DN212 - LT1777 High Voltage, Low Noise Buck Switching Regulator

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LT1777 High Voltage, Low Noise Buck Switching Regulator
Design Note 212
Ajmal Godil
The LT®1777 is a wide input range buck (step-down)
switching regulator specially designed for low noise applications. The LT1777 can be beneficial in applications
where low noise is critical, such as telecom, automotive
cellular and GPS receiver power supplies. The schematic
in Figure 1 highlights the capabilities of the LT1777.
VSW NODE
VOLTAGE
10V/DIV
SWITCH
CURRENT
200mA/DIV
The LT1777 can accept input voltages from 7.4V to 48V
and has a nominal switching frequency of 100kHz. The
monolithic die includes an onboard 700mA peak current
switch, oscillator, control and protection circuitry. It uses
current mode control that delivers excellent dynamic
input supply rejection and short-circuit protection. In
order to achieve low noise, the LT1777 is equipped with
dI/dt limiting circuitry that is programmed via a small
inductor (LSENSE in Figure 1) in the power path. It also
contains internal circuitry to limit dV/dt during switch
turn-on and turn-off.
Figure 2 shows the VSW node voltage and switch current
for the low noise LT1777. Figure 3 shows the VSW node
voltage and switch current for the high voltage LT1676
buck regulator under the same test conditions (no slew
rate limiting). It can be seen from Figures 2 and 3 that
the switch node voltage and current waveforms for
the LT1777 are more controlled and rise and fall more
slowly than those of the LT1676 regulator. Conducted
and radiated EMI are dramatically reduced by slowing
down the sharp edges during turn-on and turn-off of
VIN
7.4V TO
48V
+
C4
100μF
63V
10
12
3
14
R4
68k
C3
100pF
C6
100pF
C7
680pF
2
11
15
7
VIN
VCC
SHDN
VC
U1
LT1777
VSW
VD
FB
NC
GND
NC
GND
NC
GND
SGND
Figure 2. VSW Voltage and Switch Current
for the LT1777
VSW NODE
VOLTAGE
10V/DIV
SWITCH
CURRENT
200mA/DIV
GND
DN212 F03
500ns/DIV
Figure 3. VSW Voltage and Switch Current
for the LT1676 (No Slew Control)
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.
LSENSE
0.47μH
SML32-470k
4
SYNC
DN212 F02
500ns/DIV
6
L1
200μH
BH 510-1043
OR CTX200-4
+
5
13
1
D1
MBRS1100
C2
100μF
10V
C1
1μF
10V
VOUT
5V
500mA
R3
36.5k
1%
8
9
16
R1
21.1k
1%
DN212 F01
Figure 1. 100kHz Low Noise Step-Down Switching Regulator
09/99/212_conv
the power switch with only modest reduction in conversion efficiency.
Figure 4 shows a spectral analysis of the current wave
forms for the LT1777 and the LT1676. The horizontal
axis is 2MHz/DIV (0MHz to 20MHz), and the vertical axis
is 10dB/DIV. It can be seen from Figure 4, the LT1777
attenuates the high frequency noise by approximately
–20dB compared to the LT1676.
The LT1777 can be disabled by connecting the shutdown
(SHDN) pin to ground, reducing input current to a few
microamperes. For normal operation, decouple the
SHDN pin with a 100pF capacitor to ground. The part
also has a SYNC pin, used to synchronize the internal
oscillator to an external clock, which can range from
130kHz to 250kHz. To use the part’s internal oscillator,
simply connect the SYNC pin to ground.
Since the LT1777 allows such a wide input range, the
internal control circuitry draws power from the VCC
pin, which is normally connected to the output supply.
During start-up, the LT1777 draws power from VIN; after
the switching supply output reaches 2.9V, the LT1777
uses the output voltage to power its internal control
circuitry, thereby reducing quiescent power by hundreds
of milliwatts when operating at high line voltages.
Figure 5 shows a typical efficiency curve for the LT1777
using sense inductor, LSENSE = 0.47μH, VIN = 12V and
VOUT = 5V.
Generating Low Noise, Dual-Voltage Supplies
The circuit in Figure 6 shows a cost-effective way to
generate 5V and –5V low noise supplies from a single
10V to 28V supply using the LT1777 and a few off-theshelf components.
L1A and L1B are two windings on a single core, used
to generate ±5V. To minimize coupling mismatches
between the two windings, C2 has been added, forcing
the winding potentials to be equal and improving cross
regulation. Total available current from both outputs is
limited to 500mA. Maximum negative suppy current
is affected by the postitive 5V load; a typical limit is
one-half of the positive current.
85
EFFICIENCY (%)
80
10dB/DIV
75
70
65
VIN = 12V
VOUT = 5V
LSENSE = 0.47μH
DN212 F04
2MHz/DIV
60
Figure 4. Spectral Analysis of the Waveforms
for the LT1676 and the Low Noise LT1777
100
0
300
400
200
LOAD CURRENT (mA)
500
DC242 F05
+
C6
100μF
63V
4
10
100pF
C4
100pF
R3
22k
VCC
VIN
VSW
L1B
200μH
6
C3
100μF
10V
5
VD
13
SHDN
FB
LT1777
14
1
VC
GND
8
GND
12
9
SYNC
GND
7
12
SGND
GND
3
SHDN
LSENSE
0.47μH
D1
MBRS1100
C2
4.7μF
+
L1A
200μH
C5
2200pF
+
C1
1μF
10V
D2
MBRS1100
C8
1μF
10V
VOUT
5V
R1
36.5k
1%
L1A/B: COILTRONICS CTX200-4
OR BH ELECTRONICS 510-1043
LSENSE: GOWANDA SML32-470K
R2
12.1k
1%
+
VIN
10V TO 28V
Figure 5. LT1777 Output Efficiency
–VOUT
–5V
C7
100μF
10V
DN212 F06
Figure 6. This Cost-Effective Supply Generates ±5V from a 10V to 28V Input
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dn212f_conv LT/TP 0999 340K • PRINTED IN THE USA
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