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LTC3770EG#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LTC3770
16
3770fc
To further limit current in the event of a short circuit to
ground, the LTC3770 includes foldback current limiting.
If the output falls by more than 60%, then the maximum
sense voltage is progressively lowered to about one tenth
of its full value.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3770. The INTV
CC
pin can supply up to 50mA
RMS and must be bypassed to ground with a minimum
of 4.7μF low ESR tantalum capacitor or other low ESR
capacitor. Good bypassing is necessary to supply the high
transient currents required by the MOSFET gate drivers.
Applications using large MOSFETs with a high input voltage
and high frequency of operation may cause the LTC3770
to exceed its maximum junction temperature rating or
RMS current rating. Most of the supply current drives the
MOSFET gates. In continuous mode operation, this current
is I
GATECHG
= f(Q
g(TOP)
+ Q
g(BOT)
). The junction temperature
can be estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3770EG is
limited to less than 14mA from a 30V supply:
T
J
= 70°C + (14mA)(30V)(130°C/W) = 125°C
External Gate Drive Buffers
The LTC3770 drivers are adequate for driving up to about
50nC into MOSFET switches with RMS currents of 50mA.
Applications with larger MOSFET switches or operating
at frequencies requiring greater RMS currents will benefi t
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 6
can be used.
APPLICATIONS INFORMATION
Soft-Start and Tracking
The LTC3770 has the ability to either soft-start by itself with
a capacitor or track the output of another supply. When
the device is confi gured to soft-start by itself, a capacitor
should be connected to the TRACK/SS pin. The LTC3770 is
put in a low quiescent current shutdown state (I
Q
< 30μA)
if the RUN pin voltage is below 1.5V. The TRACK/SS pin is
actively pulled to ground in this shutdown state. Once the
RUN pin voltage is above 1.5V, the LTC3770 is powered
up. A soft-start current of 1.4μA then starts to charge the
soft-start capacitor C
SS
. Pin Z1 must be grounded for
soft-start operation. Note that soft-start is achieved not
by limiting the maximum output current of the controller
but by controlling the ramp rate of the output voltage.
Current foldback is disabled during this soft-start phase.
During the soft-start phase, the LTC3770 is ramping the
reference voltage until it is 20% below the voltage set by
the V
REFIN
pin. The forced continuous mode is also disabled
and PGOOD signal is forced low during this phase. The
total soft-start time can be calculated as:
t
SOFTSTART
= 0.8 • V
REFIN
• C
SS
/1.4μA
When the device is confi gured to track another supply,
the feedback voltage of the other supply is duplicated
by a resistor divider and applied to the TRACK/SS pin.
Pin Z1 should be tied to INTV
CC
to turn off the soft-start
current in this mode. Therefore, the voltage ramp rate on
this pin is determined by the ramp rate of the other supply
output voltage.
Output Voltage Tracking
The LTC3770 allows the user to program how its output
ramps up and down by means of the TRACK/SS pin.
Through this pin, the output can be set up to either co-
incidentally or ratiometrically track with another supply’s
output, as shown in Figure 7. In the following discussions,
V
OUT1
refers to the master LTC3770’s output and V
OUT2
refers to the slave LTC3770’s output.
To implement the coincident tracking in Figure 7a, connect
an additional resistive divider to V
OUT1
and connect its
midpoint to the TRACK/SS pin of the slave IC. The ratio
of this divider should be selected the same as that of
the slave IC’s feedback divider shown in Figure 8. In this
Figure 6. Optional External Gate Driver
Q1
FMMT619
GATE
OF M1
TG
BOOST
SW
Q2
FMMT720
Q3
FMMT619
GATE
OF M2
BG
3770 F06
INTV
CC
PGND
Q4
FMMT720
10Ω 10Ω
LTC3770
17
3770fc
tracking mode, V
OUT1
must be set higher than V
OUT2
. To
implement the ratiometric tracking, the ratio of the divider
should be exactly the same as the master IC’s feedback
divider. Note that the pin Z1 of the slave IC should be tied
to INTV
CC
so that the internal soft-start current is disabled
in both tracking modes or it will introduce a small error
on the tracking voltage depending on the absolute values
of the tracking resistive divider.
By selecting different resistors, the LTC3770 can achieve
different modes of tracking including the two in Figure 7.
So which mode should be programmed? While either
mode in Figure 7 satisfi es most practical applications,
APPLICATIONS INFORMATION
there do exist some tradeoffs. The ratiometric mode saves
a pair of resistors, but the coincident mode offers better
output regulation. This can be better understood with the
help of Figure 9. At the input stage of the slave IC’s error
amplifi er, two common anode diodes are used to clamp
the equivalent reference voltage and an additional diode
is used to match the shifted common mode voltage. The
top two current sources are of the same amplitude. In the
coincident mode, the TRACK/SS voltage is substantially
higher than 0.6V at steady state and effectively turns off
D1. D2 and D3 will therefore conduct the same current
and offer tight matching between V
FB2
and the internal
precision 0.6V reference. In the ratiometric mode, however,
TRACK/SS equals 0.6V at steady state. D1 will divert part
of the bias current to make V
FB2
slightly lower than 0.6V.
Although this error is minimized by the exponential I-V
characteristic of the diode, it does impose a fi nite amount
of output voltage deviation. Furthermore, when the master
IC’s output experiences dynamic excursion (under load
transient, for example), the slave IC output will be affected
as well. For better output regulation, use the coincident
tracking mode instead of ratiometric.
Figure 7. Two Different Modes of Output Voltage Tracking
TIME
(7a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3770 F07
(7b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
Figure 8. Setup for Coincident and Ratiometric Tracking
R3 R1
R4 R2
R3
V
OUT2
R4
(8a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TRACK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3770 F08
(8b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TRACK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
Figure 9. Equivalent Input Circuit of Error Amplifi er
–
+
II
D1
TRACK/SS2
0.6V
V
FB2
D2
D3
3770 F09
EA2
LTC3770
18
3770fc
Margining
Margining is a way to program the reference voltage to
the error amplifi er to a voltage different from the default
0.6V. Margining is useful for customers who want to
stress their systems by varying supply voltages during
testing. The reference voltage to the error amplifi er is set
according to the following equation when the margining
function is enabled:
V
REFIN
= 0.6V ±(1.18V/R4) • R3
Referring to the functional diagram, 0.6V is the buffered
system reference at the V
REFOUT
pin. R3 and R4 are resistors
used for programming the amount of margining. V
REFIN
should be a voltage between 0.5V and 1V.
There are two logic control pins, MARGIN1 and MARGIN0,
to determine whether the margining function is enabled,
Margin up(+) or Margin down(–). Table 1 summarizes
the confi gurations:
Table 1. Margining Function
MARGIN1 MARGIN0 MODE
LOW LOW No Margining
LOW HIGH Margin Up
HIGH LOW Margin Down
HIGH HIGH No Margining
The buffered reference at V
REFOUT
has the ability to source
a large amount of current. However, it can only sink a
maximum of 50μA of current. To increase the sinking
capability of this reference, connect a resistor to ground
at this pin. One may also be tempted to connect a large
capacitor to this pin to fi lter out the noise. However, it is
recommended that no larger than 100pF of capacitance
should be connected to this pin.
Phase-Locked Loop and Frequency Synchronization
The LTC3770 has a phase-locked loop comprised of 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. The frequency range
of the voltage controlled oscillator is ±30% around the
center frequency f
O
. The center frequency is the operating
frequency discussed in the previous section. The LTC3770
incorporates a pulse detection circuit that will detect a
APPLICATIONS INFORMATION
clock on the PLLIN pin. In turn, it will turn on the phase-
locked loop function. The pulse width of the clock has
to be greater than 400ns and the amplitude of the clock
should be greater than 2V.
During the start-up phase, phase-locked loop function is
disabled. When LTC3770 is not in synchronization mode,
PLLFLTR pin voltage is set to around 1.18V. Frequency
synchronization is accomplished by changing the internal
on-time current according to the voltage on the PLLFLTR
pin.
The phase detector used is an edge sensitive digital type
which provides zero degrees phase shift between the
external and internal pulses. This type of phase detector
will not lock up on input frequencies close to the harmonics
of the VCO center frequency. The PLL hold-in range, Δf
H
,
is equal to the capture range, Δf
C
:
Δf
H
= Δf
C
= ±0.3 f
O
The output of the phase detector is a complementary pair of
current sources charging or discharging the external fi lter
network on the PLLFLTR pin. A simplifi ed block diagram
is shown in Figure 10.
If the external frequency (f
PLLIN
) is greater than the oscil-
lator frequency f
O
, current is sourced continuously, pulling
up the PLLFLTR pin. When the external frequency is less
than f
O
, current is sunk continuously, pulling down the
PLLFLTR pin. If the external and internal frequencies are
the same but exhibit a phase difference, the current sources
turn on for an amount of time corresponding to the phase
difference. Thus the voltage on the PLLFLTR pin is adjusted
until the phase and frequency of the external and internal
Figure 10. Phase-Locked Loop Block Diagram
DIGITAL
PHASE/
FREQUENCY
DETECTOR
PLLIN
PLLFLTR
2.4V
C
LP
3770 F10
R
LP
VCO
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LTC3770EG#PBF

Mfr. #:
Buy LTC3770EG#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Fast Synch Controller w/ Margining, Tracking, PLL
Lifecycle:
New from this manufacturer.
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