DN332 - 4-Phase Monolithic Synchronous Boost Converter Delivers 2.5A with Output Disconnect in a 5mm x 5mm QFN Package

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4-Phase Monolithic Synchronous Boost Converter Delivers 2.5A
with Output Disconnect in a 5mm × 5mm QFN Package
Design Note 332
David Salerno
Introduction
The LTC®3425 is the industry’s first 4-phase, monolithic
synchronous boost converter. It can start up with as little
as 1V and operate with inputs up to 4.5V. The output
voltage range is 2.4V to 5.25V, making it well suited
for battery-powered applications as well as point-ofload regulation in low voltage systems. The 4-phase
architecture allows for an effective switching frequency
of up to 8MHz, which in turn reduces output ripple current and peak inductor current by a factor of four (over
an equivalent single-phase circuit). This allows the use
of small, low cost, low profile inductors and ceramic
capacitors even at high load currents.
With 5A peak current capability and an effective switch
RDS(ON) of 40mΩ (NMOS) and 50mΩ (PMOS), the
LTC3425 is capable of efficiently delivering 2.5A load
current from a 0.8mm maximum profile, 32-pin 5mm
× 5mm QFN package.
Multiple Operating Modes Optimize Performance
in Different Applications
The LTC3425 can be configured for automatic Burst
Mode® operation, fixed frequency mode with forced
continuous conduction or fixed frequency mode with
pulse skipping. Programmable automatic Burst Mode
operation is ideal for portable applications, where the
load current can vary over a wide range and efficiency
is paramount. A quiescent current of just 12μA in
Integrated output disconnect allows VOUT to go to 0V
in shutdown while eliminating the high inrush current
typical of traditional boost converters during start-up.
VIN
2V TO 3V
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
CIN
1μF
RLIM 75k
RT 15k
GND
32
CSS
0.047μF
1
2
L1
2.7μH
3
4
VOUT
5
L2
2.7μH
6
7
8
31
SS SHDN
GNDA
30
SYNCIN
29
VIN
28
RT
27
ILIM
26
COM
25
SYNCOUT
GNDD
GNDA
GNDD
SWA
SWD
VOUTA
VOUTD
LTC3425
VOUTB
VOUTC
SWB
SWC
GNDB
GNDC
GNDB
GNDC
REFEN VOUTS SGND
9
10
11
CFF 22pF
23
22
L4
2.7μH
VOUT
3.3V
2.5A
21
20
19
L3
2.7μH
R1
100k
18
17
FB COMP BURST REFOUT PGOOD
12
13
14
15
16
PGOOD
CZ
330pF
RF2 1M
RFF 10k
24
RF1
590k
RC
47k
CP
15pF
CB
0.022μF
RB
27k
CBULK
47μF
DN332 F01
CBULK: TDK C3225X5R0J476M
CIN: TDK C1608X5R1A105M
COUT: TDK C2012X5ROJ106M ¥4
L1 TO L4: TDK RLF5018T-2R7M1R8
Figure 1. 2-Cell to 3.3V/2.5A Boost Converter
03/04/332_conv
COUT
10μF
w4
GND
Burst Mode operation extends battery life during light
load operation. For noise-sensitive applications fixed
frequency mode can be selected, with either forced
conduction for low noise at light load or pulse skipping
for improved light load efficiency. Quiescent current in
shutdown is less than 1μA.
Fault Protection
The LTC3425 includes short-circuit protection, programmable peak current limit and thermal shutdown.
High Power and High Efficiency in
a Small Package
Figure 1 shows an LTC3425 application using all ceramic capacitors and low profile inductors to deliver
2.5A load current at 3.3V from a 2-cell input (VIN of 2V
to 3V). Maximum component profile is only 2.5mm. In
this example, the oscillator frequency is programmed
for 4MHz (1MHz/phase), resulting in 10mVP-P output
ripple at full load (see Figure 2). The burst resistor is
selected to automatically transition from Burst Mode
operation to fixed frequency mode when the load exceeds
100mA for optimal efficiency (peaking at 96%) over
the load range (see Figure 3). No-load input current is
only 39μA at 2.4V.
The current mode architecture results in excellent
transient response during a load step (see Figure 4).
The use of a small feed-forward RC network across
the top feedback resistor provides improved transient
response and reduced output ripple in Burst Mode
operation, especially when using low output capacitor
values. The feed-forward capacitor also compensates
for the effect of stray capacitance at the FB pin when
using large value feedback resistors.
The open-drain PGOOD output goes low when VOUT is
11.5% below its regulated value and goes high when
VOUT is within 9% of its regulated value.
Conclusion
The LTC3425’s multiphase architecture makes it possible to deliver high power in a small, low profile package
with very low output ripple. It offers features that are
demanded in high performance portable applications,
including output disconnect, automatic Burst Mode and
high efficiency over a wide load range. It also offers
flexible design features, including its programmable
soft-start, current limit and oscillator frequency, external
compensation and multiple available operating modes.
VOUT
100mV/DIV
AC COUPLED
ILOAD
0.5A/DIV
0A
Figure 2. Output Voltage Ripple at 2.5A Load
for Converter of Figure 1
100
DN332 F04
Figure 4. Response of the Circuit in Figure 1
to a 1A Load Step
VIN = 3V
90
VIN = 2V
80
EFFICIENCY (%)
VIN = 2.4V
100μs/DIV
VOUT = 3.3V
ILOAD = 0.5A TO 1.5A
VIN = 2.4V
70
60
50
40
30
20
10
0
0.1
1
10
100
1k
LOAD CURRENT (mA)
10k
Figure 5. The LTC3425 Can Deliver 10W in a
Low Profile 0.7in2 Footprint
DN332 F03
Figure 3. Efficiency vs Load of the Converter in Figure 1
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn332f_conv LT/TP 0304 344K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004
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