DN269 - 60V Step-Down DC/DC Converter Maintains High Efficiency

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60V Step-Down DC/DC Converter Maintains High Efficiency
Design Note 269
Mark W. Marosek
Introduction
Monolithic step-down converters capable of operation at
high input voltages are usually optimized for efficiency
at high input-to-output voltage differentials. At such low
duty cycle operation where DC switch losses are less
critical, the switch design is often neglected, resulting in
a switch resistance that (for some 1.5A converters) can
be as poor as 1Ω. Such converters sacrifice efficiency
when lower input-to-output voltage differentials are
required. The switch drop can also limit maximum duty
cycle—putting a limit on the minimum input voltage
for a given regulated output voltage.
The LT®1766 is designed to optimize efficiency for both
high and low input-to-output voltage differentials to
support a wide input voltage range. In addition, the
current mode topology used to provide fast transient
response and good loop stability does not suffer from
peak switch current fall off at low input-to-output voltage differentials, commonplace in most current mode
converters.
The LT1766 is a 1.5A monolithic buck switching regulator. The 5.5V to 60V input voltage range makes the
LT1766 ideal for 48V nonisolated telecom applications
D2
1N914
4
VIN
42V
C3
4.7μF
100V
CER
15
VIN
SHDN
SW
BIAS
2
C2
L1
0.33μF 68μH
10
R1
15.4k
LT1766
12
14
SYNC
FB
GND
1, 8, 9, 16
R3
2.2k
C5
0.022μF
VOUT
5V
1A
+
D1
10MQ060N
VC
11
C4
220pF
The LT1766 is also capable of excellent efficiencies at
lower input voltages. The peak efficiency for a 12V-to5V conversion is >90% (Figure 2). One key to achieving
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.
100
C1
100μF
10V
SOLID TANT
R2
4.99k
DN269 F01
C1: AVX D CASE 100μF 10V TPSD107M010R0100 (207) 282-5111
C2: AVX 0.33μF X7R 16V 0805YC334KAT1A (803) 946-0362
C3: MARCON 4.7μF 100V TCCR70E2A475M (708) 913-9980
C4: AVX 220pF X7R 50V 08055A221KAT
C5: AVX 0.022μF X7R 16V 0805YC223KAT
D1: INTERNATIONAL RECTIFIER 60V 1.5A SCHOTTKY 10MQ060N
D2: ZETEX FMMD914TA
L1: COILTRONICS 68H UP2-680 (561) 241-7876
Figure 1. 42V to 5V Step-Down Converter
02/12/269_conv
Efficiency
A typical high input voltage application, a 42V to 5V
converter, is shown in Figure 1. To achieve high efficiency
at high input voltages, fast output-switch edge rates
are required; the LT1766 achieves edge rates of 1.2V/ns
(rise) and 1.7V/ns (fall). In addition, light loads at high
input voltages require minimal quiescent current to be
drawn from the input. A BIAS pin allows the internal
control circuitry to be supplied from the regulated output
if greater than 3V. The peak efficiency for a 42V to 5V
conversion is > 80%.
VOUT = 5V
L = 68μH
90
EFFICIENCY (%)
6
BOOST
as well as 12V, 24V and (future) 42V automotive applications. These systems must survive load-dump input
transients as high as 60V. Running at a fixed frequency
of 200kHz, the LT1766 can be externally synchronized
to clock frequencies up to 700kHz. A shutdown pin
provides an accurate 2.38V undervoltage lockout
threshold in addition to a 0.4V threshold for micropower
shutdown (25μA). The LT1766 is provided in a small
16-pin SSOP (GN16) package with fused corner pins
to improve thermal performance.
VIN = 12V
80
VIN = 42V
70
60
50
0
0.25
0.75
1.00
0.50
LOAD CURRENT (A)
1.25
DN267 F02
Figure 2. LT1766 Efficiency
high efficiency for low input-to-output voltage conversions is to use a low resistance saturating switch. A
prebiased capacitor, connected between the BOOST
and SW pins, generates a boost voltage above the input
supply during switching. Driving the switch from this
boost voltage allows the 200mΩ power switch to fully
saturate. Furthermore, an output voltage as low as 3.3V
is capable of generating the required boost supply.
Output Ripple Voltage
The output ripple voltage for the circuit in Figure 1, using
a tantalum output capacitor, is approximately 35mVP-P
(Figure 4). Peak-to-peak output ripple voltage is the sum
of a triwave (created by peak-to-peak ripple current in
the inductor times the ESR of the output capacitor)
and a square wave (created by the parasitic inductance
(ESL) of the output capacitor times ripple current slew
rate). A significant reduction in output ripple voltage to
12mVP-P can be achieved using a ceramic output capacitor (Figure 4). With negligible ESR, the ceramic output
capacitor reduces the portion of output ripple voltage
generated by inductor ripple current times capacitor
D2
D1N914
6
VIN
42V
4
15
C3
4.7μF
100V
CER
14
BOOST
VIN
SHDN
SW
BIAS
2
10
LT1766
12
SYNC
FB
GND
1, 8, 9, 16
R3
2.2k
C5
0.022μF
C2
L1
0.33μF 68μH
C6
1nF
R1
15.4k
D1
10MQ060N
VC
11
C4
220pF
VOUT
5V
1A
C1
47μF
6.3V
CERAMIC
R2
4.99k
ESR. The useful feedback response zero provided by
the tantalum output capacitor ESR for loop stabilization
is now replaced by a capacitor inserted across R1 in
the feedback resistor network.
Peak Switch Current
The LT1766 maintains peak switch current over the full
duty cycle range. Although the LT1766 uses a current
mode architecture to provide fast transient response
and good loop stability, the LT1766 peak switch current
does not fall off at high duty cycles, unlike most current
mode converters. The fall off of peak switch current in
most current mode converters is due to the addition of
slope compensation to the current sensing loop of the
converter in order to prevent subharmonic oscillations
for duty cycles above 50%. The LT1766 uses patented
circuitry to cancel the effect of slope compensation on
peak switch current without affecting frequency compensation. For high duty cycle requirements, this is a
significant benefit over typical current mode converters
with similar peak switch current limits.
LT1766 Features
• Wide Input Range: 5.5V to 60V
• 1.5A Peak Switch Current
• Small 16-Pin SSOP Package
• Constant 200kHz Switching Frequency
• 0.2Ω Saturating Switch
• Peak Switch Current Maintained Over
Full Duty Cycle Range
• 25μA Shutdown Current
• 1.2V Feedback Reference
• Easily Synchronizable Up to 700kHz
DN269 F03
C1: TAIYO YUDEN 47μF X5R 6.3V JMK4328J476MM
C2: AVX 0.33μF X7R 16V 0805YC334KAT1A (803) 946-0362
C3: MARCON 4.7μF 100V TCCR70E2A475M (708) 913-9980
C4: AVX 220pF X7R 50V 08055A221KAT
C5: AVX 0.022F X7R 16V 0805YC223KAT
C6: AVX 1000pF X7R 50V 08055C102KAT
D1: INTERNATIONAL RECTIFIER 60V 1.5A SCHOTTKY 10MQ060N
D2: ZETEX FMMD914TA
L1: COILTRONICS 68H UP2-680 (561) 241-7876
Figure 3. 42V to 5V (All Ceramic) Step-Down Converter
with Low Output Ripple Voltage
Data Sheet Download
www.linear.com
Linear Technology Corporation
OUTPUT RIPPLE VOLTAGE
TANTLUM OUTPUT
CAPACITOR 20mV/DIV
OUTPUT RIPPLE VOLTAGE
CERAMIC OUTPUT
CAPACITOR 20mV/DIV
INDUCTOR CURRENT
0.5A/DIV
DN269 F04
Figure 4. Output Ripple Voltage Comparison
(Tantalum vs Ceramic Output Capacitor)
For applications help,
call (408) 432-1900
dn269f_conv LT/TP 1001 341.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2001
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