DN328 - Monolithic Synchronous Regulator Drives 4A Loads with Few External Components

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Monolithic Synchronous Regulator Drives 4A Loads
with Few External Components – Design Note 328
Joey M. Esteves
Introduction
The LTC ®3414 offers a compact and efficient voltage
regulator solution for point of load conversion in electronic systems that require low output voltages (down
to 0.8V) from a 2.5V to 5V power bus. Internal power
MOSFET switches, with only 67mΩ on-resistance, allow
the LTC3414 to deliver up to 4A of output current with
efficiency as high as 94%. The LTC3414 saves space by
operating with switching frequencies as high as 4MHz,
enabling the use of tiny inductors and capacitors.
The LTC3414 employs a constant frequency, currentmode architecture and can deliver up to 4A of output
current. The switching frequency can be set between
300kHz and 4MHz by an external resistor; alternatively,
it can be synchronized to an external clock where each
switching cycle begins at the falling edge of the external
clock signal. For improved thermal management, the
LTC3414 is offered in a 20-lead TSSOP package with
an exposed pad to facilitate heat transfer.
The LTC3414 can be configured for either Burst Mode®
operation, pulse-skipping or forced continuous operation. Burst Mode operation maximizes light-load
efficiency and extends battery life by reducing gate
charge losses at light loads—at no load, the LTC3414
consumes a mere 64μA of supply current. Forced
continuous operation maintains a constant frequency
throughout the entire load range, making it easier to
filter the switching noise and reduce RF interference—
important for EMI-sensitive applications. Pulse skip
mode provides a good compromise between light load
efficiency and output voltage ripple.
The LTC3414 provides for external control of the
burst clamp current level, in effect allowing the
burst frequency to be varied. Lower Burst Mode
operating frequencies result in improved light load
efficiencies, but there is a trade-off between light
load efficiency and output voltage ripple—as the
Burst Mode frequency decreases, the output ripple
increases slightly. In the LTC3414, the burst clamp is
01/04/328_conv
adjusted by varying the DC voltage at the SYNC/MODE
pin within a 0V to 1V range. The voltage level at this pin
sets the minimum peak inductor current during each
switching cycle in Burst Mode operation. Pulse skip
mode is implemented by connecting the SYNC/MODE
pin to zero volts. In pulse skip mode, the burst clamp
is set to zero current and the minimum peak inductor
current is determined by the minimum on-time of the
control loop. Pulse skipping minimizes the output voltage ripple by providing the lowest possible inductor
current ripple.
High Efficiency 2.5V/4A Step-Down Regulator
Figure 1 shows a 2.5V step-down DC/DC converter that
is configured for Burst Mode operation. This circuit
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and OPTI-LOOP
are registered trademarks of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
1
ROSC
294k 2
3
RSS
2.2M
CSS
1000pF
X7R
4
5
6
7
8
+
CIN2***
330μF
9
10
PGND
PGND
VFB
RT
ITH
SYNC/MODE
RUN/SS
PGOOD
SVIN
SGND
LTC3414
NC
NC
PVIN
PVIN
SW
SW
SW
SW
PGND
PGND
20
19
18
C1
10pF
X7R
R3
124k
CITH
470pF
X7R
R2
78.7k
RITH
20k
R1
432k
17
16
15
PGOOD
RPG
100k
CIN1
22μF
X5R
2X
VIN
2.7V TO 5V
14
13
12
11
L1*
0.47μH
COUT2
22μF
X5R
VOUT
2.5V
4A
+
COUT1**
470μF
DN328 F01
* VISHAY DALE IHLP-2525CZ-01
** SANYO POSCAP 4TPD470M
*** SANYO POSCAP 6TPB330M
Figure 1. 2.5V/4A Regulator, Burst Mode Operation
provides a regulated 2.5V output at up to 4A from a
2.7V to 4.2V input. Efficiency for this circuit, shown in
Figure 2, is as high as 94% for a 3.3V input voltage. The
switching frequency for this circuit is set at 1MHz by a
single external resistor, ROSC. Operating at frequencies
this high allows the use of a lower valued (and physically smaller) inductor.
1
ROSC
294k 2
3
RSS
2.2M
4
CSS
1000pF
X7R
In this particular application, Burst Mode operation
maintains the high efficiency at light loads. The burst
clamp current is set by the R2 and R3 voltage divider,
which generates a 0.49V reference at the SYNC/MODE
pin. This corresponds to approximately 1.2A minimum
peak inductor current, as shown in Figure 2.
5
6
7
8
100
+
TA = 25°C
95
CIN2***
150μF
9
EFFICIENCY (%)
90
85
10
80
PGND
PGND
VFB
RT
ITH
SYNC/MODE
RUN/SS
PGOOD
SVIN
SGND
LTC3414
NC
PVIN
NC
PVIN
SW
SW
SW
SW
PGND
PGND
20
CC
100pF
X7R
19
18
R2
200k
CITH
470pF
X7R
RITH
12.1k
R1
634k
17
16
C1
22pF
X7R
PGOOD
RPG
100k
CIN1
22μF
X5R
2x
15
VIN
5V
14
13
L1*
0.68μH
12
VOUT
3.3V
4A
COUT**
100μF
2x
11
75
DN328 F03
70
* MURATA LQH66SNR68M03L
** TDK C4532X5ROJ107M
*** SANYO POSCAP 6TPE150M
65
69
55
50
0.001
Figure 3. 3.3V/4A Regulator, Forced Continuous Mode
0.010
0.100
1.000
LOAD CURRENT (A)
10.000
DN328 F02
100
Figure 2. Efficiency vs Load Current 3.3V to 2.5V
Burst Mode Operation from Figure 1
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Linear Technology Corporation
EFFICIENCY (%)
High Efficiency 3.3V/4A Step-Down Regulator
with All Ceramic Capacitors
Figure 3 shows a 3.3V step-down DC/DC converter
using all ceramic capacitors. This circuit provides a
regulated 3.3V output at up to 4A from a 5V input
voltage. Efficiency for this circuit, shown in Figure 4,
is as high as 93%. Ceramic capacitors offer low cost
and low ESR, but many switching regulators have difficulty operating with them because the extremely low
ESR can lead to loop instability. The phase margin of
the control loop can drop to inadequate levels without
the aid of the zero that is normally generated from
the higher ESR of tantalum capacitors. The LTC3414,
however, includes OPTI-LOOP® compensation, which
allows it to operate properly with ceramic input and
output capacitors. The LTC3414 allows loop stability
to be achieved over a wide range of loads and output
capacitors with proper selection of the compensation
components on the ITH pin.
90
80
70
60
50
40
30
20
10
0
0.01
0.1
1
LOAD CURRENT (A)
10
DN328 F04
Figure 4. Efficiency vs Load Current,
5V to 3.3V, Forced Continuous
Conclusion
The LTC3414 is a monolithic, synchronous step-down
DC/DC converter that is well suited for applications
requiring up to 4A of output current. Its high switching
frequency and internal low RDS(ON) power switches
make the LTC3414 an excellent choice for compact,
high efficiency power supplies.
For applications help,
call (408) 432-1900
dn328f_conv LT/TP 0104 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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