DN383 - High Voltage Current Mode Step-Down Converter with Low Power Standby Capability

High Voltage Current Mode Step-Down Converter with Low
Power Standby Capability – Design Note 383
Jay Celani
Introduction
Low power standby requirements are typically associated with battery-powered systems. Automotive systems, for example, commonly require power supplies to
maintain output voltage regulation even under no-load
conditions—while drawing minimal quiescent current to
preserve battery life. Rising energy costs, however, have
extended the need for low current standby operation to
line-powered systems, such as small plugged-in appliances for home and business.
converter requires few external components, maintains
high conversion efficiencies over a wide load range and
supports low power standby operation for compliance
with system power-management requirements.
High Efficiency at Standby
The LT3800 is a 4VIN to 60VIN, 200kHz fixed-frequency
controller that uses synchronous operation and N-channel
MOSFETs to maximize high current efficiency. Current
mode operation with continuous high-side inductor current sensing yields fast transient response and excellent
line regulation. Low current standby requirements are met
using Burst Mode® operation. A reverse inductor current
inhibit feature also increases efficiency during light-load
conditions. The LT3800 runs directly from the converter
input supply, so there are no local supplies required to
power the IC. The IC is also designed for easy use of
output-derived power which further increases conversion
efficiency. Cycle-by-cycle current limiting maintains the
Designing a power supply that is very efficient at light
loads is particularly difficult in systems where high input
voltages and substantial load currents are required. A
common approach in such high power systems is to add
a secondary power path for low current operation—a
potentially significant increase in the cost, board space
and complexity of the power supply.
A better solution is to use the LT® 3800 as the core of a
single-supply synchronous DC/DC converter. The resulting power supply is simple and efficient. An LT3800-based
VIN
20V TO 55V
+
120mF
63V
2.2mF
100V
´2
, LTC, LT and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
0.22mF 16V
0.1mF
100V
VIN
BOOST
1M
NC
TG
SHDN
SW
Si7850DP
82.5k
CSS
130k
1%
1500pF
120pF
VC
82.5k
680pF
LT3800
BURST_EN
VFB
14.7k
1%
15mH
BAS19
200k
VCC
BG
1N4148
1mF
16V
470pF
Si7370DP
PGND
SENSE+
SENSE–
SGND
0.015W
22mF
16V
´2
+
Figure 1. LT3800 12V, 75W Buck DC/DC Converter with High Efficiency at Light Loads
02/06/383
VOUT
12V
270mF 75W
16V
DN383 F01
programmed current limit, even during instantaneous
short circuit fault conditions.
12V/75W Synchronous Buck DC/DC Converter
Figure 1 shows a 12V, 75W DC/DC converter that can
operate with input voltages from 20V to 55V. The 20V
minimum input is set by a programmable UVLO function
implemented using the precision hysteretic threshold of
the LT3800 SHDN pin. The 55V upper bound is limited
by switch FET margin. This converter provides full-load
efficiencies above 95%, as shown in Figure 2, and can
maintain a no-load output voltage with only 0.1mA of
input supply quiescent current.
The light load efficiency enhancement features of the
LT3800 produce a supply that maintains high conversion
efficiency across a 4-decade range. This DC/DC converter
also features excellent heavy-load efficiency, producing
peak conversion efficiencies as high as 96%.
8
100
95
7
VIN = 20V
6
90
85
5
VIN = 48V
4
80
3
75
VIN = 48V
70
2
65
1
POWER LOSS (W)
EFFICIENCY (%)
This DC/DC converter incorporates a controlled dV/dt
soft-start function which servos the converter output
voltage to a programmed rising rate during start-up—in
this case 1.3V/ms, yielding a start-up rise time of just
under 10ms. The LT3800 automatically resets the softstart function if the output drops out of regulation, so
recovery from a short circuit or brownout event is graceful
and controlled.
VOUT
100mV/DIV
IOUT
2A/DIV
VIN = 20V
60
1
100
1000
10
OUTPUT CURRENT (mA)
0
10000
DN383 F02
Figure 2. The DC/DC Converter in Figure 1 Exhibits High
Efficiency Across a Wide Load Range and Produces Peak
Efficiencies Above 95%
VIN = 48V
100µs/DIV
DN383 F04
Figure 4. 1A to 5A Load Step Generates
<2% VOUT Transient
VOUT
10V/DIV
IOUT
10A/DIV
VIN = 48V
2.5ms/DIV
DN383 F03
Figure 3. Current Limit is Maintained During Instantaneous
Short-Circuit and Auto-Reset Soft-Start Yields a Graceful
Recovery from the Fault Event
Figure 5. LT3800 12V, 75W DC/DC Converter Layout
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2593
www.linear.com
Linear Technology Corporation
dn383f LT/TP 0206 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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