LTC3770 Datasheet, PDF (16/24 Pages) Linear Technology – Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIO S I FOR ATIO
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 10µF 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
For applications where more current is needed than INTV_CC
could supply, INTV_CC could be driven by an external
supply with a voltage higher than 5.3V. However, the
INTV_CC pin should not exceed its absolute maximum
voltage of 7V.
External Gate Drive Buffers
The LTC3770 drivers are adequate for driving up to about
BOOST
INTV_CC
Q1
FMMT619
10Ω
TG
GATE
OF M1
Q2
FMMT720
Q3
FMMT619
10Ω
BG
GATE
OF M2
Q4
FMMT720
SW
PGND
3770 F06
Figure 6. Optional External Gate Driver
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.
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 (IQ <
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 ramp-
ing the reference voltage until it is 20% below the voltage
set by the V_REFIN pin. The force 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
coincidentally 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 tracking
3770f