LTC3770_15 Datasheet, PDF (16/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIONS INFORMATION
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 benefit
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 6
can be used.
BOOST
Q1
FMMT619
10Ω
TG
GATE
OF M1
Q2
FMMT720
INTV_CC
Q3
FMMT619
10Ω
BG
GATE
OF M2
Q4
FMMT720
SW
PGND
3770 F06
Figure 6. Optional External Gate Driver
16
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 configured 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 configured 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
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