April 2010 - Dual Output Step-Down Regulator Features Pin Selectable Outputs, DCR Sensing, Reverse Current Protection and a 5mm × 5mm QFN

design features
Dual Output Step-Down Regulator Features Pin Selectable
Outputs, DCR Sensing, Reverse Current Protection and a
5mm × 5mm QFN
Stephanie Dai
The LTC3865 is a high performance step-down controller with two constant frequency,
current mode, synchronous buck controllers and on-chip drivers. It offers high output
current capability over a wide input range. An important feature offered by the LTC3865 is its
highly accurate programmable output voltage. Internal precision feedback resistors make it
possible to select from nine different output voltages via two VID pins. The internal resistors
reduce the number of external components and assure 1% accuracy for low voltage rails.
The LTC3865 is suitable for applications
with input voltages up to 38V and output
voltages up to 5V. It can be synchronized
to a frequency of up to 750kHz and comes
in a compact 5mm × 5mm QFN package. The part is also capable of inductor DCR sensing, allowing for increased
efficiencies at higher load currents. These
features are ideal for wide input voltage
range, high current applications where
design solution footprint size is restricted.
them floating can result in nine different output voltages from 0.6V to 5V (see
Table 1). Pin programming eliminates
at least four external feedback resistors, making the overall design solution
space conservative and cost effective. If
an application requires an output voltage not supported by pin programming,
one still has the option to use external
resistors to set the output voltage. Since
the LTC3865 integrates precision feedback resistors, it can achieve 1% output
accuracy for outputs from 0.6V to 1.8V and
1.5% percent output accuracy for 2.5V to
5V, with this accuracy maintained over
the temperature range of −40°C to 85°C.
R SENSE AND DCR SENSING
VID11/VID21
VID21/VID22
V OUT1 /V OUT2 (V)
For applications requiring the highest
possible efficiency at high load currents,
a sense resistor would sacrifice several
percentage points of efficiency compared
to DCR sensing. Inductor DCR is a manifestation of the inductor’s copper winding
resistance. In high current applications,
typical inductance values are low, allowing for a high saturation current inductor
with sub 1mΩ DCR values. DCR current
sensing takes advantage of this by sensing
the voltage drop across the low copper
DCR to monitor the inductor current.
This eliminates the sense resistor and its
additional power loss, thus increasing
efficiency as well as lowering solution
size and cost. An example of a DCR sensing application is shown in Figure 1.
Figure 2 shows an efficiency comparison.
INTV CC
INTV CC
5.0
MULTIPHASE OPERATION
INTV CC
Float
3.3
INTV CC
GND
2.5
Float
INTV CC
1.8
Float
Float
0.6 or External Divider
Float
GND
1.5
GND
INTV CC
1.2
The LTC3865 operates both of channels 180° out-of-phase. This reduces the
required input capacitance and power
supply induced noise. With its current
mode architecture, it can be configured
for dual outputs, or for one output
with both power stages tied together.
GND
Float
1.0
GND
GND
1.1
PIN SELECTABLE OUTPUTS
The LTC3865 features pin programmable
output voltages. Tying each channel’s
two VID pins to INTVCC, GND or leaving
Table 1. Programming the Output Voltages
A dual-phase single output application
is easy to configure; just tie the channels’ compensation (ITH), feedback (VFB),
April 2010 : LT Journal of Analog Innovation | 9
The LTC3865 step-down regulator is ideal for
applications requiring inductor DCR sensing for
maximum efficiency under heavy load.
VIN
7V TO 20V
Figure 1. DCR sensing application
1µF
2.2Ω
4.7µF
D3
M1
0.1µF
L1
3.3µH
VIN PGOOD EXTVCC INTVCC
TG1
TG2
BOOST1
SW1
BG1
5.49k
1%
10µF
35V
×2
LTC3865
COUT1
100µF
×2
FREQ
RUN1
TK/SS1
100pF
3.65k
1%
SENSE2+
1000pF
RUN2
SENSE1–
VID11
VID12
VOSENSE1
ITH1
1800pF
4.75k
1%
L2
2.2µH
PGND
SENSE1+
VOUT1
3.3V
5A
M2
0.1µF
BG2
ILIM
1000pF
D4
BOOST2
SW2
MODE/PLLIN
1.37k
1%
22µF
50V
SENSE2–
VID21
VID22
VOSENSE2
ITH2
TK/SS2
SGND
0.1µF
0.1µF
1.58k
1%
VOUT2
1.8V
5A
2200pF
162k
1%
5.49k
1%
100pF
COUT2
100µF
×2
L1, L2: COILTRONICS HCP0703
M1, M2: VISHAY SILICONIX Si4816BDY
COUT1, COUT2: TAIYO YUDEN JMK325BJ107MM
D3, D4: CMDSH-3
The traditional way of protecting an
IC against overvoltage conditions is to use
an overvoltage comparator, which guards
against transient overshoots (>10%) as
well as other more serious conditions that
10 | April 2010 : LT Journal of Analog Innovation
100
10
90
EFFICIENCY
EFFICIENCY (%)
OUTPUT OVERVOLTAGE
PROTECTION WITH A NEGATIVE
REVERSE CURRENT LIMIT
may cause the output voltage to overshoot.
In such cases, the top MOSFET is turned off
and the bottom MOSFET is turned on and
kept on until excessive energy has been
1
80
70
60
POWER LOSS
50
40
0.01
0.1
DCR
8mΩ
0.1
1
LOAD CURRENT (mA)
10
0.01
Figure 2. Efficiency for the circuit in Figure 1
POWER LOSS (mW)
enable (RUN), power good (PGOOD) and
track/soft-start (TRK/SS) pins together. By
doubling the effective switching frequency
and interleaving phases, the single output configuration minimizes the required
input and output capacitance and voltage ripple, and allows for a fast transient
response and increased current capability. An example for a dual-phase single
output application is shown in Figure 3.
discharged from the output capacitor,
bringing the output back to regulation.
One problem with turning on the bottom
MOSFET indefinitely to clear an overvoltage condition is that sometimes excessive
reverse current is required to discharge the
output capacitor. In these cases the bottom
FET experiences extreme current stress. To
avoid this scenario, the LTC3865 adds a
–53mV of reverse current limit. By setting a floor on how much reverse current
is allowed, the LTC3865 limits how long
the bottom FET can be turned on. This
feature is important in applications that
reprogram output voltages on the fly. For
example, if the output voltage is changed
from 1.8V to 1.5V, the reverse current limit
is activated as shown in the Figure 4.
design features
The LTC3865’s VID programmable output voltage
decreases parts count while increasing design flexibility.
10µF
35V
4.7µF
RJK0305DPB
VIN PGOOD EXTVCC INTVCC
TG1
0.1µF
L1
0.47µH
1µF
2.2Ω
D3
BOOST1
SW1
RJK0330DPB
BG1
TG2
LTC3865
MODE/PLLIN
ILIM
100Ω
2mΩ
100Ω
1.21k
1%
RUN1
TK/SS1
BG2
D4
RJK0305DPB
0.1µF
L2
0.47µH
RJK0330DPB
VOUT
500mV/DIV
1.8V TO 1.5V
PGND
100Ω
SENSE2+
1000pF
100Ω
RUN2
SENSE1–
VID11
VID12
VOSENSE1
ITH1
6800pF
COUT1
220µF
BOOST2
SW2
SENSE2–
VID21
VID22
VOSENSE2
ITH2
SGND
2mΩ
IL
5A/DIV
1.2V
30A
TK/SS2
COUT2
220µF
162k
100pF
VIN
7V TO 20V
22µF
50V
FREQ
SENSE1+
1000pF
10µF
35V
0.1µF
COUT1, COUT2: SANYO 4TPE 220µF
L1, L2: VISHAY IHLP4040DZERR47M11
D3, D4: CMDSH-3
RUN1
0Ω
RUN2
Figure 3. Single output application
FREQUENCY SELECTION
AND MODE/PLLIN
To maximize efficiency at light loads,
the LTC3865 can be set for automatic
Burst Mode® operation. Alternately, to
minimize noise at the expense of light
load efficiency, it can be set to operate in
forced continuous conduction mode. For
both relatively high efficiency and low
noise operation, it can be set to operate
with a hybrid of the two, namely pulseskipping mode. Pulse-skipping mode,
like forced continuous mode, exhibits
lower output ripple as well as low audio
noise and reduced RF interference as
compared to Burst Mode operation. It
also improves light load efficiency, but
not as much as Burst Mode operation.
A clock on the MODE/PLLIN pin forces the
controller into forced continuous mode
and synchronizes the internal oscillator
with the clock on this pin. The phaselocked loop integrated at this pin is composed of an internal voltage-controlled
oscillator and a phase detector. This allows
the turn-on of the top MOSFET of controller 1 to be locked to the rising edge of
an external clock signal applied to the
MODE/PLLIN pin. The frequency range for
the LTC3865 is from 250kHz to 750kHz.
If no external synchronization signal is
applied, there is a precision 7.5µA current
flow out of the FREQ pin that can be used
to program the operating frequency of the
LTC3865 from 250kHz to 750kHz through
a single resistor from the pin to SGND.
1ms/DIV
REVERSE CURRENT LIMIT = –53mV
RSENSE = 10mΩ
Figure 4. As VOUT transitions from 1.8V to 1.5V, with
–53mV reverse current limit and 10mΩ sense resistor, the reverse inductor current is limited at around
5.3A.
CONCLUSION
The LTC3865 step-down regulator is
ideal for applications requiring inductor DCR sensing for maximum efficiency
under heavy load. It can regulate two
separate outputs and can be configured
for higher load current capability by
tying its channels together, and/or by
paralleling additional LTC3865 power
stages. The LTC3865’s VID programmable
output voltage decreases parts count
while increasing design flexibility. These
features, along with its additional negative reverse current limit and integrated
PLL features, make the LTC3865 an easy
fit in a wide variety of applications. n
April 2010 : LT Journal of Analog Innovation | 11
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