DN492 - 36V, 3.5A Dual Monolithic Buck with Integrated Die Temperature Monitor and Standalone Comparator Block

36V, 3.5A Dual Monolithic Buck with Integrated Die Temperature
Monitor and Standalone Comparator Block
Design Note 492
Edwin Li
INTRODUCTION
HIGH INPUT VOLTAGE WITH HIGH TRANSIENT
CAPABILITY
Multioutput monolithic regulators are easy to use and fit
into spaces where multichip solutions cannot. Nevertheless, the popularity of multioutput regulators is tempered
by a lack of options for input voltages above 30V and
support of high output currents. The LT3692A fills this
gap with a dual monolithic regulator that operates from
inputs up to 36V. It also includes a number of channel
optimization features that allow the LT3692A’s per-channel
performance to rival that of multichip solutions.
The LT3692A can operate up to an input voltage of 36V
and can sustain a transient voltage up to 60V for 1 second,
making it suitable for harsh operating environments such
as those commonly found in automotive environments.
ON-DIE TEMPERATURE MONITORING
The LT3692A provides an on-die temperature monitoring
function which facilitates the application circuit design,
debugging and package thermal optimization. The voltage
at TJ pin is directly proportional to the die temperature in
Celsius (i.e., 250mV equals 25°C and 1.5V equals 150°C).
The LT3692A is available in two packages: a 5mm × 5mm
QFN and a 38-lead plastic TSSOP. Although Both include
the full feature set, the TSSOP package enhances the
thermal performance of the dual buck.
The measured temperature of the LT3692A TSSOP die
tops out at 80°C* with the two outputs each supporting
3A loads at 5V and 3.3V from an input voltage of 18V,
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VIN
6V TO 32V
4.7μF
×2
VIN2
VIN1
SHDN1
BST1
6.8μH
0.47μF
VOUT1
5V 3A
fSW1 = 400kHz
47μF
SW1
SW2
IND1
IND2
FB1
8.06k
SS2
ILIM2
CMPI2
CMPO1
CMPO2
SS1
0.47μF
8.06k
ILIM2
CLKOUT
VC2
RT/SYNC
GND
10.2k
24.9k
VOUT2
3.3V 3A
47μF fSW2 = 400kHz
SS2
ILIM1
VC1
680pF
FB2
CMPI1
DIV
17.8k
6.8μH
LT3692A
VOUT2
VOUT1
42.2k
47pF
SHDN2
BST2
TJ
0.1μF
CLOCKOUT
400kHz
60.4k
330pF
0.1μF
33pF
36.5k
dn492 F01
Figure 1. Dual 5V/3A/400kHz, 3.3V/3A/400kHz Application Keeps Temperature Rise Low at a VIN of 18V
07/11/492
with a switching frequency of 400kHz. Figure 1 shows
the schematic of the measured application circuit. The
same setup, but with 2.5A loads, drops the max die
temperature to 68°C.*
STANDALONE COMPARATOR BLOCK
The LT3692A also includes a standalone comparator block,
which provides a 720mV threshold with hysteresis and
outputs an open-collector signal. This comparator can
be configured as a power good flag signal by connecting
CMPI pin to FB pin to monitor the output voltage. It can
also be configured as a temperature flag, which gives a
warning signal when the die temperature rises to a preset
point. This function is realized together with the on-die
temperature monitor. Figure 2 shows how to configure
a 100°C temperature flag.
OTHER FEATURES
180°. The phase difference on the LT3692A is adjusted by
controlling the duty cycle of the synchronization signal.
Frequency Division
Frequency division makes it possible to tune the operating
frequency of each channel to optimize overall performance
and size. The frequency of channel 1 can be programmed
to run at 1, 1/2, 1/4 or 1/8 the frequency of channel 2.
Figure 3 shows the layout of a 3.3V/2.5A/550kHz channel
and a 1.2V/1A/2.2MHz channel application. The relatively
low 550kHz frequency of VOUT1 maximizes channel 1’s
input voltage to 36V while meeting minimum on-time
requirements and keeping the efficiency high. The high
2.2MHz frequency of VOUT2 allows the use of smaller
components for channel 2 as shown in Figure 3. Despite
the reduction in size, electrical and thermal performance
is uncompromised.
CONCLUSION
Independent Adjustable Current Limit
The switch current limit on each output can be programmed from 2A to 4.8A. This expands the number of
loading combinations that can be safely implemented
without risking thermal overload of the package under
extreme conditions, such as a short-circuit. Likewise, the
current limit can be used to protect the part in compact
designs where the saturation margin on inductors is
lowered to meet size constraints.
The LT3692A is a dual output monolithic regulator that
combines the ease-of-use and compact solution size of
typical monolithic regulators with the flexibility of discrete,
multichip solutions. Its high transient voltage capability,
die temperature monitor, standalone comparator block,
adjustable current limit, adjustable switching frequency
and frequency division function and independent synchronization enable the LT3692A to work in many applications
that other monolithic chips cannot.
Independent Synchronization
Independent synchronization allows any phase difference
between the two outputs besides the standard 0° and
VOUT2
3.3V
VOUT2
100k
47μF
CMPI2
CMPO2
100°C TEMP FLAG
TJ
GND
28k
10nF
R3
73.2k
dn492 F02
Figure 2. Temperature Flag Using Comparator Block and
Temperature Monitor
Figure 3. Dual 3.3V/2.5A/500kHz, 1.2V/1A/2.2MHz
Layout. Channel 2 Requires Half of the Area of Channel 1
*TJ pin reading on a standard demo board (DC1403A) running in a 25°C ambient
temperature environment.
Data Sheet Download
www.linear.com
For applications help,
call (408) 432-1900, Ext. 3209
Linear Technology Corporation
dn492f LT/TP 0711 226K • PRINTED IN THE USA
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1630 McCarthy Blvd., Milpitas, CA 95035-7417
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