DN544 - 42V Quad Monolithic Synchronous Step-Down Regulator with 30μA Quiescent Current

42V Quad Monolithic Synchronous Step-Down Regulator
with 30µA Quiescent Current
Design Note 544
Zhongming Ye
Introduction
Industrial and automotive applications require robust,
easy-to-use, compact DC/DC converters to produce
a variety of low voltage power supplies. The LT8602
monolithic quad synchronous step-down converter in
a 6mm × 6mm package is capable of providing four
outputs from a wide input range of 3V to 42V. It is
versatile and easy to use with features such as low
EMI and adjustable operating frequency up to 2.2MHz.
It consumes only 30µA quiescent current from the
input source even when regulating the outputs.
Small Size, Low EMI, Quad Step-Down
Solution
The LT®8602 integrates two high voltage (HV) and
two low voltage (LV) synchronous regulators in a
QFN (6mm × 6mm) package. Replacing external
Schottky diodes with internal synchronous switches
minimizes the solution size and also increases efficiency, reducing power dissipation.
Figure 1. Top Center View of
the Demo Board DC1949A
11/15/544
The two HV channels (1 and 2) are capable of supporting 1.5A and 2.5A loads respectively, from an
input of 3V to 42V. The two LV channels (3 and 4)
can support up to 1.8A each from an input of 2.6V to
5.5V. Figure 1 shows the top center view of the demo
circuit DC1949A. A simplified schematic with four
outputs: 5V, 3.3V, 1.8V and 1.2V is shown in Figure 3.
One LT8602 replaces four individual regulators, reducing overall solution size and cost. All four regulators
in the LT8602 are synchronized to a single oscillator programmed at 2MHz, allowing the channels to
operate anti-phase. The LT8602 reduces the input
ripple current by operating channels 1 and 3 out of
phase from channels 2 and 4, simplifying EMI filter
design. The demonstration circuit DC1949A includes
a small EMI filter. Figure 2 shows the radiated EMI
performance of the board. It passes the CISPR25
class 5 peak limits with good margin.
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.
Figure 2. Radiated EMI Performance (CISPR25, Class 5, Peak
Detector), 14VIN, 1A at All Outputs, Switching Frequency = 2MHz
Wide Input Range Even at High Switching
Frequencies
A high switching frequency can be used to minimize
DC/DC solution size, but increasing frequency usually comes with a trade-off: reduced input voltage
range. This is a significant concern in automotive
and industrial environments, giving designers pause
when considering the advantages of increasing the
switching frequency.
With low minimum on-time and low dropout, the
LT8602 allows a wide input range, even at 2MHz.
Figure 4 shows the dropout performance of channel
1. As the input voltage decreases toward the programmed output voltage, LT8602 maintains regulation by skipping switch-off times and decreasing the
switching frequency up to a maximum duty cycle of
99.6%. If the input voltage decreases further, the
output voltages remain 50mV–550mV below the input
voltage, depending on the load. The boost capacitor
VIN
6V TO 42V
VIN
C12
4.7µF
BST1
PVIN1
SW1
LT8602
C11
10µF
PVIN2
EN/UVLO
SW2
TRKSS1
C13
2200pF
TRKSS2
L2
2.2µH
4.7pF
R3
316k
22pF
PVIN4
IL
0.2A/DIV
VIN
2V/DIV
C6
4.7µF
PG4
SW3
POREN
CPOR
SW4
RT
R9
28.9k
L3
1µH
10pF
FB3
RST
GND
R5
249k
C3
47µF
R6
200k
L4
1µH
22pF
FB4
INTVCC
OUT2
C2 3.3V, 0.8A
47µF
C5
4.7µF
PG3
SYNC
OUT1
C1 5V, 1.5A
47µF
R4
137k
PVIN3
PG2
Conclusion
The LT8602 integrates four synchronous buck regulators in a 6mm × 6mm QFN package, enabling compact,
low EMI, high efficiency, fault robust solutions requiring only 30µA quiescent current. Individual inputs for
each converter allow design freedom, while separate
PG indicators and TRK/SS pins further extend tracking
and sequencing flexibility. These features make the
LT8602 ideal for the harsh environments common
to automotive and industrial applications.
R2
113k
C8
0.1µF
PG1
OUT2
R1
453k
FB2
RUN4
R10
20k
L1
4.7µH
BIAS
RUN3
Flexible Sequence Control
The LT8602 has a track and soft-start pin for each
HV channel. For each LV channel, it has an internal
soft-start of ~1ms. Each channel also has a power
good indicator. Those pins simplify output tracking
or sequencing.
C7
0.1µF
FB1
BST2
is charged during dropout conditions, maintaining
high efficiency.
C9
4.7µF
R7
100k
OUT3
1.8V, 1.7A
VOUT
2V/DIV
VSW1
5V/DIV
10µs/DIV
C4
47µF
OUT4
1.2V, 1.8A
DN544 F04
Figure 4. Dropout Performance
at IOUT = 1.5A
R8
200k
DN544 F03
Figure 3. Schematic. 5V, 3.3V, 1.8V, 1.2V, fSW = 2MHz Outputs
Data Sheet Download
www.linear.com/LT8602
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 3798
dn544f LT/AP 1115 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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 LINEAR TECHNOLOGY CORPORATION 2015
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