LTC1778EGN Linear Technology, LTC1778EGN Datasheet - Page 15

• 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: -

APPLICATIO S I FOR ATIO
INTV
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC1778. The INTV
50mA RMS and must be bypassed to ground with a
minimum of 4.7µF low ESR tantalum capacitor. Good
bypassing is necessary to supply the high transient cur-
rents required by the MOSFET gate drivers. Applications
using large MOSFETs with a high input voltage and high
frequency of operation may cause the LTC1778 to exceed
its maximum junction temperature rating or RMS current
rating. Most of the supply current drives the MOSFET
gates unless an external EXTV
tinuous mode operation, this current is I
+ Q
from the equations given in Note 2 of the Electrical
Characteristics. For example, the LTC1778CGN is limited
to less than 14mA from a 30V supply:
For larger currents, consider using an external supply with
the EXTV
EXTV
The EXTV
and control power from the output or another external
source during normal operation. Whenever the EXTV
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
pin to INTV
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV
ing list summarizes the possible connections for EXTV
1. EXTV
internal 5V regulator.
2. EXTV
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
T
g(BOT)
J
CC
CC
= 70°C + (14mA)(30V)(130°C/W) = 125°C
Regulator
CC
CC
CC
Connection
CC
CC
). The junction temperature can be estimated
CC
CC
grounded. INTV
connected to an output derived boost network.
connected to an external supply. A high effi-
pin and ensure that EXTV
pin.
pin can be used to provide MOSFET gate drive
. INTV
CC
U
power is supplied from EXTV
U
CC
is always powered from the
CC
CC
source is used. In con-
W
pin can supply up to
CC
GATECHG
≤ V
IN
. The follow-
U
= f(Q
CC
g(TOP)
until
CC
CC
CC
:
will start-up using the internal linear regulator until the
boosted output supply is available.
External Gate Drive Buffers
The LTC1778 drivers are adequate for driving up to about
30nC 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 7
can be used. Note that the bipolar devices reduce the
signal swing at the MOSFET gate, and benefit from an
increased EXTV
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC1778 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC1778 into a low quiescent current shutdown (I
30µA). Releasing the pin allows an internal 1.2µA current
source to charge up the external timing capacitor C
RUN/SS has been pulled all the way to ground, there is a
delay before starting of about:
When the voltage on RUN/SS reaches 1.5V, the LTC1778
begins operating with a clamp on I
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
is raised until its full 2.4V range is available. This takes an
additional 1.3s/µF, during which the load current is folded
back until the output reaches 75% of its final value. The pin
can be driven from logic as shown in Figure 7. Diode D1
t
DELAY
TG
=
Figure 7. Optional External Gate Driver
10Ω
1 2
1 5
.
BOOST
.
CC
SW
µ
V
Q1
FMMT619
Q2
FMMT720
A
voltage of about 6V.
C
LTC1778/LTC1778-1
SS
GATE
OF M1
=
(
1 3
. /
s F C
BG
µ
)
TH
10Ω
SS
of approximately
INTV
PGND
Q3
FMMT619
Q4
FMMT720
CC
GATE
OF M2
15
1778 F07
SS
1778fb
Q
. If
TH
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