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LTC1628IG-PG#PBF

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16
LTC1628/LTC1628-PG
1628fb
Medium voltage (20V to 35V) ceramic, tantalum, OS-CON
and switcher-rated electrolytic capacitors can be used as
input capacitors, but each has drawbacks: ceramic voltage
coefficients are very high and may have audible piezoelec-
tric effects; tantalums need to be surge-rated; OS-CONs
suffer from higher inductance, larger case size and limited
surface-mount applicability; electrolytics’ higher ESR and
dryout possibility require several to be used. Multiphase
systems allow the lowest amount of capacitance overall.
As little as one 22µF or two to three 10µF ceramic capaci-
tors are an ideal choice in a 20W to 35W power supply due
to their extremely low ESR. Even though the capacitance
at 20V is substantially below their rating at zero-bias, very
low ESR loss makes ceramics an ideal candidate for
highest efficiency battery operated systems. Also con-
sider parallel ceramic and high quality electrolytic capaci-
tors as an effective means of achieving ESR and bulk
capacitance goals.
In continuous mode, the source current of the top N-chan-
nel MOSFET is a square wave of duty cycle V
OUT
/V
IN
. To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current of one channel must
be used. The maximum RMS capacitor current is given by:
C quiredI I
VVV
V
IN RMS MAX
OUT IN OUT
IN
Re
/
≈
−
()
[]
12
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on only 2000 hours
of life. This makes it advisable to further derate the
capacitor, or to choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
paralleled to meet size or height requirements in the
design. Always consult the manufacturer if there is any
question.
The benefit of the LTC1628 multiphase can be calculated
by using the equation above for the higher power control-
ler and then calculating the loss that would have resulted
if both controller channels switch on at the same time. The
total RMS power lost is lower when both controllers are
operating due to the interleaving of current pulses through
the input capacitor’s ESR. This is why the input capacitor’s
requirement calculated above for the worst-case control-
ler is adequate for the dual controller design. Remember
that input protection fuse resistance, battery resistance
and PC board trace resistance losses are also reduced due
to the reduced peak currents in a multiphase system.
The
overall benefit of a multiphase design will only be fully
realized when the source impedance of the power supply/
battery is included in the efficiency testing.
The drains of
the two top MOSFETS should be placed within 1cm of each
other and share a common C
IN
(s). Separating the drains
and C
IN
may produce undesirable voltage and current
resonances at V
IN
.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The output ripple (∆V
OUT
) is determined by:
∆∆V I ESR
fC
OUT L
OUT
≈+
1
8
Where f = operating frequency, C
OUT
= output capacitance,
and ∆I
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆I
L
increases
with input voltage. With ∆I
L
= 0.3I
OUT(MAX)
the output
ripple will typically be less than 50mV at max V
IN
assum-
ing:
C
OUT
Recommended ESR < 2 R
SENSE
and C
OUT
> 1/(8fR
SENSE
)
The first condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran-
tees that the output capacitance does not significantly
discharge during the operating frequency period due to
ripple current. The choice of using smaller output capaci-
tance increases the ripple voltage due to the discharging
term but can be compensated for by using capacitors of
very low ESR to maintain the ripple voltage at or below
50mV. The I
TH
pin OPTI-LOOP compensation compo-
nents can be optimized to provide stable, high perfor-
mance transient response regardless of the output capaci-
tors selected.
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LTC1628/LTC1628-PG
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Manufacturers such as Nichicon, United Chemicon and
Sanyo can be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON capacitors is recommended to reduce the
inductance effects.
In surface mount applications multiple capacitors may
need to be used in parallel to meet the ESR, RMS current
handling and load step requirements of the application.
Aluminum electrolytic, dry tantalum and special polymer
capacitors are available in surface mount packages. Spe-
cial polymer surface mount capacitors offer very low ESR
but have lower storage capacity per unit volume than other
capacitor types. These capacitors offer a very cost-effec-
tive output capacitor solution and are an ideal choice when
combined with a controller having high loop bandwidth.
Tantalum capacitors offer the highest capacitance density
and are often used as output capacitors for switching
regulators having controlled soft-start. Several excellent
surge-tested 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. Aluminum
electrolytic capacitors can be used in cost-driven applica-
tions providing that consideration is given to ripple current
ratings, temperature and long term reliability. A typical
application will require several to many aluminum electro-
lytic capacitors in parallel. A combination of the above
mentioned capacitors will often result in maximizing per-
formance and minimizing overall cost. Other capacitor
types include Nichicon PL series, NEC Neocap, Pansonic
SP and Sprague 595D series. Consult manufacturers for
other specific recommendations.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV
CC
pin from the V
IN
supply pin. INTV
CC
powers
the drivers and internal circuitry within the LTC1628. The
INTV
CC
pin regulator can supply a peak current of 50mA
and must be bypassed to ground with a minimum of
4.7µF tantalum, 10µF special polymer, or low ESR type
electrolytic capacitor. A 1µF ceramic capacitor placed
directly adjacent to the INTV
CC
and PGND IC pins is highly
recommended. Good bypassing is necessary to supply
the high transient currents required by the MOSFET gate
drivers and to prevent interaction between channels.
Higher input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1628 to be
exceeded. The system supply current is normally domi-
nated by the gate charge current. Additional external
loading of the INTV
CC
and 3.3V linear regulators also
needs to be taken into account for the power dissipation
calculations. The total INTV
CC
current can be supplied by
either the 5V internal linear regulator or by the EXTV
CC
input pin. When the voltage applied to the EXTV
CC
pin is
less than 4.7V, all of the INTV
CC
current is supplied by the
internal 5V linear regulator. Power dissipation for the IC in
this case is highest: (V
IN
)(I
INTVCC
), and overall efficiency
is lowered. The gate charge current is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC1628 V
IN
current is limited to less than 24mA from a 24V supply
when not using the EXTV
CC
pin as follows:
T
J
= 70°C + (24mA)(24V)(95°C/W) = 125°C
Use of the EXTV
CC
input pin reduces the junction tempera-
ture to:
T
J
= 70°C + (24mA)(5V)(95°C/W) = 81°C
Dissipation should be calculated to also include any added
current drawn from the internal 3.3V linear regulator. To
prevent maximum junction temperature from being ex-
ceeded, the input supply current must be checked operat-
ing in continuous mode at maximum V
IN
.
EXTV
CC
Connection
The LTC1628 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 the switch closes,
connecting the EXTV
CC
pin to the INTV
CC
pin thereby
supplying internal power. 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
APPLICATIO S I FOR ATIO
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LTC1628/LTC1628-PG
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derived from the output during normal operation (4.7V <
V
OUT
< 7V) and from the internal regulator when the output
is out of regulation (start-up, short-circuit). If more cur-
rent is required through the EXTV
CC
switch than is speci-
fied, an external Schottky diode can be added between the
EXTV
CC
and INTV
CC
pins. Do not apply greater than 7V to
the EXTV
CC
pin and ensure that EXTV
CC␣
<␣ V
IN
.
Significant efficiency gains can be realized by powering
INTV
CC
from the output, since the V
IN
current resulting
from the driver and control currents will be scaled by a
factor of (Duty Cycle)/(Efficiency). For 5V regulators this
supply means connecting the EXTV
CC
pin directly to V
OUT
.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
CC
power
from the output.
The following list summarizes the four possible connec-
tions for EXTV
CC:
1. EXTV
CC
Left Open (or Grounded). This will cause INTV
CC
to be powered from the internal 5V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTV
CC
Connected directly to V
OUT
. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
CC
Connected to an External supply. If an external
supply is available in the 5V to 7V range, it may be used to
power EXTV
CC
providing it is compatible with the MOSFET
gate drive requirements.
Figure 6a. Secondary Output Loop & EXTV
CC
Connection
Figure 6b. Capacitive Charge Pump for EXTV
CC
4. EXTV
CC
Connected to an Output-Derived Boost Net-
work. For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with either the inductive boost
winding as shown in Figure 6a or the capacitive charge
pump shown in Figure 6b. The charge pump has the
advantage of simple magnetics.
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
B
in the functional diagram is charged though
external diode D
B
from INTV
CC
when the SW pin is low.
When one of the topside MOSFETs is to be turned 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 follows. With the topside MOSFET
on, the boost voltage is above the input supply: V
BOOST
=
V
IN
+ V
INTVCC
. The value of the boost capacitor C
B
needs
to be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the exter-
nal Schottky diode must be greater than V
IN(MAX)
. When
adjusting the gate drive level, the final arbiter is the total
input current for the regulator. If a change is made and the
input current decreases, then the efficiency has improved.
If there is no change in input current, then there is no
change in efficiency.
EXTV
CC
FCB
SGND
V
IN
TG1
SW
BG1
PGND
LTC1628
R
SENSE
V
OUT
V
SEC
+
C
OUT
+
1µF
1628 F06a
N-CH
N-CH
R6
+
C
IN
V
IN
T1
1:N
OPTIONAL EXTV
CC
CONNECTION
5V < V
SEC
< 7V
R5
EXTV
CC
V
IN
TG1
SW
BG1
PGND
LTC1628
R
SENSE
V
OUT
VN2222LL
+
C
OUT
1628 F06b
N-CH
N-CH
+
C
IN
+
1µF
V
IN
L1
BAT85 BAT85
BAT85
0.22µF
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LTC1628IG-PG#PBF

Mfr. #:
Buy LTC1628IG-PG#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual 2-phase Step-dn + Pgood
Lifecycle:
New from this manufacturer.
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