DN78 - Triple Output 3.3V, 5V, and 12V High Efficiency Notebook Power Supply

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Triple Output 3.3V, 5V, and 12V High Efficiency
Notebook Power Supply – Design Note 78
Randy G. Flatness
The operating current levels for both regulator sections are user programmable, via external current
sense resistors, to set current limit. A wide input
voltage range for the LTC1142 allows operation
from 4V to 16V. The LTC1142HV extends this voltage range to 20V, permitting operation with up to
12-cell battery packs.
Both regulator blocks in the LTC1142 and LTC1142HV
use a constant off-time current mode architectures
with Burst Mode operation. This results in a power
supply that has very high efficiency over a wide load
current range, fast transient response, and very low
dropout. The LTC1142 is ideal for applications requiring 5V and 3.3V output voltages with high conversion
efficiencies over a wide load current range in a small
amount of board space.
The application circuit in Figure 2 is configured to
provide output voltages of 3.3V, 5V, and 12V. The
current capability of both the 3.3V and 5V outputs
is 2A (2.5A peak). The logic controlled 12V output
can provide 150mA (200mA peak), which is ideal for
flash memory applications. The operating efficiency
shown in Figure 1 exceeds 90% for both the 3.3V
and 5V sections.
100
LTC1142
VIN = 8V
5V SECTION
95
90
EFFICIENCY (%)
The new LTC®1142 is a dual 5V and 3.3V synchronous
step-down switching regulator controller featuring
automatic Burst Mode® operation to maintain high
efficiencies at low output currents. Two independent
regulator sections, each driving a pair of complementary MOSFETs, may be shut down separately to
less than 20μA /output. This feature is an absolute
necessity to maximize battery life in portable applications. Additionally, the input voltage to each
regulator section can be individually connected to
different potentials (20V maximum) allowing a wide
range of novel applications.
85
LTC1142
VIN = 8V
3.3V SECTION
80
75
70
65
60
1
10
100
1000 2500
OUTPUT CURRENT (mA)
%/t'
Figure 1. LTC1142 Efficiency
The 3.3V section of the circuit in Figure 2 is comprised
of the main switch Q4, synchronous switch Q5, inductor L1, and current shunt RSENSE3. The current
sense resistor RSENSE monitors the inductor current
and is used to set the output current according to
the formula IOUT = 100mV/RSENSE. Advantages of
current control include excellent line and load transient rejection, inherent short-circuit protection and
controlled start-up currents. Peak inductor currents
for L1 and T1 of the circuit in Figure 2 are limited
to 150mV/RSENSE or 3.0A and 3.75A respectively.
When the output current for either regulator section
drops below approximately 15mV/RSENSE, that section automatically enters Burst Mode operation to
reduce switching losses. In this mode the LTC1142
holds both MOSFETs off and sleeps at 160μA supply current while the output capacitor supports the
load. When the output capacitor discharges 50mV,
the LTC1142 briefly turns this section back on, or
“bursts” to recharge the output capacitor. The timing capacitor pins, which go to 0V during the sleep
interval, can be monitored with an oscilloscope to
observe burst action. As the load current is decreased
the circuit will burst less and less frequently.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
01/94/78_conv
The timing capacitors CT3 and CT5 set the off-time
according to the formula tOFF = 1.3 × 104 × CT. The
constant off-time architecture maintains a constant
ripple current while the operating frequency varies
with input voltage. The 3.3V section has an off-time of
approximately 5μs resulting in a operating frequency
of 120kHz at 8V input voltage. The 5V section has
an off-time of 3.5μs and a switching frequency of
107kHz at 8V input voltage.
The LTC1142 synchronous switch removes the normal limitation that power must be drawn from the
primary 5V inductor winding in order to extract power
from the auxiliary winding. With synchronous switching the auxiliary 12V output may be loaded without
regard to the 5V primary output load providing that
the loop remains in continuous mode operation.
When the 12V output is activated by a TTL high (6V
maximum) on the 12V enable line, the 5V section
of the LTC1142 is forced into continuous mode. A
resistor divider composed of R1, R5 and switch Q1
forces an offset subtracting from the internal 25mV
offset at pin 14. When this external offset cancels the
built-in 25mV offset Burst Mode operation is inhibited.
The operation of the 5V section is identical to the 3.3V
section with inductor L1 replaced by transformer T1.
The 12V output voltage is derived from an auxiliary
winding on the 5V inductor T1. The output from this
additional winding is rectified by diode D3 and applied
to the input of an LT1121 regulator. The 12V output
voltage is set by resistors R3 and R4. A turns ratio of
1:1.8 is used for T1 to ensure that the input voltage
to the LT1121 is high enough to keep the regulator
out of dropout while maximizing efficiency.
22μF
VIN
(6.5V TO 35V
× 2
18V)
+
+
24
VIN3
VOUT3
3.3V/2A
23
L1
33μH
RSENSE3
0.05Ω
1
2
P-DRIVE 5
SENSE + 3
SENSE + 5
+
SENSE
SENSE
–5
N-DRIVE 5
P-GND3 S-GND3 CT3
4
3
25
ITH3
ITH5
27
510Ω
13
T1
CT5
S-GND5 P-GND5
17
11
18
1.8 T
30μH
RSENSE5
0.04Ω
R2
100<
R1
100Ω
VOUT5
5V/2A
15
1000pF
–3
N-DRIVE 3
6
Q5
Si9410DY
220μF
10V
× 2
Q2
Si9430DY
9
LTC1142HV
28
D1
MBRS140
10
VIN5
SHUTDOWN 5
P-DRIVE 3
2000pF
22μF
35V
× 2
1μF
16
SHUTDOWN 3
+
+
0V = NORMAL
>1.5V = SHUTDOWN
1μF
Q4
Si9430DY
For additional high efficiency circuits see Application Note 54.
14
D2
MBRS140
20
Q3
Si9410DY
Q1
VN7002
R5
18k
510Ω
+
220μF
10V
× 2
CT3
3300pF 3300pF CT5
270pF
390pF
12V ENABLE
0V = 12V OFF
>3V = 12V ON
(6V MAX)
12V/150mA
RSENSE3:
RSENSE5:
L1:
T1:
KRL SL-1R050J
KRL SL-1R040J
COILTRONICS CTX-33-4
PRIMARY = 30RH
PRIMARY: SECONDARY = 1:1.8
DALE LPE-6562-A026
22μF
25V
+
+
20pF
VOUT
R3
660k
SHUTDOWN
ADJ
R4
300k
LT1121
D3
MBRS140
22μF
35V
22Ω
1000pF
VIN
DN78 • F02
GND
Figure 2. LTC1142 Triple Output High Efficiency Power Supply
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 361
dn78f_conv LT/TP 0194 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 1994
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