5818 diode
maintains current flow when the LT1072 is off.
The 0.02μF capacitor combines wth L1’s characteristics
to produce sine wave voltage drive at the Q1 and Q2
collectors. L1 furnishes voltage step-up, and about
1400VP-P appears at its secondary. Current flows
through the 33pF capacitor into the lamp. On negative
waveform cycles the lamp’s current is steered to ground
via D1. Positive waveform cycles are directed, via D2,
to the ground referred 562Ω to 50k potentiometer
chain. The positive half-sine appearing across these
resistors represents one-half the lamp current. This
signal is filtered by the 10k-1μF pair and presented to
the LT1072’s feedback pin. This connection closes a
control loop which regulates lamp current. The 2μF
capacitor at the LT1072’s VC pin provides stable loop
compensation. The loop forces the LT1072 to switchmode modulate L2’s average current to whatever value is
required to maintain a constant current in the lamp. The
constant current’s value, and hence lamp intensity, may
be varied with the potentiometer. The constant current
drive allows full 0-100% intensity control with no lamp
dead zones or “pop-on” at low intensities. Additionally,
LAMP
33pF
3kV
5
1
2
3
4
+
+VIN
10μF
D2
1N4148
C1
0.02μF
Q1
MPS650
1k
Q2
MPS650
1N5818
+VIN
4.5V TO 20V
562 Ω*
L2
300μH
5
6
VIN
E1
VSW
50k
INTENSITY
ADJUST
7
LT1072
8
E2
VFB
GND
1
VC
10k
3
+
+
1μF
2μF
C1 = MUST BE A LOW LOSS CAPACITOR.
METALIZED POLYCARB
WIMA FPK 2 (GERMAN) RECOMMENDED.
L1 = SUMIDA 6345-020 OR COILTRONIX CTX110092-1.
PIN NUMBERS SHOWN FOR COILTRONIX UNIT
L2 = COILTRONIX CTX300-4
* = 1% FILM RESISTOR
DO NOT SUBSTITUTE COMPONENTS
-5%/t
Figure 3. Cold Cathode Fluorescent Lamp Power Supply
For applications help,
call (408) 432-1900
dn52f_conv BA/GP 0791 180K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1991
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