Feb 1998 LT1533 Ultralow Noise Switching Regulator for High Voltage or High Current Applications

DESIGN IDEAS
LT1533 Ultralow Noise Switching
Regulator for High Voltage or
High Current Applications
by Jim Williams
High Voltage Input Regulator
The LT1533 switching regulator1, 2
achieves 100µV output noise by using
closed-loop control around its output switches to tightly control
switching transition time. Slowing
down switch transitions eliminates
high frequency harmonics, greatly
reducing conducted and radiated
noise.
The part’s 30V, 1A output transistors limit available power. It is possible
to exceed these limits while maintaining low noise performance by
using suitably designed output
stages.
The LT1533’s IC process limits
collector breakdown to 30V. A complicating factor is that the transformer
causes the collectors to swing to twice
the supply voltage. Thus, 15V represents the maximum allowable input
supply. Many applications require
higher voltage inputs; the circuit in
Figure 1 uses a cascoded3 output
stage to achieve such high voltage
capability. This 24V to 5V (VIN = 20V–
50V) converter is reminiscent of
previous LT1533 circuits, except for
6
the presence of Q1 and Q2.4 These
devices, interposed between the IC
and the transformer, constitute a cascoded high voltage stage. They provide
voltage gain while isolating the IC
from their large drain voltage swings.
Normally, high voltage cascodes
are designed to simply supply voltage
isolation. Cascoding the LT1533 presents special considerations because
the transformer’s instantaneous voltage and current information must be
accurately transmitted, albeit at lower
amplitude, to the LT1533. If this is
not done, the regulator’s slew-control
T1
7
5
8
24VIN
(20V TO 50V)
+
4
10µF
9
3
MBRS140
10
1
0.002µF
220Ω
10k
Q3
MPSA42
Q4
2N2222
10k
Q1
(
L3
OPTIONAL
100µH SEE TEXT
)
+
220µF
100µF
12
0.002µF
Q2
1k
5VOUT
+
10k
220Ω
L1
100µH
1k
2
+
4.7µF
2
14
4
3
1500pF
11
5
18k
6
15
COL A
VIN
0.01µF
MBRS140
SYNC
DUTY
SHDN
CT
LT1533
L2
PGND
RT
NFB
10
11
COL B
VC
RVSL
FB
GND
RCSL
9
12
13
12k
10k
16
8
7
7.5k
1%
2.49k
1%
AN70 F40
L1, L3: COILTRONICS CTX100-3
L2: 22nH TRACE INDUCTANCE, FERRITE BEAD OR
INDUCTOR COILCRAFT B-07T TYPICAL
Q1, Q2: MTD6N15
T1: COILTRONICS VP4-0860
Figure 1. A low noise 24V to 5V converter (VIN = 20V–50V): cascoded MOSFETs withstand 100V transformer swings, permitting the LT1533 to
control 5V/2A output.
24
Linear Technology Magazine • February 1998
DESIGN IDEAS
A = 20V/DIV
B = 5V/DIV
(AC COUPLED)
A = 5mV/DIV
C = 100V/DIV
B = 100µV/DIV
10µs/DIV
2µs/DIV
Figure 2. MOSFET-based cascode permits the regulator to control
100V transformer swings while maintaining a low noise 5V output.
Trace A is Q1’s source, Trace B is Q1’s gate and Trace C is the drain.
Waveform fidelity through cascode permits proper slew-control
operation.
Figure 4. Waveforms for Figure 3 at 10W output: Trace A shows
fundamental ripple with higher frequency residue just discernible. The
optional LC section results in Trace B’s 180µVP-P wideband noise
performance.
Current Boosting
loops will not function, causing a
dramatic output noise increase. The
AC-compensated resistor dividers
associated with the Q1–Q2 gate-drain
biasing serve this purpose, preventing transformer swings coupled via
gate-channel capacitance from
corrupting the cascode’s waveformtransfer fidelity. Q3 and associated
components provide a stable DC termination for the dividers while
protecting the LT1533 from the high
voltage input.
Figure 2 shows that the resultant
cascode response is faithful, even with
100V swings. Trace A is Q1’s source;
traces B and C are its gate and drain,
respectively. Under these conditions,
at 2A output, noise is inside 400µV
peak.
Figure 3 boosts the regulator’s 1A
output capability to over 5A. It does
this with simple emitter followers (Q1–
Q2). Theoretically, the followers
preserve T1’s voltage and current
waveform information, permitting the
LT1533’s slew-control circuitry to
function. In practice, the transistors
must be relatively low beta types. At
3A collector current, their beta of 20
sources ≈150mA via the Q1–Q2 base
paths, adequate for proper slew-loop
operation.5 The follower loss limits
efficiency to about 68%. Higher input
voltages minimize follower-induced
loss, permitting efficiencies in the low
70% range.
Figure 4 shows noise performance.
Ripple measures 4mV (Trace A) using
a single LC section, with high fre-
1N4148
1N5817
0.05Ω
T1
Q1
4.7µF
14
11
3
1500pF
4
5
18k
6
0.003µF
VIN
SHDN
COL A
DUTY
COL B
SYNC
CT
PGND
LT1533
RVSL
RT
RCSL
10
0.01µF
VC
Notes:
1 Witt, Jeff. The LT1533 Heralds a New Class of
Low Noise Switching Regulators. Linear Technology VII:3 (August 1997).
2 Williams, Jim. LTC Application Note 70: A Monolithic Switching Regulator with 100µ V Output
Noise. October 1997.
3 The term “cascode,” derived from “cascade to
cathode,” is applied to a configuration that places
active devices in series. The benefit may be higher
breakdown voltage, decreased input capacitance,
bandwidth improvement or the like. Cascoding
has been employed in op amps, power supplies,
oscilloscopes and other areas to obtain performance enhancement.
4 This circuit derives from a design by Jeff Witt of
Linear Technology Corp.
5 Operating the slew loops from follower base current was suggested by Bob Dobkin of Linear
Technology Corp.
330Ω
5V
+
quency content just discernible. Adding the optional second LC section
reduces ripple to below 100µV (trace
B), and high frequency content is
seen to be inside 180µV (note ×50
vertical scale-factor change).
GND
NFB
9
8
FB
+
2
4.7µF
15
Q2
0.05Ω
330Ω
16
L2
7
12V
L3
33µH
+
(
OPTIONAL FOR
LOWEST RIPPLE
)
+
100µF
100µF
1N5817
1N4148
13 10k
12 10k
680Ω
L1
300µH
R1
21.5k
1%
AN70 F42
R2
2.49k
1%
L1: COILTRONICS CTX300-4
L2: 22nH TRACE INDUCTANCE, FERRITE BEAD OR
INDUCTOR. COILCRAFT B-07T TYPICAL
L3: COILTRONICS CTX33-4
Q1, Q2: MOTOROLA D45C1
T1: COILTRONICS CTX-02-13949-X1
: FERRONICS FERRITE BEAD 21-110J
Figure 3. A 10W low noise 5V to 12V converter: Q1–Q2 provide 5A output capacity while preserving the LT1533’s voltage/current slew control.
Efficiency is 68%. Higher input voltages minimize follower loss, boosting efficiency above 71%.
Linear Technology Magazine • February 1998
25