16
LTC3819
3819f
C
OUT
required ESR < 4(R
SENSE
) and
C
OUT
> 1/(16f)(R
SENSE
)
The emergence of very low ESR capacitors in small,
surface mount packages makes very physically small
implementations possible. The ability to externally com-
pensate the switching regulator loop using the I
TH
pin(OPTI-LOOP compensation) allows a much wider se-
lection of output capacitor types. OPTI-LOOP compensa-
tion effectively removes constraints on output capacitor
ESR. The impedance characteristics of each capacitor
type are significantly different than an ideal capacitor and
therefore require accurate modeling or bench evaluation
during design.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance
through-hole capacitors. The OS-CON semiconductor
dielectric capacitor available from Sanyo and the Panasonic
SP surface mount types have the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON type capacitors is recommended to reduce
the inductance effects.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer sur-
face mount capacitors offer very low ESR also but have
much lower capacitive density per unit volume. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. Several excellent
choices are the AVX TPS, AVX TPSV or the KEMET T510
series of surface mount tantalums, available in case heights
ranging from 2mm to 4mm. Other capacitor types include
Sanyo OS-CON, POSCAPs, Panasonic SP caps, Nichicon
PL series and Sprague 595D series. Consult the manufac-
turer for other specific recommendations. A combination
of capacitors will often result in maximizing performance
and minimizing overall cost and size.
APPLICATIO S I FOR ATIO
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Figure 4. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages
DUTY FACTOR (V
OUT
/V
IN
)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.9
0.6
0.5
0.4
0.3
0.2
0.1
0
3819 F04
RMS INPUT RIPPLE CURRNET
DC LOAD CURRENT
2-PHASE
1-PHASE
It is important to note that the efficiency loss is propor-
tional to the input RMS current
squared
and therefore a
2-phase implementation results in 75% less power loss
when compared to a single phase design. Battery/input
protection fuse resistance (if used), PC board trace and
connector resistance losses are also reduced by the
reduction of the input ripple current in a 2-phase system.
The required amount of input capacitance is further
reduced by the factor, 2, due to the effective increase in
the frequency of the current pulses.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment has been met, the RMS current rating generally far
exceeds the I
RIPPLE(P-P)
requirements. The steady state
output ripple (V
OUT
) is determined by:
∆∆V I ESR
fC
OUT RIPPLE
OUT
+
1
16
Where f = operating frequency of each stage, C
OUT
=
output capacitance and I
RIPPLE
= combined inductor
ripple currents.
The output ripple varies with input voltage since I
L
is a
function of input voltage. The output ripple will be less than
50mV at max V
IN
with I
L
= 0.4I
OUT(MAX)
/2 assuming:
17
LTC3819
3819f
INTV
CC
Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV
CC
pin from the V
IN
supply pin. The INTV
CC
regulator powers the drivers and internal circuitry of the
LTC3819. The INTV
CC
pin regulator can supply up to 50mA
peak and must be bypassed to power ground with a
minimum of 4.7µF tantalum or electrolytic capacitor. An
additional 1µF ceramic capacitor placed very close to the
IC is recommended due to the extremely high instanta-
neous currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3819 to be
exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTV
CC
pin. When the voltage applied to the EXTV
CC
pin
is less than 4.7V, all of the INTV
CC
load current is supplied
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (I
IN
)(V
IN
– INTV
CC
) and
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC3819 V
IN
current is limited to less than 24mA from a 24V supply:
T
J
= 70°C + (24mA)(24V)(85°C/W) = 119°C
Use of the EXTV
CC
pin reduces the junction temperature to:
T
J
= 70°C + (24mA)(5V)(85°C/W) = 80.2°C
The input supply current should be measured while the
controller is operating in continuous mode at maximum
V
IN
and the power dissipation calculated in order to
prevent the maximum junction temperature from being
exceeded.
EXTV
CC
Connection
The LTC3819 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
When the voltage applied to EXTV
CC
rises above
4.7V, the
internal regulator is turned off and an internal switch
closes, connecting the EXTV
CC
pin to the INTV
CC
pin
thereby supplying internal and MOSFET gate driving power
to the IC. The switch remains closed as long as the voltage
applied to EXTV
CC
remains above 4.5V. This allows the
MOSFET driver and control power to be derived from a
separate 5V supply during normal operation (4.7V <
V
EXTVCC
< 7V) and from the internal regulator when the
external 5V supply is not available. Do not apply greater
than 7V to the EXTV
CC
pin and ensure that EXTV
CC
< V
IN
+
0.3V when using the application circuits shown.
If an
external voltage source is applied to the EXTV
CC
pin when
the V
IN
supply is not present, a diode can be placed in
series with the LTC3819’s V
IN
pin and a Schottky diode
between the EXTV
CC
and the V
IN
pin, to prevent current
from backfeeding V
IN
.
Topside MOSFET Driver Supply (C
B
,D
B
) (Refer to
Functional Diagram)
External bootstrap capacitors C
B1
and C
B2
connected to
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C
B
in the
Functional Diagram is charged though diode D
B
from
INTV
CC
when the SW pin is low. When the topside MOSFET
turns on, the driver places the C
B
voltage across the gate-
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to V
IN
and the BOOST pin rises to V
IN
+ V
INTVCC
.
The value of the boost capacitor C
B
needs to be 30 to 100
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of D
B
must be greater
than V
IN(MAX).
The final arbiter when defining the best gate drive ampli-
tude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
APPLICATIO S I FOR ATIO
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18
LTC3819
3819f
APPLICATIO S I FOR ATIO
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Table 1. VID Output Voltage Programming
CODE VID4 VID3 VID2 VID1 VID0 OUTPUT
00000 GND GND GND GND GND 1.4125V
00001 GND GND GND GND Float 1.4000V
00010 GND GND GND Float GND 1.3875V
00011 GND GND GND Float Float 1.3750V
00100 GND GND Float GND GND 1.3625V
00101 GND GND Float GND Float 1.3500V
00110 GND GND Float Float GND 1.3375V
00111 GND GND Float Float Float 1.3250V
01000 GND Float GND GND GND 1.3125V
01001 GND Float GND GND Float 1.3000V
01010 GND Float GND Float GND 1.2875V
01011 GND Float GND Float Float 1.2750V
01100 GND Float Float GND GND 1.2625V
01101 GND Float Float GND Float 1.2500V
01110 GND Float Float Float GND 1.2375V
01111 GND Float Float Float Float 1.2250V
10000 Float GND GND GND GND 1.2125V
10001 Float GND GND GND Float 1.2000V
10010 Float GND GND Float GND 1.1875V
10011 Float GND GND Float Float 1.1750V
10100 Float GND Float GND GND 1.1625V
10101 Float GND Float GND Float 1.1500V
10110 Float GND Float Float GND 1.1375V
10111 Float GND Float Float Float 1.1250V
11000 Float Float GND GND GND 1.1125V
11001 Float Float GND GND Float 1.1000V
11010 Float Float GND Float GND 1.0875V
11011 Float Float GND Float Float 1.0750V
11100 Float Float Float GND GND 1.0625V
11101 Float Float Float GND Float 1.0500V
11110 Float Float Float Float GND 1.0375V
11111 Float Float Float Float Float 1.0250V
Output Voltage
The LTC3819 has a true remote voltage sense capablity.
The sensing connections should be returned from the load
back to the differential amplifier’s inputs through a com-
mon, tightly coupled pair of PC traces. The differential
amplifier corrects for DC drops in both the power and
ground paths. The differential amplifier output signal is
divided down and compared with the internal precision
0.6V voltage reference by the error amplifier.
Output Voltage Programming
The output voltage is digitally programmed as defined in
Table 1 using the VID0 to VID4 logic input pins. The VID
logic inputs program a precision, 0.25% internal feedback
resistive divider. The LTC3819 has an output voltage range
of 1.025V to 1.4125V in 12.5mV steps.
Between the ATTENOUT
pin and ground is a variable
resistor, R1, whose value is controlled by the five VID input
pins (VID0 to VID4). Another resistor, R2, between the
ATTENIN and the ATTENOUT pins completes the resistive
divider. The output voltage is thus set by the ratio of
(R1 + R2) to R1.
Each VID digital input is pulled up by a 40k resistor in series
with a diode from V
BIAS
. Therefore, it must be grounded to
get a digital low input, and can be either floated or con-
nected to V
BIAS
to get a digital high input. The series diode
is used to prevent the digital inputs from being damaged
or clamped if they are driven higher than V
BIAS
. The digital
inputs accept CMOS voltage levels.
V
BIAS
is the supply voltage for the VID section. It is
normally connected to INTV
CC
but can be driven from
other sources. If it is driven from another source, that
source
must
be in the range of 2.7V to 5.5V and
must
be
alive prior to enabling the LTC3819.

LTC3819EG#PBF

Mfr. #:
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 2-Phase Synch Controller w/ VID
Lifecycle:
New from this manufacturer.
Delivery:
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