LTC3704_15 Datasheet, PDF (8/28 Pages) Linear Technology – Wide Input Range, No RSENSE Positive-to-Negative DC/DC Controller

LTC3704
U
OPERATIO
Main Control Loop
The LTC3704 is a constant frequency, current mode
controller for DC/DC positive-to-negative converter appli-
cations. The LTC3704 is distinguished from conventional
current mode controllers because the current control loop
can be closed by sensing the voltage drop across the
power MOSFET switch instead of across a discrete sense
resistor, as shown in Figure 2. This sensing technique
improves efficiency, increases power density, and re-
duces the cost of the overall solution.
V_IN
V_SW
V_IN
SENSE
GATE
GND
GND
2a. SENSE Pin Connection for
Maximum Efficiency (V_SW < 36V)
V_IN
V_SW
V_IN
GATE
SENSE
GND
R_SENSE
GND
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2b. SENSE Pin Connection for Precise
Control of Peak I_IN/I_OUT or for V_SW > 36V
Figure 2. Using the SENSE Pin On the LTC3704
For circuit operation, please refer to the Block Diagram of
the IC and Figure 1. In normal operation, the power
MOSFET is turned on when the oscillator sets the PWM
latch and is turned off when the current comparator C1
resets the latch. The divided-down output voltage is com-
pared to an internal 1.230V reference by the error amplifier
EA, which outputs an error signal at the I_TH pin. The voltage
on the I_TH pin sets the current comparator C1 input
threshold. When the load current increases, a fall in the
NFB voltage relative to the reference voltage causes the I_TH
pin to rise, which causes the current comparator C1 to trip
at a higher peak inductor current value. The average
inductor current will therefore rise until it equals the load
current, thereby maintaining output regulation.
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The nominal operating frequency of the LTC3704 is pro-
grammed using a resistor from the FREQ pin to ground
and can be controlled over a 50kHz to 1000kHz range. In
addition, the internal oscillator can be synchronized to an
external clock applied to the MODE/SYNC pin and can be
locked to a frequency between 100% and 130% of its
nominal value. When the MODE/SYNC pin is left open, it is
pulled low by an internal 50k resistor and Burst Mode
operation is enabled. If this pin is taken above 2V or an
external clock is applied, Burst Mode operation is disabled
and the IC operates in continuous mode. With no load (or
an extremely light load), the controller will skip pulses in
order to maintain regulation and prevent excessive output
ripple.
The RUN pin controls whether the IC is enabled or is in a
low current shutdown state. A micropower 1.248V refer-
ence and comparator C2 allow the user to program the
supply voltage at which the IC turns on and off (compara-
tor C2 has 100mV of hysteresis for noise immunity). With
the RUN pin below 1.248V, the chip is off and the input
supply current is typically only 10μA.
The LTC3704 can be used either by sensing the voltage
drop across the power MOSFET or by connecting the
SENSE pin to a conventional shunt resistor in the source
of the power MOSFET, as shown in Figure 2. Sensing the
voltage across the power MOSFET maximizes converter
efficiency and minimizes the component count, but limits
the output voltage to the maximum rating for this pin
(36V). By connecting the SENSE pin to a resistor in the
source of the power MOSFET, the user is able to program
output voltages significantly greater than the 36V maxi-
mum input voltage rating for the IC.
Programming the Operating Mode
For applications where maximizing the efficiency at very
light loads (e.g., <100μA) is a high priority, Burst Mode
operation should be applied (i.e., the MODE/SYNC pin
should be connected to ground). In applications where
fixed frequency operation is more critical than low cur-
rent efficiency, or where the lowest output ripple is
desired, pulse-skip mode operation should be used and
the MODE/SYNC pin should be connected to the INTV_CC
pin. This allows discontinuous conduction mode (DCM)
operation down to near the limit defined by the chip’s
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