LTC3703_15 Datasheet, PDF (10/34 Pages) Linear Technology – 100V Synchronous Switching Regulator Controller

LTC3703
Operation (Refer to Functional Diagram)
side MOSFETs can occur. To prevent this from occurring,
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_GS of the bottom MOSFET has
more than 0V across it.
DRV_CC
V_IN
duty 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 is in full operation. When the
RUN/SS is low, the supply current is reduced to 50µA.
LTC3703
DRV_CC
BOOST
TG
SW
BG
BGRTN
0V TO –5V
D_B
+
C_IN
C_B
MT
L
V_OUT
MB
+
C_OUT
3703 F03
Figure 3. Floating TG Driver Supply and Negative BG Return
Constant Frequency
The internal oscillator can be programmed with an external
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 automatically 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 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_SS. When C_SS
reaches 0.9V, the main control loop is enabled with the
10
V_OUT
0V
SHUTDOWN START-UP
NORMAL OPERATION
4V
3V
V_RUN/SS
1.4V
0.9V
0V
MIN COMPARATOR ENABLED
OUTPUT VOLTAGE
IN REGULATION
RUN/SS SOFT-STARTS
OUTPUT VOLTAGE AND
INDUCTOR CURRENT
MINIMUM
DUTY CYCLE
LTC3703 POWER
ENABLE DOWN MODE
CURRENT
LIMIT
3703 F04
Figure 4. Soft-Start Operation in Start-Up and Current Limit
Current Limit
The LTC3703 includes an onboard current limit circuit that
limits the maximum output current to a user-programmed
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 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_MAX pin. The I_MAX pin includes a 12µA
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.
3703fc