LTC3703EGN Linear Technology, LTC3703EGN Datasheet - Page 10

• Manufacturer Part Number: LTC3703EGN
• Description: IC BUCK/BOOST SYNC ADJ 5A 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LTC3703EGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 93 V
• Current - Output: 5A
• Frequency - Switching: 100kHz ~ 600kHz
• Voltage - Input: 9.3 ~ 100 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
• Power - Output: -

LTC3703
OPERATION
side 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 sup-
ply is used on BGRTN, the switch node dV/dt could pull
the gate up 2V before the V
more than 0V across it.
Constant Frequency
The internal oscillator can be programmed with an external
resistor connected from f
100kHz and 600kHz, thereby optimizing component size,
effi ciency, and noise for the specifi c 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 automatically disabled.
Constant frequency operation brings with it a number of
benefi ts: inductor and capacitor values can be chosen for
a precise operating frequency and the feedback loop can
be similarly tightly specifi ed. Noise generated by the circuit
will always be at known frequencies. Subharmonic oscil-
lation and slope compensation, common headaches with
constant frequency current mode switchers, are absent in
voltage mode designs like the LTC3703.
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 0.9V, the main control loop is enabled with the
10
Figure 3. Floating TG Driver Supply and Negative BG Return
LTC3703
0V TO –5V
DRV
BGRTN
DRV
CC
CC
BOOST
SW
BG
TG
(Refer to Functional Diagram)
SET
GS
D
C
B
of the bottom MOSFET has
B
to ground to run between
V
IN
MT
MB
L
SS
3703 F03
. When C
+
+
C
C
V
IN
OUT
OUT
SS
duty cycle control set to 0%. As C
the 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 is in full operation. When the
RUN/SS is low, the supply current is reduced to 50μA.
Current Limit
The LTC3703 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
bottom MOSFET looks like a low value resistor during
its on-time, the voltage drop across it is proportional to
the current fl owing in it. In a buck converter, the average
current in the inductor is equal to the output current. This
current also fl ows through the bottom MOSFET during its
on-time. Thus by watching the drain-to-source voltage
when the bottom MOSFET is on, the LTC3703 can monitor
the output current. The LTC3703 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
V
V
OUT
RUN/SS
Figure 4. Soft-Start Operation in Start-Up and Current Limit
1.4V
0.9V
0V
4V
3V
0V
SHUTDOWN
LTC3703
ENABLE
DOWN MODE
START-UP
POWER
DUTY CYCLE
MINIMUM
IMAX
MAX
NORMAL OPERATION
) to ground. See the Current
pin. The I
MIN COMPARATOR ENABLED
OUTPUT VOLTAGE
IN REGULATION
RUN/SS SOFT-STARTS
OUTPUT VOLTAGE AND
INDUCTOR CURRENT
IMAX
SS
continues to charge,
MAX
MAX
selection.
pin. Since the
pin includes a
CURRENT
LIMIT
3703fb
MAX
3703 F04