LTC3703-5
10
37035fa
OPERATIO
U
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
GS
of the bottom MOSFET has more
than 0V across it.
cycle control set to 0%. As C
SS
continues to charge, 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 LTC3703-5 is in full operation. When the
RUN/SS is low, the supply current is reduced to 25
µ
A.
CURRENT
LIMIT
NORMAL OPERATION
START-UP
0V
4V
3V
1.4V
1V
0V
POWER
DOWN MODE
MINIMUM
DUTY CYCLE
OUTPUT VOLTAGE
IN REGULATION
LTC3703-5
ENABLE
MIN COMPARATOR ENABLED
37035 F03
RUN/SS SOFT-STARTS
OUTPUT VOLTAGE AND
INDUCTOR CURRENT
SHUTDOWN
V
OUT
V
RUN/SS
(Refer to Functional Diagram)
BOOST
TG
SW
BG
BGRTN
DRV
CC
DRV
CC
LTC3703-5
M1
M2
+
+
V
IN
C
IN
V
OUT
C
OUT
D
B
C
B
L
37035 F02
0V TO –5V
Figure 3. Soft-Start Operation in Start Up and Current Limit
Figure 2. Floating TG Driver Supply and Negative BG Return
Constant Frequency
The internal oscillator can be programmed with an exter-
nal resistor connected from f
SET
to ground to run between
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 4
µ
A current
source to charge the soft-start capacitor C
SS
. When C
SS
reaches 1V, the main control loop is enabled with the duty
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
MAX
pin. Since the bottom
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
MAX
pin. The I
MAX
pin includes a
12
µ
A pull-up, enabling the user to set the voltage at I
MAX
with a single resistor (R
IMAX
) to ground. See the Current
Limit Programming section for R
IMAX
selection.