LTC1778EGN Linear Technology, LTC1778EGN Datasheet - Page 9

• Manufacturer Part Number: LTC1778EGN
• Description: IC CONTROLLR STP-DWN SYNC 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1778EGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Current - Output: 2A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Power - Output: -
• Frequency - Switching: -

OPERATIO
Main Control Loop
The LTC1778 is a current mode controller for DC/DC
step-down converters. In normal operation, the top
MOSFET is turned on for a fixed interval determined by a
one-shot timer OST. When the top MOSFET is turned off,
the bottom MOSFET is turned on until the current com-
parator I
ating the next cycle. Inductor current is determined by
sensing the voltage between the PGND and SW pins using
the bottom MOSFET on-resistance . The voltage on the I
pin sets the comparator threshold corresponding to in-
ductor valley current. The error amplifier EA adjusts this
voltage by comparing the feedback signal V
output voltage with an internal 0.8V reference. If the load
current increases, it causes a drop in the feedback voltage
relative to the reference. The I
average inductor current again matches the load current.
At low load currents, the inductor current can drop to zero
and become negative. This is detected by current reversal
comparator I
discontinuous operation. Both switches will remain off
with the output capacitor supplying the load current until
the I
initiate another cycle. Discontinuous mode operation is
disabled by comparator F when the FCB pin is brought
below 0.8V, forcing continuous synchronous operation.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to
maintain regulation. The one-shot timer generates an on-
time that is proportional to the ideal duty cycle, thus
holding frequency approximately constant with changes
in V
external resistor R
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a ±7.5% window around the regulation point.
IN
TH
. The nominal frequency can be adjusted with an
voltage rises above the zero current level (0.8V) to
CMP
trips, restarting the one-shot timer and initi-
REV
U
which then shuts off M2, resulting in
ON
.
TH
voltage then rises until the
FB
from the
TH
Furthermore, in an overvoltage condition, M1 is turned off
and M2 is turned on and held on until the overvoltage
condition clears.
Foldback current limiting is provided if the output is
shorted to ground. As V
threshold voltage I
level set by Q4 and Q6. This reduces the inductor valley
current level to one sixth of its maximum value as V
approaches 0V.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2µA current source to charge
up an external soft-start capacitor C
reaches 1.5V, the controller turns on and begins switch-
ing, but with the I
0.6V below the RUN/SS voltage. As C
charge, the soft-start current limit is removed.
INTV
Power for the top and bottom MOSFET drivers and most
of the internal controller circuitry is derived from the
INTV
floating bootstrap capacitor C
charged from INTV
D
pin is grounded, an internal 5V low dropout regulator
supplies the INTV
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTV
a high efficiency source connected to EXTV
external 5V supply or a secondary output from the
converter, to provide the INTV
7V can be applied to EXTV
the input voltage is low and INTV
undervoltage lockout circuitry prevents the power
switches from turning on.
B
when the top MOSFET is turned off. When the EXTV
CC
CC
/EXTV
pin. The top MOSFET driver is powered from a
CC
Power
THB
TH
CC
CC
LTC1778/LTC1778-1
through an external Schottky diode
voltage clamped at approximately
power from V
is pulled down by clamp Q3 to a 1V
FB
CC
drops, the buffered current
for additional gate drive. If
CC
CC
B
. This capacitor is re-
to INTV
power. Voltages up to
CC
SS
IN
. When this voltage
drops below 3.5V,
. If EXTV
SS
CC
CC
continues to
. This allows
, such as an
CC
rises
1778fb
9
CC
FB