Low IQ, High Efficiency Dual Output Controllers for Wide Ranging Input and Output Voltages

Low IQ, High Efficiency Dual Output Controllers for
Wide Ranging Input and Output Voltages
Jason Leonard
The LTC3857 and LTC3858 are versatile low quiescent
current (IQ), 2-phase dual output synchronous buck
controllers that are ideal for applications demanding
high efficiency and minimal power loss over a broad
range of load currents, from virtually no load to many
amps. They feature a wide input supply range of 4V
to 38V (40V abs max) that covers a broad range of
battery chemistries and power sources. Each output
can be set from 0.8V to 24V (28V abs max).
RB1
215k
CF1
15pF
RA1
68.1k
C1
1nF
LTC3857
SENSE1+
PGOOD2
SENSE1–
PGOOD1
VFB1
RITH2 27k
CITH2A 100pF
RA2
44.2k
CF2
39pF
MB1
L1
3.3µH
C2
1nF
RSENSE1
5mΩ
CB1
0.1µF
BOOST1
ITH1
+
MT1
TG1
COUT1
150µF
VOUT1
3.3V
5A
COUT1A
100µF
D1
CSS1 0.1µF
CSS2 0.1µF
CITH2 680pF
100k
SW1
RITH1 15k
CITH1 820pF
LOW I Q EXTENDS
BATTERY RUN TIME
INTVCC
100k
BG1
CITH1A 150pF
The LTC3857 and LTC3858 are pin-compatible parts that differ only slightly,
most notably in short-circuit operation
and at light load operation. These differences are discussed below and summarized in Table 1. A “-1” version of each
is available with slightly fewer features
(Table 2). The LTC3857/LTC3858 are also
mostly pin compatible with the popular
LTC3827/LTC3826 family of low IQ controllers. Figure 1 shows a typical application circuit in which the LTC3857 is used
to convert a widely varying car battery
voltage to two well regulated outputs.
VIN
TRACK/SS1
INTVCC
ILIM
PHASMD
CLKOUT
PGND
PLLIN/MODE
SGND
TG2
EXTVCC
RUN1
BOOST2
RUN2
FREQ
TRACK/SS2
SW2
ITH2
BG2
CIN2
47µF
×2
CINT
4.7µF
+
CIN1
47µF
VIN
4.5V TO 38V
D2
MT2
CB2
0.1µF
L2
7.2µH
RSENSE2
8mΩ
+
MB2
VOUT2
8.5V
3A
COUT2
68µF
10µF
In many applications, one or more supplies remain active at all times, often in
a standby mode where little or no load
current is drawn. In these “always-on”
systems, the quiescent current of the
power supply circuit represents the vast
majority of the current drawn from the
input supply (battery). Having a low
IQ power supply is crucial to extending
battery run times. In Burst Mode® operation, the LTC3857 draws only 50µ A when
one output is active and only 65µ A when
both outputs are enabled, while the
LTC3858 draws 170µ A when one output is
active and only 300µ A when both outputs
are enabled. Both devices consume only
8µ A when both outputs are shut down.
VFB2
SENSE2–
SENSE2+
RB2
422k
COUT1: SANYO 4TPE220MF
COUT1A: TAIYO YUDEN JMK325BJ107MM-T
COUT2: SANYO 10TPC68M
D1, D2: CENTRAL SEMI CMDSH-4E
L1: SUMIDA CDEP105NP-3R2MC-88
L2: SUMIDA CDEP105NP-7R2MC-88
16 | October 2011 : LT Journal of Analog Innovation
MT1, MT2, MB1, MB2: INFINEON BSZ097NO4LS
Figure 1. High efficiency dual output
3.3V/8.5V step-down converter. The 8.5V
output follows V when V is less than 8.5V.
design features
In “always-on” systems, the quiescent current of the power
supply circuit represents the vast majority of the current
drawn from the input supply (battery). Having a low IQ
power supply is crucial to extending battery run times.
CURRENT SENSING, CURRENT LIMIT
AND SHORT-CIRCUIT PROTECTION
Table 1. Key differences between the LTC3857 and LTC3858
FEATURE
LTC3857
LTC3858
Quiescent Current
(One Channel On)
50µA
170µA
Quiescent Current
(Both Channels On)
65µA
300µA
Burst Mode Operation
Lowest Ripple
Highest Midrange Efficiency
Short-Circuit Protection
Cycle-by-Cycle Current Limiting
with Current Foldback
Cycle-by-Cycle Current Limiting with
Current Foldback and
Optional Short-Circuit Latchoff
Output Voltage Tracking
During Start-Up
Yes, Tracking or Soft-Start
(TRACK/SS Pin)
No, Soft-Start Only (SS Pin also Used
for Short-Circuit Latchoff Timer)
The LTC3857 and LTC3858 operate similarly
to each other in forced continuous and
pulse-skipping mode. There are differences
in Burst Mode operation, however. The
LTC3857 is optimized for the lowest quiescent current and for relatively low ripple.
The LTC3858 is optimized for the highest
efficiency over a broad range of load current. This means the LTC3857 transitions
to constant frequency operation (with
lower ripple) at a lower load current than
the LTC3858, while the LTC3858 maintains
higher efficiency (with higher ripple) at
intermediate light loads (Figures 4 and 5).
The LTC3858 has an additional feature
that further protects during a short-circuit
event. The SS pin can be used as a shortcircuit timer. If the short circuit lasts long
enough, the output “latches off” and stops
100
100
90
90
80
70
80
BURST MODE
OPERATION
EFFICIENCY (%)
The PLLIN/MODE pin is used to program
one of three modes at low load currents—
Burst Mode operation, pulse-skipping
mode, or forced continuous mode. Forced
continuous mode maintains constant
frequency operation from no load to full
load, at the expense of light load efficiency. Burst Mode operation is the most
efficient mode at light loads, albeit with
slightly higher ripple and features the
lowest quiescent current. Pulse-skipping
mode is somewhat of a compromise,
maintaining very low ripple and moderate
efficiency at light loads. Figures 2 and 3
show the efficiencies in these three modes.
EFFICIENCY (%)
OPERATING MODES
The LTC3857/LTC3858 each uses a peak
current mode architecture. A high speed
rail-to-rail differential current sense comparator constantly monitors the voltage
across a current sense element, either a
sense resistor or the inductor’s DC resistance (as derived from an R-C network).
The peak sense voltage is set by the threestate ILIM pin (fixed on the “-1” version).
If a short circuit occurs, current limit
foldback reduces the peak current to minimize the dissipation in the power components. Foldback is disabled during start-up
for predictable tracking or soft-start.
60
PULSESKIPPING
MODE
50
40
30
20
70
60
PULSESKIPPING
MODE
50
40
30
20
10
0
0.1m
BURST MODE
OPERATION
FORCED CONTINUOUS MODE
1m
10m
0.1
ILOAD (A)
1
10
Figure 2. Efficiency of in the circuit of Figure 1 using
the LTC3857. Efficiencies are shown for the three
modes of operation (forced continuous, pulseskipping, and Burst Mode operation). At heavier
loads, the efficiency is independent of the mode.
10
0
0.1m
FORCED CONTINUOUS MODE
1m
10m
0.1
ILOAD (A)
1
10
Figure 3. Efficiency of in the circuit of Figure 1
using the LTC3858. Note that the efficiencies in
forced continuous and pulse-skipping modes are
essentially identical to the LTC3857’s in Figure 2—
only Burst Mode operation is different at light to
intermediate loads.
October 2011 : LT Journal of Analog Innovation | 17
switching. The latch can only be reset by
cycling the RUN pin or by cycling the input
power (VIN). This latchoff feature can be
defeated by connecting a resistor between
the SS and INTVCC pins. The two channels
of the LTC3858 operate independently;
i.e., a short-circuit latchoff on one channel does not affect the other channel.
OTHER IMPORTANT FEATURES
The FREQ pin is used to set the switching
frequency. Tying this pin to ground selects
350kHz while tying it to INTVCC selects
535kHz. Connecting a single resistor
from this pin to ground allows the frequency to be set anywhere from 50kHz
to 900kHz. A short minimum on-time
of 95ns allows low duty cycle operation even at high frequencies. The maximum 99% duty cycle capability allows
low dropout operation for low input/
high output voltage applications.
An internally compensated phase-locked
loop (PLL) enables the LTC3857/LTC3858 to
synchronize to an external clock source
(applied to the PLLIN/MODE pin) from
75kHz to 850kHz. When synchronized,
the LTC3857/LTC3858 operates in forced
continuous mode to maintain constant
frequency operation independent of
the load current. When the external
clock is absent or momentarily interrupted, the LTC3857/LTC3858 operates at
the frequency set by the FREQ pin. The
internal PLL filter is prebiased to a voltage corresponding to this free-running
frequency. When an external clock is
detected, the PLL is enabled. Since the
PLL filter is prebiased and barely has to
18 | October 2011 : LT Journal of Analog Innovation
LTC3858
IL
2A/DIV
FORCED CONTINUOUS MODE
IL
2A/DIV
PULSE SKIPPING MODE
IL
2A/DIV
BURST MODE OPERATION
VIN = 12V
VOUT = 3.3V
ILOAD = 100µA
2µs/DIV
2µs/DIV
Figure 4. Inductor current ripple at 12V to 3.3V at 100µA load current. The LTC3857 and LTC3858 differ only in
Burst Mode operation.
charge or discharge during this transition, synchronization is achieved quickly,
with only small changes in frequency
and minimal output voltage ripple.
The MOSFET drivers and control circuits
are powered by INTVCC , which by default
is generated from an internal low dropout (LDO) regulator from the main input
supply pin (VIN). The strong gate drivers
with optimized dead time control provide
high efficiency at heavy loads. To reduce
power dissipation due to MOSFET gate
charge losses and improve efficiency at
high input voltages, a supply between
5V and 14V (abs max) can be connected to
Figure 5. Comparison of the LTC3857 and LTC3858
efficiency when configured for Burst Mode operation
(PLLIN/MODE pin connected to ground) for the
Figure 1 circuit. At very light to virtually no load, the
LTC3857 has the lowest power loss due to its low IQ.
At intermediate loads, the LTC3858 is more efficient,
at the expense of ripple.
100
90
80
EFFICIENCY (%)
Although both current foldback and
latchoff provide additional levels of
protection during a short-circuit event,
the LTC3857/LTC3858 is fundamentally
protected by its current mode architecture. The current comparator is always
active, meaning switching cycles can be
gracefully skipped as needed to keep the
inductor current under control at all times.
LTC3857
LTC3858
70
LTC3857
60
50
40
30
20
10
0
0.1m
1m
10m
0.1
ILOAD (A)
1
10
the EXTVCC pin. When a supply is detected
on EXTVCC , the VIN LDO is disabled
and another LDO between EXTVCC and
INTVCC is enabled. EXTVCC is commonly
connected to one of the output voltages generated by the LTC3857/LTC3858.
The LTC3857/LTC3858 features a RUN pin
for each channel. RUN enables the output
and the INTVCC supply. The LTC3857 has
a TRACK/SS pin for each channel, which
acts as a soft-start or allows the output to
track an external reference (e.g., another
supply). The LTC3858 has a dual-function
SS pin for each channel. SS is used for
soft-start (like the TRACK/SS pin on the
LTC3857 but without tracking) and also as
the optional short-circuit latchoff timer.
IDEAL FOR AUTOMOTIVE
APPLICATIONS
The LTC3857, in particular, is well suited
for automotive applications, including
navigation, telematics and infotainment
systems. The wide input voltage range is
high enough to protect against double
battery and load dump transients, while
low enough to allow continuous operation during cold crank and engine start.
The ultralow 50µ A quiescent current
is ideal for always-on supplies that are
enabled even when the ignition is off.
The wide output voltage range supports
the higher voltage rails often used for
audio systems, CD/DVD players, and disk
design features
RB1
698k
CF1
10pF
C1
1nF
RA1
49.9k
SENSE1+
PGOOD2
SENSE1–
PGOOD1
VFB1
CITH1 3300pF
MB1
L1
6µH
CSS1 0.1µF
VIN
TRACK/SS1
INTVCC
ILIM
PHASMD
CLKOUT
PGND
PLLIN/MODE
SGND
TG2
EXTVCC
RUN1
BOOST2
RUN2
FREQ
SS2
SW2
ITH1
ITH2
BG2
VFB1
VFB2
C2
1nF
SENSE2–
SENSE2
COUT1
22µF
16V
10µF
16V
VOUT
12V
12.5A
D1
LTC3857
SS1
Figure 6. 2-phase single output application. Multiple
LTC3857/LTC3858 controllers can be cascaded to
drive a single output with up to 12 power stages
operating out-of-phase for very high power
applications.
MT1
TG1
ITH1
RSENSE1
5mΩ
CB1
0.47µF
BOOST1
VOUT
The LTC3857/LTC3858 is normally configured for two independent outputs
that run 180° out-of-phase. Operating
the channels out-of-phase minimizes the required input capacitance.
However, the LTC3857/LTC3858 can also
be configured with both power stages
100k
SW1
RITH1 2.94k
MULTIPHASE SINGLE OUTPUT
APPLICATIONS
INTVCC
BG1
CITH1A 68pF
drives. The 99% duty cycle capability
provides a low dropout voltage for these
rails when the battery voltage dips.
100k
CIN
10µF
50V
CINT
4.7µF
10µF
50V
VIN
19V TO 28V
D2
MT2
CB2
0.47µF
L2
6µH
RSENSE2
5mΩ
COUT2
22µF
16V
MB2
10µF
16V
COUT1, COUT2: SANYO 16T0C22M
D1, D2: CMDSH-4E
L1, L2: SUMIDA CDEP106-6ROM
MT1, MT2, MB1, MB2: INFINEON BSZ097NO4LS
+
driving a single output. Figure 6 depicts
a 19V–28V input supply generating a
regulated 12V/150W output. In this configuration, both channels’ compensation
(ITH), feedback (VFB), enable (RUN) and
soft-start (TRACK/SS or SS) pins are tied
together. Since the channels operate outof-phase, the effective switching frequency
is doubled, minimizing the required
input and output capacitance and voltage ripple, while allowing for even faster
transient response. The LTC3857/LTC3858
provides inherently fast, accurate
Table 2. Key differences between the standard and “-1” parts
LTC3857/LTC3858
LTC3857-1/LTC3858-1
Current Sense Voltage
Adjustable 30mV/50mV/75mV
(ILIM pin)
Fixed 50mV
Power Good Output
Voltage Monitor
Independent Monitors for Each Channel
(PGOOD1 and PGOOD2 pins)
Monitor for Channel 1 Only
(PGOOD1 pin)
CLKOUT/PHASMD Pins for
Three or More Phases
Yes
No
Package
5mm × 5mm QFN
28-Lead Narrow SSOP
cycle-by-cycle current sharing due to its
peak current mode control architecture.
The LTC3857/LTC3858 can also be
used in designs with three or more
phases. The CLKOUT pin can drive the
PLLIN/MODE pin of other controllers,
while the PHASMD pin adjusts the relative phases of each controller. This allows
3-, 4-, 6- and 12-phase operation.
CONCLUSION
The LTC3857 and LTC3858 are nearly
pin-compatible parts, ideal for converters
requiring high efficiency over a broad load
range, from no load to full load. Their low
quiescent current extends operating life
in battery-powered systems. They each
regulate two separate outputs from 0.8V to
24V from inputs of 4V to 38V. The short
minimum on-time and 99% duty cycle
capability allows high frequency operation
from very low to very high duty cycles.
The LTC3857 and LTC3858 incorporate
these features and more in 5mm × 5mm
QFN and 28-lead narrow SSOP packages. n
October 2011 : LT Journal of Analog Innovation | 19