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Reduce Component Count and Improve Efficiency in
SLIC and RF Power Supplies – Design Note 282
Tick Houk
Introduction
Nonisolated power supplies with negative output
voltages require op amps and other “glue” circuitry in
order to level-shift the feedback voltage to the error
amplifier. In applications such as SLIC (subscriber line
interface circuit) power supplies, this extra circuitry
adds unwanted cost and requires more space on the
PC board, with no increase in system performance.
A Dual Output SLIC Supply With Simplified
Feedback Using the LTC3704
Figure 1 illustrates a dual output telecom power supply
designed for use in subscriber line interface circuits
or SLICs. The input to the SLIC power supply is some
form of battery (lead-acid or lithium-ion, for example)
so that extended talk-time can be provided to POTS
(plain old telephone system) phones during an AC line
failure (or rolling blackout). The output voltage is typically proportional to the distance the subscriber line
runs from the local hub to a house or office, in order
to compensate for the impedance of the loop. Multiple
output supplies are used to supply groups of users at
different distances from the hub.
The LTC ®3704 is a high performance, single-ended
current mode DC/DC controller IC that allows the user
to directly sense a negative output voltage using the
NFB pin. A unity-gain inverting amplifier within the
IC, combined with a precision bandgap voltage reference, give users a –1.230V input that can be directly
connected to a resistor divider between the negative
output voltage and ground. Additional features, such
as No RSENSE ™ current mode control, programmable
operating frequency (50kHz to 1MHz), synchronization
capability and a wide input range (2.5V to 36V) improve
efficiency and greatly ease design. The LTC3704 is
housed in a small 10-lead MSOP package.
This SLIC power supply takes advantage of the negative
feedback input (NFB) on the LTC3704 and therefore,
eliminates the need for an additional op amp. The
–24V output is regulated directly and the –72V output
is obtained by stacking additional windings on the
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and No RSENSE is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
GND
t
D1 +
4
+
CC2
100pF
VIN
7V TO 12V
UV + = 5.4V
UV – = 5.0V
R2
150k
1%
R1
49.9k
1%
CIN
220μF
16V
TPS
VOUT1
–24V
200mA
t
T1
1, 2, 3
NFB
t
6
INTVCC
FREQ
GATE
MODE/SYNC
RFB1
2.49k
1%
D3
C5
10μF
25V
X5R
t
LTC3704
RT
120k
C4
10μF
25V
X5R
+
COUT
3.3μF
100V
VIN
ITH
CC1
1nF
D2 +
5
SENSE
RUN
RC
82k
C3
10μF
25V
X5R
f = 200kHz
RFB2
45.3k
1%
GND
M1
+
C1
4.7μF
X5R
RS
0.012Ω
+
VOUT2
–72V
200mA
C2
4.7μF
50V
X5R
DN282 F01
D1 TO D3: INTERNATIONAL RECTIFIER 10BQ060
M1: INTERNATIONAL RECTIFIER IRL2910S
T1: COILTRONICS VP5-0155 (PRIMARY = 3 WINDINGS IN PARALLEL)
Figure 1. Dual Output SLIC Supply with the LTC3704 Simplifiers Feedback
04/02/282_conv
–24V output. The –24V output of this supply uses one
secondary winding in a SEPIC-type configuration in
order to reduce the input ripple current, whereas the
–72V output uses the other two windings in a conventional flyback mode. A standard, 6-winding VERSA-PAC
transformer (VP5-0155) was used, with 3 windings
connected in parallel on the primary in order to satisfy
the high current demand (the peak switch current can
be almost 4A at an input of 7V). A 5.2V LDO within the
LTC3704 provides a regulated supply for the gate driver
which is capable of driving very large power MOSFETs
(50nC to 100nC).
Improved Battery Protection Using the LTC3704’s
Programmable Undervoltage Lockout
With most lead-acid or lithium-ion chemistries, deep
discharging of the battery cells can severely reduce the
life of the battery. As a result, it is important to monitor
the battery and turn the converter off before the battery
voltage reaches an unsafe level. In Figure 1, the RUN
pin on the LTC3704 monitors the battery voltage using
resistors R1 and R2 and turns off the power supply
when the input supply drops below 5V. A hysteresis
level of 8% is provided in order to increase noise
immunity (UV+ is 5.4V).
A Current Mode, –8.0V, 1.2A RF Power Supply
with No Current Sense Resistor
The design illustrated in Figure 2 takes advantage of
LTC’s proprietary No RSENSE technology to provide true
current mode control without a discrete current sense
resistor. The voltage drop across the power MOSFET
is sensed during the on-time, thereby providing the
control loop with a “lossless” method of measuring the
switch current. This technique provides the maximum
efficiency possible for a single-ended current mode
converter, reduces board space and reduces the overall
cost of the power supply in applications where the drain
of the power MOSFET is less than 36V (the absolute
maximum rating of the SENSE pin). It should be noted
that the output voltage and maximum output current
of this supply can easily be scaled by the choice of the
components around the chip without modifying the
basic design.
Figures 3 and 4 illustrate the maximum output current
vs input voltage for the supply and the efficiency vs load
current at input voltages of 3V and 5V, respectively.
3
t
t
L1*
2
SENSE
RUN
VIN
ITH
3
4
5
NFB
INTVCC
FREQ
GATE
MODE/SYNC
CC1
4.7nF
GND
L2*
10
0
9
3
CDC
22μF
X5R
LTC3704
RC
14.7k
1
8
7
6
f = 300kHz
COUT
100μF
X5R
M1
CIN
47μF
X5R
RFB1
2.49k
1%
5
DN182 F03
D1
100
CVCC
4.7μF
X5R
RT
80.6k
1%
4.5
3.5
4
INPUT VOLTAGE (V)
Figure 3. Maximum Output Current vs
Input Voltage for the –8V RF Power Supply
95
90
GND
RFB2
13.7k
1%
DN282 F02
D1: DIODES INC B320B
L1, L2: BH ELECTRONICS BH 510-1009
M1: SILICONIX Si9426
Figure 2. A High Efficiency –8V RF Power Supply
EFFICIENCY (%)
1
VOUT
–8V
1.2A TO
2.5A
IO(MAX) (A)
2
VIN
3V TO
5V
VIN = 5V
85
80
VIN = 3V
75
70
65
60
55
50
0.001
0.01
0.1
1
OUTPUT CURRENT (A)
10
DN282 F04
Figure 4. Efficiency vs Output Current
for the –8V RF Power Supply
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