DN537 - 17V Input, Dual 1A Output Synchronous Step-Down Regulator with Ultralow Quiescent Current

17V Input, Dual 1A Output Synchronous Step-Down
Regulator with Ultralow Quiescent Current
Design Note 537
Haoran Wu
Introduction
The LTC ®3622 dual 1A synchronous monolithic stepdown regulator enables compact, high efficiency supplies for battery powered systems and portable devices
as well as general purpose point-of-load regulation.
The tiny 14-pin, 3mm × 4mm DFN package takes input
voltages from 2.7V to 17V, producing two adjustable
±1% accurate outputs from 0.6V to VIN while delivering
up to 1A output current on both channels.
The two channels can operate in phase or with 180°
phase shift, selected by tying the PHASE pin to GND
or INTVCC. This provides users the flexibility to
separate the channels’ switching edges to minimize
any noise coupling. There are two peak current limit
levels for the LTC3622 – 1.8A or 1A, selected via the
ILIM pin. This selectable current limit allows smaller
size inductors in lower current designs to minimize
solution size and cost.
The LTC3622’s quiescent current with both channels
enabled is as low as 5µA in Burst Mode ® operation
and less than 0.1µA in shutdown. The switching
frequency is fixed at 1MHz or 2.25MHz with a ±50%
synchronization range to an external clock. Selectable
Burst Mode operation yields the highest efficiency,
while pulse-skipping mode minimizes ripple for noisesensitive applications.
Under heavy load conditions, the regulator operates in
continuous inductor current mode with small output
ripple and high efficiency. Two discontinuous conduction modes (DCM) are available at light load to achieve
high efficiency and minimize energy consumption. To
further reduce power loss and extend battery life at
very light loads or no load standby conditions, Burst
Mode operation can be selected by tying the MODE/
SYNC pin to INTVCC. In this case, the LTC3622 IC only
consumes 5μA at no load. Burst Mode operation may
Dual 1A Outputs, 3.3V and 5V, at 1MHz
Figure 1 shows a dual output converter that takes an
input voltage range of 5V to 17V, and produces 1A
outputs at 3.3V and 5V.
VIN
5V TO 17V MAX
CIN
10µF
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and LTspice
are registered trademarks and LTpowerCAD is a trademark of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
VIN1
INTVCC
VIN2 MODE/SYNC
RUN1
PHASE
RUN2
ILIM
LTC3622
C1
1µF
PGOOD1 PGOOD2
3.3µH
VOUT1
3.3V
1A
COUT1
47µF
22pF
SW1
SW2
619k
619k
FB1
137k
4.7µH
GND
VOUT2
5V
1A
22pF
COUT2
47µF
FB2
82.5k
DN537 F01
Figure 1. A Dual-1A Converter with 3.3V and 5V Outputs, fSW = 1 MHz
04/15/537
100
90
80
EFFICIENCY (%)
increase output voltage ripple. On the other hand,
if minimizing VOUT ripple is critical, pulse-skipping
mode can be selected by grounding the MODE/SYNC
pin. This results in lower ripple than in Burst Mode
operation with the trade-off of slightly lower efficiency.
Furthermore, connecting the MODE/SYNC pin to an
external clock synchronizes the switch clock to the
external clock and puts the part in pulse-skipping
mode. Figure 2 shows the efficiency from 12VIN to
5VOUT in burst and pulse-skipping modes.
70
60
50
40
30
20
10
0
0.0001
100
VIN = 5V
VOUT = 5V
fSW = 2.25MHz
Burst Mode OPERATION
PULSE-SKIPPING MODE
0.001
0.01
0.1
LOAD CURRENT (A)
1
DN537 G02
90
Figure 3. Efficiency vs Load Current in Dropout,
VIN = 5V, VOUT2 = 5V, fSW = 1MHz
EFFICIENCY (%)
80
70
60
50
40
30
20
0.001
Burst Mode OPERATION
PULSE-SKIPPING MODE
0.01
0.1
LOAD CURRENT (A)
1
DN537 F02
Figure 2. Efficiency of the 5V Output of the Dual Supply
Shown in Figure 1, VIN = 12V, VOUT2 = 5V, fSW = 1MHz
The LTC3622 is a reliable current mode regulator with
fast, cycle-by-cycle overcurrent protection and excellent line and load transient response. When the input
voltage decreases, the duty cycle increases and slope
compensation is required to maintain a stable current
feedback loop. The LTC3622 has internal circuitry to
accurately maintain a constant peak current limit and
stable loop even at high duty cycles.
In applications such as battery powered systems,
the input voltage can have a very wide range. As VIN
drops close to VOUT and the converter duty cycle approaches 100%, the LTC3622 enters dropout operation
to maintain VOUT regulation. During dropout, the part
transitions in and out of sleep mode depending on the
output load current. This significantly reduces the
quiescent current while keeping VOUT regulated, thus
prolonging the run time of the input battery supply.
Figure 3 shows that the LTC3622 can achieve high efficiency from microamps to full load while in dropout.
To simplify design and minimize component count, the
LTC3622 has internal loop compensation. If needed,
a feedforward capacitor in parallel with upper side
Data Sheet Download
www.linear.com/LTC3622
feedback resistor can be added to further increase
the phase margin. Thanks to its current mode control, the LTC3622 supply is stable with a wide range
of output capacitance. In the dual supply shown in
Figure 1, each channel output only needs one small
1206 size, 47µF ceramic capacitor. Figure 4 shows the
transient response of the 5V output. With a 10% to
100% load step, the peak-to-peak voltage excursion
is about ±330mV. More capacitors can be added to
further reduce the VOUT transient. In this case, the
LTpowerCAD™ design tool and LTspice® simulation
tool can be used to optimize the design with the builtin LTC3622 model. Both tools are available as a free
download at www.linear.com.
VOUT2
200mV/DIV
IOUT2: 100mA – 1A
IOUT2
0.5A/DIV
100µs/DIV
DN537 F03
Figure 4. Transient Response of the 5V Supply Shown
in Figure 1, VIN = 12V, VOUT2 = 5V, IOUT2 = 100mA-1A,
Burst Mode Operation
Conclusion
The LTC3622 is a dual 1A, high efficiency synchronous
monolithic step-down regulator with ultralow quiescent
current. It solves the problems of converter efficiency
and space constraints posed by battery-powered
systems, point-of-load supplies and portable devices.
For applications help,
call (408) 432-1900 EXT 3061
Linear Technology Corporation
dn537 LT/AP 0415 11K • PRINTED IN THE USA
(408) 432-1900
 LINEAR TECHNOLOGY CORPORATION 2015
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
FAX: (408) 434-0507 ● www.linear.com
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