Sep 2006 High Voltage Step-Down Controller Delivers High Power with Minimum Number of Components

DESIGN IDEAS L
High Voltage Step-Down Controller
Delivers High Power with Minimum
Number of Components
by Stephanie Dai and Theo Phillips
Introduction
The LTC3824 is a non-synchronous
buck controller that accepts inputs
from 4V to 60V and is robust in the
presence of large input transient
voltages (Figure 1). It draws just
40µA of quiescent current in Burst
Mode operation, prolonging run time
in battery-powered applications. To
handle the wide range of temperatures
found in automotive and industrial
applications, the LTC3824 comes in
a thermally enhanced 10-pin MSE
package.
12V/2A from a
Wide Input Voltage Range
A typical LTC3824 application (Figure 2) can deliver up to 2A of
continuous load current, and provides
up to 90% efficiency at 1A (Figure 3).
Sturdy 2A, 8V gate drivers accommodate industrial high voltage P-channel
MOSFETs. By using a P-FET as a main
switch, the controller is able to operate
up to 100% duty cycle, and does not
require the boost capacitor and diode
VIN
12.5V to
60V
CIN1
33µF
100V
CIN2
2.2µF
100V
CCAP
0.1µF
CAP
+
VCC
100pF
SYNC/MODE SENSE
and rapid overvoltage and short circuit
protection.
VIN
12V TO 48V
10V/DIV
Two Modes of Operation
SW
20V/DIV
VOUT
200mV/DIV
20µs/DIV
Figure 1. The output voltage remains steady
regardless of wide swings in the input voltage.
found in N-channel buck regulators.
This means that whenever the input
voltage dips below the programmed
output voltage, the output voltage
gracefully follows the input voltage
(reduced by I2R losses through the
sense resistor, P-FET and the inductor)
(Figure 4). LTC3824’s current mode
architecture provides excellent line
and load transient response with few
compensation components. Input current is continuously sensed through
a resistor in series with the P-FET,
providing accurate current limiting
CIN1: SANYO 63MV33AX
CIN2: TDK C4532X7R2A225M
COUT: SANYO OSCON, 16SP270M, TDKC2012X7RIC105K
L1: D104C919AS-330M
D1: SS3H9
Q1: Si7465DP
RS
0.025Ω
1/2W
The LTC3824’s SYNC/MODE pin
allows the user to select between operating modes that improve efficiency at
light loads. If the pin is left open or held
above 2V, the part commences Burst
Mode operation at about 1/3 of the programmed current limit. During Burst
Mode operation, switching cycles are
skipped to reduce switching losses,
especially important to extend battery
life in mobile applications. Grounding
or applying an external clock to the
SYNC/MODE pin forces the controller
into pulse skip mode at light load. In
pulse skip mode, the burst clamp is
set to zero current, which limits the
minimum peak inductor current to a
level set by the minimum on-time of
the control loop. Although pulse skip
mode is not quite as efficient as Burst
Mode operation at very light loads, it
reduces VOUT ripple while operating at
a constant frequency, thus reducing
possible noise in the radio and audio
ranges and simplifying noise filtering. The operating frequency can be
programmed with a single resistor
RSET, or it can synchronize to an ex100
LTC3824
GATE
Q1 L1
33µH
+
1000pF
D1
68k
113k
VFB
VC
8.06k
0.1µF
15k
1000pF
Figure 2. A typical LTC3824 application
Linear Technology Magazine • September 2006
COUT
270µF
16V
VOUT 12V
AT 2A
1µF
16V
X7R
VIN = 12V
VIN = 40V
80
70
2.0
1.5
1.0
VIN = 40V
60
POWER LOSS
50
0
POWER LOSS (W)
GND
SS
EFFICIENCY
90
EFFICIENCY (%)
RSET
301k
2.5
0.5
VIN = 12V
100
LOAD CURRENT (mA)
1000
0
2000
Figure 3. Efficiency for the circuit of Figure 2
is as high as 90% at moderate loads.
27
L DESIGN IDEAS
ternal clock from 200kHz to 600kHz.
Synchronization facilitates integration
into applications using other switching
regulators.
Essential Soft-Start,
Short Circuit and
Overvoltage Protection
VOUT
12V
SW
The LTC3827 includes a programmable soft-start time, which requires only
a single external capacitor between
the SS pin and ground. At high input
voltages, a relatively large capacitor prevents inrush currents during
start-up. This in turn prevents output
overvoltage and sudden drops in VIN,
which in the extreme case could force
the LTC3824 below its 4V undervoltage
lockout. During soft-start, the voltage
on the SS pin, VSS, acts as the reference voltage that controls the output
voltage ramp-up. The effective range
of VSS during ramp-up is 0V to 0.8V.
The typical time for the output to reach
the programmed level is determined by
the selected soft-start capacitor and
the SS pin’s 7µA pull-up current: TSS
= (C • 0.8V)/7µA.
Short circuit and overvoltage
protection are designed to keep the
LTC3824 operating normally even
LTC3828, continued from page when the channel 2 feedback voltage
is within ±7.5% window.
The LTC3828 incorporates protection features such as current
limit, short circuit current foldback
limit, input undervoltage lockout and
output overvoltage protection. The
current comparators have a maximum
sense voltage of 75mV resulting in a
maximum MOSFET current of 75mV/
RSENSE. If the output falls below 70%
of its nominal output level, then the
maximum sense voltage is progressively lowered from 75mV to 25mV.
Table 1. The phase relationships of the two
output channels and the clock out (CLKOUT)
pin depend on the voltage at the PHSMD pin.
VPHSMD
GND
OPEN
INTVCC
Controller 1
0°
0°
0°
Controller 2
180°
180°
240°
CLKOUT
60°
90°
120°
28
VIN
20V TO 12V
400µs/DIV
Figure 4. When the input voltage drops to
below the programmed output voltage, the
output voltage gracefully tracks the input
voltage.
under extreme conditions. In normal
operation, the feedback voltage VFB
is regulated to 0.8V. If VFB drops
below 0.5V, the LTC3824’s switching
frequency folds back to 50kHz on the
assumption that inductor current
is ramping up too quickly during
the MOSFET’s on-time. Runaway is
avoided by providing extra time for
the inductor current to discharge. An
overvoltage comparator monitors the
voltage at VFB, and in the event of an
overshoot adjusts the VC voltage downward, keeping the MOSFET off. The
overvoltage protection (OVP) threshold
is lowered during light load Burst
Mode operation, which causes cycles
to be skipped. The OVP threshold
A comparator monitors the output for
overvoltage condition. When the comparator detects the feedback voltage
higher than 7.5% of reference voltage,
the top MOSFET is turned off and the
bottom MOSFET is turned on.
Phase-Locked Loop and
Phase Mode Selection
The LTC3828 includes a phase-locked
loop comprising an internal voltage
controlled oscillator and phase detector. This allows the top MOSFET
turn-on to be locked to the rising edge
of an external source, where the frequency range of the voltage controlled
oscillator is ±50% around the center
frequency. A voltage applied to the
PLLFLTR pin of 1.2V corresponds to
a frequency of approximately 400kHz.
The nominal operating frequency
range is 260kHz to 550kHz.
In the LTC3828, there is an internal master oscillator running at a
frequency twelve times that of each
Figure 5. The LTC3824 comes in a small,
thermally enhanced MSE package.
goes up when load current increases.
This scheme maintains protection yet
ensures the tightest possible output
voltage regulation.
Conclusion
LTC 3824 is a high voltage step-down
controller with essential features for
many sophisticated industrial and
automotive systems. It comes in a
tiny thermally enhanced 10-pin MSE
package (Figure 5) to save space, and
is highly configurable, including the
ability to synchronize with external
frequency sources, two modes of light
load operation, and programmable
soft-start and current limit. L
controller’s frequency. The PHSMD
pin (UH package only) determines the
relative phases between the internal
controllers as well as the CLKOUT
signal as shown in Table 1. The phases
tabulated are relative to zero phase
being defined as the rising edge of
the top gate (TG1) driver output of
controller 1. The CLKOUT signal can
be used to synchronize additional
power stages in a multiphase (3-, 4-,
or 6-phase) power supply solution
feeding a single, high current output
or separate outputs. In the G28 package, CLKOUT is 90° out of phase with
channel 1 and channel 2.
Conclusion
The LTC3828 is a constant-frequency
dual high performance step-down
switching regulator controller. Its
high efficiency, high power density,
current mode architecture make this
product ideal for automotive, telecom
and battery systems. L
Linear Technology Magazine • September 2006