DN467 - 15VIN, 4MHz Monolithic Synchronous Buck Regulator Delivers 5A in 4mm × 4mm QFN

15VIN, 4MHz Monolithic Synchronous Buck Regulator Delivers
5A in 4mm × 4mm QFN – Design Note 467
Tom Gross
Introduction
The LTC®3605 is a high efficiency, monolithic synchronous
step-down switching regulator that is capable of delivering
5A of continuous output current from input voltages of
4V to 15V. Its compact 4mm × 4mm QFN package has
very low thermal impedance from the IC junction to the
PCB, such that the regulator can deliver maximum power
without the need of a heat sink. A single LTC3605 circuit
can power a 1.2V microprocessor directly from a 12V
rail—no need for an intermediate voltage rail.
The LTC3605 employs a unique controlled on-time/constant frequency current mode architecture, making it ideal
for low duty cycle applications and high frequency operation. There are two phase-lock loops inside the LTC3605:
one servos the regulator on-time to track the internal
oscillator frequency, which is determined by an external
timing resistor, and the other servos the internal oscillator
to an external clock signal if the part is synchronized. Due
to the controlled on-time design, the LTC3605 can achieve
very fast load transient response while minimizing the
number and value of external output capacitors.
The LTC3605’s switching frequency is programmable from
800kHz to 4MHz, or the regulator can be synchronized to
an external clock for noise-sensitive applications.
Furthermore, multiple LTC3605s can be used in parallel
to increase the available output current. The LTC3605
R5
100k
VIN
12V
C2
22μF
16V
C1
22μF
16V
R3
10Ω
C6
0.1μF
25V
R4
71.5k
PGOOD
PVIN
PVIN
produces an out-of-phase clock signal so that parallel
devices can be interleaved to reduce input and output
current ripple. A multiphase, or PolyPhase ®, design
also generates lower high frequency EMI noise than a
single-phase design, due to the lower switching currents
of each phase. This configuration also helps with the
thermal design issues normally associated with a single
high output current device.
1.8VOUT , 2.25MHz Buck Regulator
The LTC3605 is specifically designed for high efficiency
at low duty cycles such as 12VIN-to-1.8VOUT at 5A, as
shown in Figure 1. High efficiency is achieved with a low
RDS(ON) bottom synchronous MOSFET switch (35mΩ) and
a 70mΩ RDS(ON) top synchronous MOSFET switch.
This circuit runs at 2.25MHz, which reduces the value and
size of the output capacitors and inductor. Even with the
high switching frequency, the efficiency of this circuit is
about 80% at full load.
Figure 2 shows the fast load transient response of the
application circuit shown in Figure 1. It takes only 10μs
to recover from a 4A load step with less than 100mV
of output voltage deviation and only two 47μF ceramic
output capacitors. Note that compensation is internal, set
up by tying the compensation pin (ITH) to the internal
L, LT, LTC, LTM, PolyPhase, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
C7
2.2μF
D1
PHMODE ITH INTVCC
BOOST
SW
SW
SW
LTC3605
SVIN
SW
RUN
SW
SW
CLKIN
VON
CLKOUT
FB
RT TRACK MODE SGND PGND PGND
C9
0.1μF
C5
L1
0.1μF
0.33μH
25V
C3
47μF
6.3V
R2
20.0k
R1
10.0k
DN467 F01
Figure 1. 12V to 1.8V at 5A Buck Converter Operating at 2.25MHz
06/09/467
VOUT
1.8V
C4 5A
47μF
6.3V
VSW1
10V/DIV
ΔVO
100mV/DIV
IL2
5A/DIV
IL1
5A/DIV
IO
2A/DIV
VSW2
10V/DIV
Figure 2. Load Step Response of the Circuit in Figure 1
Figure 3. Multiphase Operation Waveforms of the
Circuit in Figure 4. The Switch Voltage and Inductor
Ripple Currents Operate 180° Out of Phase with
Respect to Each Other
3.3V regulator rail (INTVCC). This connects an internal
series RC to the compensation point of the loop, while
introducing active voltage positioning to the output
voltage: 1.5% at no load and –1.5% at full load. The
hassle of using external components for compensation
is eliminated. If one wants to further optimize the loop,
and remove voltage positioning, an external RC filter can
be applied to the ITH pin.
where each slave’s CLKIN pin takes the CLKOUT signal
of its respective master. To produce the required phase
offsets, simply set the voltage level on the PHMODE pin
of each device to INTVCC, SGND or INTVCC/2 for 180°,
120° or 90° out-of-phase signals, respectively, at the
CLKOUT pin.
1.2VOUT , 10A, Dual-Phase Supply
Several LTC3605 circuits can run in parallel and out of
phase to deliver high total output current with a minimal
amount of input and output capacitance—useful for
distributed power systems.
Conclusion
The LTC3605 offers a compact, monolithic, regulator solution for high current applications. Due to its
PolyPhase capability, up to 12 LTC3605s can run in parallel
to produce 60A of output current. PolyPhase operation
can also be used in multiple output applications to lower
the amount of input ripple current, reducing the necessary
input capacitance. This feature, plus its ability to operate
at input voltages as high as 15V, make the LTC3605 an
ideal part for distributed power systems.
The 1.2VOUT dual-phase LTC3605 regulator shown in
Figure 4 can support 10A of output current. Figure 3 shows
the 180° out-of-phase operation of the two LTC3605s. The
LTC3605 requires no external clock device to operate up
to 12 devices synchronized out of phase—the CLKOUT
and CLKIN pins of the devices are simply cascaded,
RPG
100k
DBST1
CLKIN PGOOD PHMODE INTVCC
BOOST
PVIN
SVIN
VIN
12V
CIN1 RFILT1
22μF 10Ω
RITH
8k
CFILT1
0.1μF
RUN
PGND
LTC3605
SW
CITH
CC1
390pF 10pF
CSS
0.1μF
RFILT2
10Ω
TRACK CLKIN
ITH
CFILT2
0.1μF
PVIN
SVIN
L1
CBST1
0.1μF 0.33μH
COUT1
47μF
VOUT
1.2V
10A
FB
TRACK CLKOUT RT MODE SGND
CC2
10pF
CINTVCC1
2.2μF
VON
ITH
CIN2
22μF
DN467 F03
500ns/DIV
DN467 F02
20μs/DIV
RFB2
10.0k
RFB1
10.0k
RT1
162k
FB
PHMODE
INTVCC
LTC3605
BOOST
RUN
SW
PGOOD
VON
CLKOUT
RT MODE SGND PGND
CINTVCC2
2.2μF
DBST2
L2
CBST2
0.1μF 0.33μH
COUT2
47μF
RT2
162k
DN467 F04
Figure 4. 12V to 1.2V at 10A 2-Phase Buck Converter
Data Sheet Download
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For applications help,
call (408) 432-1900, Ext. 3229
Linear Technology Corporation
dn467 LT/TP 0609 155K • PRINTED IN THE USA
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