40µA IQ, P-Channel Step-Down Controller Operates from 60V to 3.5V VIN and Maintains High Efficiency at Light Loads

40µA IQ, P-Channel Step-Down Controller Operates from
60V to 3.5V VIN and Maintains High Efficiency at Light Loads
Terry Groom
The LTC3864 is low IQ step-down DC/DC controller. It controls an external
P-channel MOSFET to provide excellent light load efficiency, wide input voltage
range (3.5V–60V) including low dropout operation, reliability and functional simplicity
in an easy-to-use 12-pin package. The LTC3864 is capable of 100% duty factor
operation, allowing continued operation with input supply voltage droop.
HIGH EFFICIENCY
PMOS CONTROLLER
The LTC3864 offers high efficiency at
full and light load by virtue of a strong
0.9Ω turn-on and 2Ω turn-off gate driver
and 40µ A low IQ Burst Mode® operation. Modern automotive always-on
applications often require less than
70µ A total supply current to prevent
battery drain. Burst Mode switching
and a low IQ of only 40µ A allows high
efficiency at these very low currents.
Figure 1 shows a typical efficiency graph,
showing very little decline as the load
current is reduced. Light load efficiency
is achieved in two ways: first by low
frequency Burst Mode switching, and
second by low VIN IQ. In light load Burst
Mode operation, the load current is
supported by multiple switching pulses
generated in a “burst” of activity, with
periods of no switching in between bursts.
12 | April 2013 : LT Journal of Analog Innovation
lockout condition is actually set by the
differential voltage from the VIN pin to
the CAP pin of 3.5V. This voltage is used
to drive the gate of the power FET.
100
90
EFFICIENCY (%)
This combination of features makes it
ideal for automotive applications such
as always-on power in an electronic
control unit (ECU). Low dropout performance is guaranteed down to 3.5V over
the full operating temperature range.
The LTC3864 is offered in automotive
temperature and reliability grade and
has been verified to strict failure mode
and effects analysis, or (FMEA) criteria.
Burst Mode
OPERATION
80
PULSE-SKIPPING
70
60
50
0.01
VIN = 12V
VOUT = 5V
0.1
LOAD CURRENT (A)
1
Figure 1. LTC3864 pulse-skip and Burst Mode
efficiency
This effectively lowers the switching
frequency. Power FET switching losses are
a significant loss component when loads
are light. Reducing the effective operating frequency reduces switching losses
and increases efficiency. The efficiency’s
lower limit is ultimately determined by
the VIN quiescent current, or IQ, of 40µ A,
which enables efficient standby operation in always-on power applications.
WIDE V IN OPERATING RANGE
The LTC3864 has a high voltage PMOS gate
driver capable of operating continuously
up to 60V and down to 3.5V. This input
voltage operating range is guaranteed over
the full temperature range up to a military
grade from –55ºC to 150ºC. The minimum
input voltage operation or undervoltage
The LTC3864’s internal linear regulator
maintains 8V between VIN and CAP. When
VIN is less than 8V, the VCAP regulator
is in dropout and the CAP pin is held at
ground. In this condition, the VIN undervoltage is set by the VIN -to-CAP undervoltage. The LTC3864 guarantees a
3.5V minimum from VIN to CAP to assure
adequate PMOS switch gate voltage. For
low VIN operating conditions, we recommend choosing an external P-channel
MOSFET that has a threshold voltage of
less than 2V to assure adequate overdrive when approaching minimum VIN .
100% DUTY CYCLE OPERATION
The LTC3864 naturally and easily handles
100% duty factor operation with an
external P-channel simply by forcing the
gate on. No boost drive or additional
circuitry is needed. While there is efficiency loss by using a P-channel at high
current as opposed to an N-channel,
the simplicity of the solution makes the
LTC3864 ideally suited for many low and
medium current level applications.
One important function in automotive applications is output voltage
design features
One important function in automotive applications is output
voltage dropout during a cold crank condition. With the
LTC3864, the output simply tracks the input voltage when it
is below the regulation output. The output quickly recovers
to the regulation once the cold crank condition is over.
VBATTERY
12V
VOLTAGE
Figure 2. Typical automotive cold
crank from 12V to below 5V
VOUT
5V
LTC3864’s 100% DUTY CYCLE CAPABILITY ALLOWS
VOUT TO RIDE VIN WITHOUT SIGNIFICANT DROPOUT
TIME
dropout during a cold crank condition. Figure 2 shows how the regulated
5V output drops out and recovers during a cold crank condition. The output
simply tracks the input voltage under
the output regulation voltage. The output quickly recovers to the regulated
5V once the cold crank condition is over.
SOFT-START, FAULT PROTECTION
AND RECOVERY
The LTC3864 includes soft-start, tracking,
fault protection and recovery features
to assure robust operation even under
extreme conditions. The SS pin provides
both soft-start and tracking features.
To set the soft-start ramp-up time,
simply tie a capacitor from the SS pin to
ground and the internal 10µ A charging
current sets the SS voltage ramp from 0
to 0.8V. At 0.8V on the SS pin the output is at the full regulation voltage.
The LTC3864 can track another input
source or supply by overdriving the
10µ A current and forcing the SS pin
input voltage. The output tracks the
SS pin until the signal exceeds 0.8V.
Fault protection features include power
good, undervoltage lockout, short-circuit
recovery and frequency foldback during
start-up and short-circuit conditions.
The LTC3864 includes an internal soft-start
ramping feature, which sets the maximum
output ramping rate under all operational
conditions including short-circuit recovery.
The internal soft-start ramp sets the minimum output voltage ramp time to approximately 650µs. An external capacitor to
the SS pin determines the SS ramp once the
internal minimum of 650µs is exceeded.
The internal soft-start ramp also determines the maximum output voltage
ramp from a short-circuit recovery.
Without this feature, the output recovery would be limited only by current
SHORTCIRCUIT
TRIGGER
SHORT-CIRCUIT REGION
VOUT
5V/DIV
SOFT RECOVERY
FROM SHORT
IL
2A/DIV
500µs/DIV
VIN = 12V, VOUT = 5V
limit. An output recovery rise without
soft-start leads to high transient current
and possible output voltage overshoot.
Figure 3 shows a short-circuit event
including recovery. When the output is
shorted, the output drops near zero and
the current is regulated to the programmed
short-circuit value. The first VOUT rise in
recovery is a result of the energy in the
inductor being transferred to the output once the short is removed. Next, the
internal regulation ramp prevents switching until the ramp exceeds the regulation
point, and then ramps monotonically
once switching begins. Figure 3 shows
a smooth output recovery from a shortcircuit without exceeding current limit
and without output voltage overshoot.
VERIFIED FAILURE MODE AND
EFFECTS ANALYSIS (FMEA)
The LTC3864 is designed to meet the
most stringent automotive requirements and to satisfactorily survive an
FMEA to adjacent-pin short and pin open
operations in a typical configuration.
The purpose of this test is to emulate the
effects of typical PCB defects and determine if they are destructive. In the test,
the LTC3864 was configured for a VIN of
12V and VOUT of 5V with an output load
of 1A. Each pin was then systematically
opened and adjacent pins shorted and
the results measured. In all instances, the
LTC3864 recovered when the FMEA conditions were removed. The results can
be found in the LCT3864 data sheet.
Figure 3. Short-circuit operation including soft
recovery from short
April 2013 : LT Journal of Analog Innovation | 13
SIMPLE AND EASY TO USE
The LTC3864 is a nonsynchronous
PMOS DC/DC controller and can be used in
a variety of low to medium current level
applications. Figure 4 shows a typical
5V output automotive application. This
is a minimum component count solution. Include input and output capacitors,
PMOS switch, nonsynchronous diode,
sense resistor, bias caps and compensation and the application is complete.
CCAP
0.1µF
RRUN
100k
VIN
RITH 9.53k
The LTC3864 fits a wide variety of applications where size and light load efficiency are paramount. The output can be
programmed from 0.8V up to a maximum
of 60V. Output currents typically range
up to 5A depending on the application.
Figure 5 shows 24V output voltage, 750kHz
application with 92% peak efficiency at
1A and greater than 72% at low current
efficiency in Burst Mode operation.
LTC3864
SW
RPGD
100k
FREQ
PGOOD
SGND
CIN1: NICHICON UPJ1J120MDD
D1: DIODES INC SBR3U100LP
L1: TOKO 1217AS-H-100M
MP: FAIRCHILD FDMC5614P
RFB1
80.6k
LTspice IV
circuits.linear.com/622
SUMMARY
The LTC3864 is a versatile, easy-to-use high
voltage PMOS controller with excellent
light load efficiency. Its 40µ A low IQ Burst
Mode operation is suited to applications
where standby light load efficiency is
important such as in always-on power
systems. The 100% duty cycle capability allows the output voltage to ride
through severe input voltage droop such
as in a cold crank condition. The LTC3864
is designed to operate in low VIN droop
conditions where minimum VIN is 3.5V over
the full temperature range. The LTC3864
provides high input voltage capability
CIN2
2.2µF
CAP
MODE/PLLN
CVIN
0.1µF
+
CIN1
33µF
63V
and excellent light load efficiency in a
simple and easy-to-use 12-pin package.
The LTC3864E and LTC3864I versions
operate from –40°C to 125°C junction
temperature. The LTC3864H is guaranteed to operate from a –40°C to 150°C
operating junction temperature. The
LTC3864MP is 100% tri-temperature tested
and guaranteed to operate from –55°C to
150°C operating junction temperature.
Visit www.linear.com/LTC3864 for
data sheets, demo boards and other
applications information. n
VIN
24V TO 60V
RSENSE
50mΩ
SENSE
CITH1
6.8nF
RITH 30.1k
SS
ITH
LTC3864
GATE
CITH2 100pF
RFREQ 97.6k
FREQ
CIN1: NICHICON UPJ1J100MPD
D1: DIODES INC SBR3U100LP
L1: TOKO 1217AS-H-470M
MP: VISHAY/SILICONIX SI7113DN
14 | April 2013 : LT Journal of Analog Innovation
MP
L1
47µH
D1
SGND
RPGD2
768k
VFB
*VOUT FOLLOWS VIN WHEN 3.5V ≤ VIN ≤ 24V
10µF
RFB2
887k
PGOOD
PGND
RFB2
422k
VFB
Figure 4. Typical 5V output automotive application
VIN
CFF
47pF
VOUT*
5V
47µF 2A
×2
*VOUT FOLLOWS VIN WHEN 3.5V ≤ VIN ≤ 5.2V
SEE DROPOUT BEHAVIOR IN TYPICAL PERFORMANCE CHARACTERISTICS
CCAP
0.1µF
circuits.linear.com/623
L1
10µH
D1
RUN
VIN*
5.2V TO 55V
RSENSE
25mΩ
MP
GATE
ITH
CITH2 100pF
Figure 5. 24V to 60V input, 24V/1A
output at 750kHz
LTspice IV
CIN1
12µF
63V
SENSE
PGND
This 5V, 2A output solution achieves an
efficiency of around 90% near maximum
load and maintains this high efficiency
all the way down through Burst Mode
operation at light loads, as shown in
Figure 1. The output voltage is programmed using feedback resistors RFB2
and RFB1 with an optional CFF available to
speed up transient response, if desired.
CVIN
0.1µF
MODE/PLLN
SS
CITH1
3.3nF
+
CAP
RUN
CSS
0.1µF
CIN2
4.7µF
RPGD1
200k
RFB1
30.1k
VOUT*
24V
1A
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