µPower Buck Regulator Safely Rides Through Automotive
Load Dumps – Design Note 475
Juan G. Aranda
Introduction
When a buck regulator is used in applications where output
short-circuit and overload conditions occur, the duty cycle
required to keep the inductor current under control can
be lower than the regulator’s minimum duty cycle when
operating at full speed. Therefore, to effectively protect a
traditional regulator under such conditions, its switching
frequency must be reduced to a speed that can safely
handle the maximum expected input voltage.
In some cases, frequency foldback can help reduce the
effective duty cycle by reducing the switching frequency
as the output voltage falls out of regulation. However,
this technique might not provide enough protection if the
folded frequency is not low enough. In the end, the duty
cycle problem imposes a limit on the maximum switching
frequency at which the regulator can be safely operated,
especially in automotive applications where the input
supply can see positive voltages several times higher
than the normal 12V operating voltage.
The LT®3682 is a new 1A buck regulator that overcomes
the duty cycle limitation by monitoring the current through
the external catch diode, and delaying the generation
of new switch pulses if this current exceeds a defined
value. The LT3682 safely accommodates output shorts
and overload conditions up to its maximum adjustable
operating frequency of 2.2MHz regardless of input voltage. This added level of protection allows automotive
systems designers to take advantage of the maximum
SW INHIBIT
VIDA
DA
The LT3682 accepts input voltages from 3.6V to 36V and
transients up to 60V. Additional features such as softstart, power good flag, frequency foldback and thermal
shutdown are all included in the thermally enhanced
12-lead 3mm × 3mm DFN package.
Minimum Switch On-Time
Under normal load conditions the internal switch current
limit is regulated to meet the required peak inductor current. Due to internal delays, the power switch does not turn
off immediately when the internal current limit is reached,
but instead takes the minimum on-time (tON(MIN)) to do so.
This delay allows the inductor current to continue rising
to values that depend on the current slope and the value
of tON(MIN). During positive input voltage transients the
slope of the rising inductor current is greatly increased,
thus raising its peak value. A well designed buck converter
must skip switch pulses to maintain regulation if the new
duty cycle requirements cannot be met due to minimum
on-time limitations.
Overload conditions during the input transients may aggravate the situation further. Under extreme overloads
the internal current limit is ultimately clamped to its
maximum value, ILIM, and the output voltage falls out of
regulation, reducing the negative slope of the inductor
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VIN
6.9V TO 36V
TRANSIENT
TO 60V
LT3682
SW
– +
switching frequency of the regulator without concern for
transients on the input supply.
CATCH
DIODE
RSENSE
ON OFF
CIN
2.2μF
16.2k
40.2k
PGND
470pF
BD
VIN
RUN/SS BOOST
VC
RT
LT3682
PG
SYNC
GND
0.22μF
L
10μH
SW
B150
DA
FB
PGND
536k
102k
DN475 F01
Figure 1. The LT3682 Monitors the Current Flowing
Through the External Catch Diode at the DA Pin
02/10/475
VOUT
5V
0.9A, VIN 6.9V
1A, VIN 12V
f = 800kHz
DN475 F02
Figure 2. A Typical 800kHz Application
COUT
10μF
current accordingly. If during the switch off-time the
inductor current does not return to the same or smaller
values it had at the end of the previous cycle, its peak
value will ratchet higher in every cycle and increase to
unacceptably high levels.
Monitoring Current Via the DA Pin
The LT3682 constantly monitors the inductor current
during the switch off-time by looking at the current flowing through the external catch diode via the DA (diode
anode) pin (see Figure 1) and delays the generation of
new switch pulses if this current does not fall below a
defined threshold, IDA, thus reducing the regulator’s effective duty cycle.
Thus, it is now possible to use the small footprint inductors
found in high frequency applications without sacrificing
robustness during a number of fault conditions. Figures 2
and 5 show how to configure the LT3682 in a 5VOUT
application with a 800kHz and 1.7MHz programmed
switching frequency, respectively. The resistive load at
VOUT is increased until the regulator hits its maximum
current limit. Figures 3, 4, 6 and 7 show the DA pin current sense protection for input voltages of 12V and 36V
for both applications. In all cases the lowest value of the
inductor current is pinned to about 1.1A, which keeps its
peak value well under control. By delaying the generation
of new switch pulses the switching frequency is effectively reduced to satisfy the new duty cycle requirements
introduced by the fault condition.
Conclusion
The LT3682 is a 1A monolithic buck switching regulator that accepts input voltages from 3.6V to 36V and
transients up to 60V. It features an adjustable and
synchronizable switching frequency from 250kHz to
2.2MHz. It also has the ability to monitor the current
flowing through the external catch diode, thus providing
an extra level of protection against output fault conditions
over the entire operating frequency range, regardless of
input voltage. These features, together with its typical
75μA no load quiescent current makes the LT3682 the
right choice in high frequency automotive and batterypowered applications.
VIN
9V TO 19.5V
TRANSIENT
TO 60V
ON OFF
CIN
2.2μF
13.3k
13.7k
680pF
BD
VIN
RUN/SS BOOST
VC
RT
LT3682
0.22μF
SW
B150
PG
SYNC
GND
0A
VSW
20V/DIV
0V
1μs/DIV
DN475 F03
Figure 3. The 800kHz Application with VIN = 12V. Overload
Condition Forces VOUT to Drop to About 3.2V
102k
DN475 F05
VOUT
2V/DIV
0V
IL
1A/DIV
0A
VSW
20V/DIV
0V
DN475 F06
Figure 6. The 1.7MHz Application with VIN = 12V.
Overload Condition Forces VOUT to Drop to About 4.4V
VOUT
2V/DIV
0V
IL
1A/DIV
0A
VSW
20V/DIV
VOUT
2V/DIV
COUT
10μF
Figure 5. A Typical 1.7MHz Application
500ns/DIV
0V
536k
DA
FB
PGND
f = 1.7MHz
VOUT
2V/DIV
IL
1A/DIV
L
4.7μH
VOUT
5V
0.9A
0V
500ns/DIV
DN475 F07
0V
IL
1A/DIV
0A
VSW
20V/DIV
Figure 7. The 1.7MHz Application with VIN = 36V.
Overload Condition Forces VOUT to Drop to About 4.4V
0V
1μs/DIV
DN475 F04
Figure 4. The 800kHz Application with VIN = 36V. Overload
Condition Forces VOUT to Drop to About 3.5V
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call (408) 432-1900, Ext. 3513
Linear Technology Corporation
dn475 LT/TP 0210 116K • PRINTED IN THE USA
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