LTC3703EG-5#TR Linear Technology, LTC3703EG-5#TR Datasheet - Page 10

• Manufacturer Part Number: LTC3703EG-5#TR
• Description: IC BUCK/BOOST SYNC ADJ 5A 28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LTC3703EG-5#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 55.8 V
• Current - Output: 5A
• Frequency - Switching: 100kHz ~ 600kHz
• Voltage - Input: 9.3 ~ 60 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATIO
LTC3703-5
MOSFETs can occur. To prevent this from occuring, the
bottom driver return is brought out as a separate pin
(BGRTN) so that a negative supply can be used to reduce
the effect of the Miller pull-up. For example, if a –2V supply
is used on BGRTN, the switch node dV/dt could pull the
gate up 2V before the V
than 0V across it.
Constant Frequency
The internal oscillator can be programmed with an exter-
nal resistor connected from f
100kHz and 600kHz, thereby optimizing component size,
efficiency, and noise for the specific application. The
internal oscillator can also be synchronized to an external
clock applied to the MODE/SYNC pin and can lock to a
frequency in the 100kHz to 600kHz range. When locked to
an external clock, Pulse Skip Mode operation is automati-
cally disabled. Constant frequency operation brings with it
a number of benefits: Inductor and capacitor values can be
chosen for a precise operating frequency and the feedback
loop can be similarly tightly specified. Noise generated by
the circuit will always be at known frequencies.
Subharmonic oscillation and slope compensation, com-
mon headaches with constant frequency current mode
switchers, are absent in voltage mode designs like the
LTC3703-5.
Shutdown/Soft-Start
The main control loop is shut down by pulling RUN/SS pin
low. Releasing RUN/SS allows an internal 4µA current
source to charge the soft-start capacitor C
reaches 1V, the main control loop is enabled with the duty
10
Figure 2. Floating TG Driver Supply and Negative BG Return
LTC3703-5
0V TO –5V
DRV
BGRTN
DRV
CC
CC
U
BOOST
(Refer to Functional Diagram)
SW
BG
TG
GS
of the bottom MOSFET has more
SET
D
C
B
B
to ground to run between
V
IN
M1
M2
L
SS
37035 F02
. When C
+
+
C
C
V
IN
OUT
OUT
SS
V
V
Current Limit
The LTC3703-5 includes an onboard current limit circuit
that limits the maximum output current to a user-pro-
grammed level. It works by sensing the voltage drop across
the bottom MOSFET and comparing that voltage to a user-
programmed voltage at the I
MOSFET looks like a low value resistor during its on-time,
the voltage drop across it is proportional to the current
flowing in it. In a buck converter, the average current in the
inductor is equal to the output current. This current also
flows through the bottom MOSFET during its on-time.
Thus by watching the drain-to-source voltage when the
bottom MOSFET is on, the LTC3703-5 can monitor the
output current. The LTC3703-5 senses this voltage and
inverts it to allow it to compare the sensed voltage (which
becomes more negative as peak current increases) with a
positive voltage at the I
12µA pull-up, enabling the user to set the voltage at I
with a single resistor (R
Limit Programming section for R
cycle control set to 0%. As C
duty cycle is gradually increased, allowing the output
voltage to rise. This soft-start scheme smoothly ramps the
output voltage to its regulated value, with no overshoot.
The RUN/SS voltage will continue ramping until it reaches
an internal 4V clamp. Then the MIN feedback comparator
is enabled and the LTC3703-5 is in full operation. When the
RUN/SS is low, the supply current is reduced to 25µA.
OUT
RUN/SS
Figure 3. Soft-Start Operation in Start Up and Current Limit
1.4V
0V
4V
3V
1V
0V
SHUTDOWN
LTC3703-5
ENABLE
DOWN MODE
START-UP
POWER
DUTY CYCLE
MINIMUM
MAX
IMAX
NORMAL OPERATION
pin. The I
) to ground. See the Current
MIN COMPARATOR ENABLED
SS
MAX
OUTPUT VOLTAGE
IN REGULATION
RUN/SS SOFT-STARTS
OUTPUT VOLTAGE AND
INDUCTOR CURRENT
continues to charge, the
IMAX
pin. Since the bottom
MAX
selection.
pin includes a
CURRENT
LIMIT
37035fa
37035 F03
MAX