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LTC1435IG#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
7
LTC1435
(Refer to Functional Diagram)
OPERATION
U
Main Control Loop
The LTC1435 uses a constant frequency, current mode
step-down architecture. During normal operation, 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 controlled by the
voltage on the I
TH
pin , which is the output of error amplifier
EA. The V
OSENSE
pin, described in the Pin Functions
section, allows EA to receive an output feedback voltage
V
FB
from an external resistive divider. When the load
current increases, it causes a slight decrease in V
FB
relative to the 1.19V reference, which in turn causes the I
TH
voltage to increase until the average inductor current
matches the new load current. While the top MOSFET is
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 driver is biased from floating bootstrap
capacitor C
B
, which normally is recharged during each off
cycle. However, when V
IN
decreases to a voltage close to
V
OUT
, the loop may enter dropout and attempt to turn on
the top MOSFET continuously. The dropout detector 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/SS
pin low. Releasing RUN/SS 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
TH
voltage clamped at approximately 30% of its maximum
value. As C
SS
continues to charge, I
TH
is gradually re-
leased allowing normal operation to resume.
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
The LTC1435 is capable of Burst Mode operation in which
the external MOSFETs operate intermittently based on
load demand. 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 following cycles the top and
bottom drives are disabled. This continues until an induc-
tor current peak exceeds 20mV/R
SENSE
or the I
TH
voltage
exceeds 0.6V, either of which causes drive to be returned
to the TG 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
+
and SENSE
–
pins is below 1.4V and
the other is when the SFB pin is below 1.19V. The latter
condition is used to assist in secondary winding regulation
as described in the Applications Information section.
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the other LTC1435 circuitry is derived from the INTV
CC
pin. The bottom MOSFET driver supply pin is internally
connected to INTV
CC
in the LTC1435. 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.
8
LTC1435
APPLICATIONS INFORMATION
WUU
U
The basic LTC1435 application circuit is shown in Figure
1, High Efficiency Step-Down Converter. External compo-
nent 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 configured
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 LTC1435 current comparator has a maximum thresh-
old of 150mV/R
SENSE
and an input common mode range
of SGND to INTV
CC
. The current comparator 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
.
Allowing a margin for variations in the LTC1435 and
external component values yields:
R
mV
I
SENSE
MAX
=
100
The LTC1435 works well with values of R
SENSE
from
0.005Ω to 0.2Ω.
C
OSC
Selection for Operating Frequency
The LTC1435 uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
OSC
. Each time the topside MOSFET turns on, the
voltage C
OSC
is reset to ground. During the on-time, C
OSC
is charged by a fixed current. 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:
CpF
OSC
() –=
1.37(10 )
Frequency (kHz)
4
11
A graph for selecting C
OSC
vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
OPERATING FREQUENCY (kHz)
C
OSC
VALUE (pF)
300
250
200
150
100
50
0
100 200 300 400
LTC1435 • F02
5000
Figure 2. Timing Capacitor Value
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz.
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 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
when the inductor current reaches zero while the bottom
9
LTC1435
APPLICATIONS INFORMATION
WUU
U
MOSFET is on. Lower inductor values (higher ∆I
L
) will
cause this to occur at higher load currents, which can
cause a dip in efficiency in the upper range of low current
operation. In Burst Mode operation, lower inductance
values will cause the burst frequency to decrease.
The Figure 3 graph gives a range of recommended induc-
tor values vs operating frequency and V
OUT
.
Kool Mµ is a registered trademark of Magnetics, Inc.
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mµ. Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they generally lack a bobbin, mounting is more
difficult. However, designs for surface mount are available
which do not increase the height significantly.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for use
with the LTC1435: an N-channel MOSFET for the top
(main) switch and an N-channel MOSFET for the bottom
(synchronous) switch.
The peak-to-peak gate drive levels are set by the INTV
CC
voltage. This voltage is typically 5V during start-up (see
EXTV
CC
Pin Connection). Consequently, logic level thresh-
old MOSFETs must be used in most LTC1435 applica-
tions. The only exception is applications in which EXTV
CC
is powered from an external supply greater than 8V (must
be less than 10V), in which standard threshold MOSFETs
(V
GS(TH)
< 4V) may be used. Pay close attention to the
BV
DSS
specification for the MOSFETs as well; many of the
logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
SD(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC1435 is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
Main Switch Duty Cycle =
V
V
Synchronous Switch Duty Cycle =
V
OUT
IN
IN
−
()
V
V
OUT
IN
The MOSFET power dissipations at maximum output
current are given by:
P
V
V
IR
ICf
P
VV
V
IR
MAIN
OUT
IN
MAX DS ON
MAX RSS
SYNC
IN OUT
IN
MAX DS ON
=
()
+
()
+
() ( )( )()
=
−
()
+
()
()
()
2
185
2
1
1
δ
δ
k V
IN
.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ
®
cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires more
turns of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
Figure 3. Recommended Inductor Values
OPERATING FREQUENCY (kHz)
0
0
INDUCTOR VALUE (µH)
10
20
30
40
60
50
100 150 200
1435 F03
250 300
50
V
OUT
= 5.0V
V
OUT
= 3.3V
V
OUT
= 2.5V
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LTC1435IG#PBF

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Manufacturer:
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Description:
Switching Voltage Regulators See P/N LTC1435AIG for Upgrade/
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