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LTC1539CGW#TRPBF

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10
LTC1538-AUX/LTC1539
OPERATION
U
Main Control Loop
The LTC1538-AUX/LTC1539 use a constant frequency,
current mode step-down architecture. During normal op-
eration, the top MOSFET is turned on each cycle when the
oscillator sets the RS latch and turned off when the main
current comparator I1 resets the RS latch. The peak
inductor current at which I1 resets the RS latch is con-
trolled by the voltage on the I
TH1
(I
TH2
) pin, which is the
output of each error amplifier (EA). The V
PROG1
pin,
described in the Pin Functions, allows the EA to receive a
selectively attenuated output feedback voltage V
FB1
from
the SENSE
–
1 pin while V
PROG2
and V
OSENSE2
allow EA to
receive an output feedback voltage V
FB2
from either inter-
nal or external resistive dividers on the second controller.
When the load current increases, it causes a slight de-
crease in V
FB
relative to the 1.19V reference, which in turn
causes the I
TH1
(I
TH2
) voltage to increase until the average
inductor current matches the new load current. After the
large top MOSFET has turned off, the bottom MOSFET is
turned on until either the inductor current starts to reverse,
as indicated by current comparator I2, or the beginning of
the next cycle.
The top MOSFET drivers are biased from floating boot
strap capacitor C
B
, which normally is recharged during
each Off cycle. When V
IN
decreases to a voltage close to
V
OUT
, however, the loop may enter dropout and attempt to
turn on the top MOSFET continuously. The dropout detec-
tor counts the number of oscillator cycles that the top
MOSFET remains on and periodically forces a brief off
period to allow C
B
to recharge.
The main control loop is shut down by pulling the RUN/
SS1 (RUN/SS2) pin low. Releasing RUN/SS1 (RUN/SS2)
allows an internal 3µA current source to charge soft start
capacitor C
SS
. When C
SS
reaches 1.3V, the main control
loop is enabled with the I
TH1
(I
TH2
) voltage clamped at
approximately 30% of its maximum value. As C
SS
contin-
ues to charge, I
TH1
(I
TH2
) is gradually released allowing
normal operation to resume. When both RUN/SS1 and
RUN/SS2 are low, all LTC1538-AUX/LTC1539 functions
are shut down except for the 5V standby regulator, internal
reference and a comparator. Refer to the LTC1438/LTC1439
for applications which do not require a 5V standby regulator.
Comparator OV guards against transient overshoots
> 7.5% by turning off the top MOSFET and keeping it off
until the fault is removed.
Low Current Operation
Adaptive Power Mode allows the LTC1539 to automati-
cally change between two output stages sized for different
load currents. The TGL1 (TGL2) and BG1 (BG2) pins drive
large synchronous N-channel MOSFETs for operation at
high currents, while the TGS1 (TGS2) pin drives a much
smaller N-channel MOSFET used in conjunction with a
Schottky diode for operation at low currents. This allows
the loop to continue to operate at normal operating fre-
quency as the load current decreases without incurring the
large MOSFET gate charge losses. If the TGS1 (TGS2) pin
is left open, the loop defaults to Burst Mode operation in
which the large MOSFETs operate intermittently based on
load demand. Adaptive Power mode provides constant
frequency operation down to approximately 1% of rated
load current. This results in an order of magnitude reduc-
tion of load current before Burst Mode operation com-
mences. Without the small MOSFET (ie: no Adaptive
Power mode) the transition to Burst Mode operation is
approximately 10% of rated load current. The transition to
low current operation begins when comparator I2 detects
current reversal and turns off the bottom MOSFET. If the
voltage across R
SENSE
does not exceed the hysteresis of
I2 (approximately 20mV) for one full cycle, then on follow-
ing cycles the top drive is routed to the small MOSFET at
the TGS1 (TGS2) pin and the BG1 (BG2) pin is disabled.
This continues until an inductor current peak exceeds
20mV/R
SENSE
or the I
TH1
(I
TH2
) voltage exceeds 0.6V,
either of which causes drive to be returned to the TGL1
(TGL2) pin on the next cycle.
Two conditions can force continuous synchronous opera-
tion, even when the load current would otherwise dictate
low current operation. One is when the common mode
voltage of the SENSE
+
1 (SENSE
+
2) and SENSE
–
1
(SENSE
–
2) pins are below 1.4V, and the other is when the
SFB1 pin is below 1.19V. The latter condition is used to
assist in secondary winding regulation, as described in the
Applications Information section.
(Refer to Functional Diagram)
11
LTC1538-AUX/LTC1539
OPERATION
U
Frequency Synchronization
A Phase-Locked Loop (PLL) is available on the LTC1539
to allow the oscillator to be synchronized to an external
source connected to the PLLIN pin. The output of the
phase detector at the PLL LPF pin is also the control input
of the oscillator, which operates over a 0V to 2.4V range
corresponding to –30% to 30% in frequency. When
locked, the PLL aligns the turn-on of the top MOSFET to
the rising edge of the synchronizing signal. When PLLIN
is left open, PLL LPF goes low, forcing the oscillator to
minimum frequency.
Power-On Reset
The POR1 pin is an open drain output which pulls low
when the main regulator output voltage of the LTC1539
first controller is out of regulation. When the output
voltage rises to within 5% of regulation, a timer is started
which releases POR1 after 2
16
(65536) oscillator cycles.
Auxiliary Linear Regulator
The auxiliary linear regulator in the LTC1538-AUX and
LTC1539 controls an external PNP transistor for operation
up to 500mA. A precise internal AUXFB resistive divider is
invoked when the AUXDR pin is above 9.5V to allow
regulated 12V VPP supplies to be easily implemented.
When AUXDR is below 8.5V an external feedback divider
may be used to set other output voltages. Taking the
AUXON pin low shuts down the auxiliary regulator provid-
ing a convenient logic-controlled power supply.
The AUX block can be used as a comparator having its
inverting input tied to the internal 1.19V reference. The
AUXDR pin is used as the output and requires an external
pull-up to a supply of less than 8.5V in order to inhibit the
invoking of the internal resistive divider.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the other LTC1538-AUX/LTC1539 circuitry is derived
from the INTV
CC
pin. The bottom MOSFET driver supply is
also connected to INTV
CC
. When the EXTV
CC
pin is left
open, an internal 5V low dropout regulator supplies INTV
CC
power. If EXTV
CC
is taken above 4.8V, the 5V regulator is
turned off and an internal switch is turned on to connect
EXTV
CC
to INTV
CC
. This allows the INTV
CC
power to be
derived from a high efficiency external source such as the
output of the regulator itself or a secondary winding, as
described in the Applications Information section.
The 5V/20mA INTV
CC
regulator can be used as a standby
regulator when the two controllers are in shutdown or
when either or both controllers are on. Irrespective of the
signals on the RUN/SS pins, the INTV
CC
pin will follow the
voltage applied to the EXTV
CC
pin when the voltage applied
to the EXTV
CC
pin is taken above 4.8V. The externally
applied voltage is required to be less than the voltage
applied to the V
IN
pin at all times, even when both control-
lers are shut down. This prevents a voltage backfeed
situation from the source applied to the EXTV
CC
pin to the
V
IN
pin. If the EXTV
CC
pin is tied to the first controller’s 5V
output, the nominal INTV
CC
pin voltage will stay in the
guaranteed range of 4.7V to 5.2V.
(Refer to Functional Diagram)
APPLICATIONS INFORMATION
WUU
U
The basic LTC1539 application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection of R
SENSE
. Once
R
SENSE
is known, C
OSC
and L can be chosen. Next, the
power MOSFETs and D1 are selected. Finally, C
IN
and C
OUT
are selected. The circuit shown in Figure 1 can be config-
ured for operation up to an input voltage of 28V (limited by
the external MOSFETs).
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
The LTC1538-AUX/LTC1539 current comparator has a
maximum threshold of 150mV/R
SENSE
and an input com-
mon mode range of SGND to INTV
CC
. The current com-
parator threshold sets the peak of the inductor current,
yielding a maximum average output current I
MAX
equal to
the peak value less half the peak-to-peak ripple current, ∆I
L
.
12
LTC1538-AUX/LTC1539
APPLICATIONS INFORMATION
WUU
U
Allowing some margin for variations in the LTC1538-AUX/
LTC1539 and external component values yield:
R
mV
I
SENSE
MAX
=
100
The LTC1538-AUX/LTC1539 work well with values of
R
SENSE
from 0.005Ω to 0.2Ω.
C
OSC
Selection for Operating Frequency
The LTC1538-AUX/LTC1539 use a constant frequency
architecture with the frequency determined by an external
oscillator capacitor on C
OSC
. Each time the topside MOSFET
turns on, the voltage on C
OSC
is reset to ground. During the
on-time, C
OSC
is charged by a fixed current plus an
additional current which is proportional to the output
voltage of the phase detector (V
PLLLPF
)(LTC1539 only).
When the voltage on the capacitor reaches 1.19V, C
OSC
is
reset to ground. The process then repeats.
The value of C
OSC
is calculated from the desired operating
frequency. Assuming the phase-locked loop has no exter-
nal oscillator input (V
PLLLPF
= 0V):
CpF
Frequency kHz
OSC
()
.
–=
()
()
13710
11
4
A graph for selecting C
OSC
vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz. When using Figure 2 for
synchronizable applications, choose C
OSC
corresponding
to a frequency approximately 30% below your center
frequency. (See Phase-Locked Loop and Frequency
Sychronization).
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge losses. In addition to this basic trade
off, the effect of inductor value on ripple current and low
current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current ∆I
L
decreases with higher induc-
tance or frequency and generally increases with higher V
IN
or V
OUT
:
∆I
fL
V
V
V
L OUT
OUT
IN
=
1
1
()()
–
Accepting larger values of ∆I
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I
L
= 0.4(I
MAX
). Remember, the
maximum ∆I
L
occurs at the maximum input voltage.
The inductor value also has an effect on low current
operation. The transition to low current operation begins
OPERATING FREQUENCY (kHz)
C
OSC
VALUE (pF)
300
250
200
150
100
50
0
100 200 300 400
LTC1538 • F02
5000
V
PLLLPF
= 0V
Figure 2. Timing Capacitor Value
OPERATING FREQUENCY (kHz)
0
0
INDUCTOR VALUE (µH)
10
20
30
40
60
50
100 150 200
1538 F03
250 300
50
V
OUT
= 5.0V
V
OUT
= 3.3V
V
OUT
= 2.5V
Figure 3. Recommended Inductor Values
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LTC1539CGW#TRPBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual Const Freq Syn Sw Reg Cntrl
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