ltc3770 Linear Technology Corporation, ltc3770 Datasheet - Page 16

• Manufacturer Part Number: ltc3770
• Description: Fast No Rsense Step-down Synchronous Controller With Margining, Tracking And Pll
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3770
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
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3770. The INTV
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
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:
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.
16
TG
T
J
CC
= 70°C + (14mA)(30V)(130°C/W) = 125°C
10Ω
Regulator
BOOST
SW
Figure 6. Optional External Gate Driver
Q1
FMMT619
Q2
FMMT720
GATECHG
GATE
OF M1
U
= f(Q
U
BG
g(TOP)
10Ω
CC
+ Q
INTV
PGND
pin can supply up to
W
g(BOT)
Q3
FMMT619
Q4
FMMT720
CC
). The junction
GATE
OF M2
3770 F06
U
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
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
disabled and PGOOD signal is forced low during this
phase. The total soft-start time can be calculated as:
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
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
refers to the slave LTC3770’s output.
To implement the coincident tracking in Figure 7a, connect
an additional resistive divider to V
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
OUT1
refers to the master LTC3770’s output and V
t
SOFTSTART
REFIN
pin. The force continuous mode is also
= 0.8 • V
CC
to turn off the soft-start current
REFIN
• C
SS
OUT1
SS
. Pin Z1 must be
/1.4µA
and connect its
OUT2
3770fb