LT3483_15 Datasheet, PDF (6/12 Pages) Linear Technology – Inverting Micropower DC/DC Converters with Schottky

LT3483/LT3483A
OPERATION
The LT3483/LT3483A use a constant off-time control
scheme to provide high efficiency over a wide range of
output currents. Operation can be best understood by
referring to the Block Diagram. When the voltage at the
FB pin is approximately 0V, comparator A3 disables most
of the internal circuitry. Output current is then provided
by external capacitor C_OUT, which slowly discharges until
the voltage at the FB pin goes above the hysteresis point
of A3. Typical hysteresis at the FB pin is 10mV. A3 then
enables the internal circuitry, turns on power switch Q1,
and the currents in external inductors L1A and L1B be-
gin to ramp up. Once the switch current reaches 200mA
(LT3483) or 400mA (LT3483A), comparator A1 resets
the latch, which turns off Q1 after about 80ns. Inductor
current flows through the internal Schottky D1 to GND,
charging the flying capacitor. Once the 300ns off-time has
elapsed, and internal diode current drops below 250mA
(as detected by comparator A2), Q1 turns on again and
ramps up to the switch current limit. This switching action
continues until the output capacitor charge is replenished
(until the FB pin decreases to 0V), then A3 turns off the
internal circuitry and the cycle repeats. The inverting charge
pump topology replaces L1B with the series combination
D2 and R2.
APPLICATIONS INFORMATION
CHOOSING A REGULATOR TOPOLOGY
Inverting Charge Pump
The inverting charge pump regulator combines an induc-
tor-based step-up with an inverting charge pump. This
configuration usually provides the best size, efficiency
and output ripple and is applicable where the magnitude of
V_OUT is greater than V_IN. Negative outputs to –38V can be
produced with the LT3483/LT3483A in this configuration.
For cases where the magnitude of V_OUT is less than or
equal to V_IN, use a 2-inductor or transformer configuration
such as the inverting flyback.
In the inverting charge pump configuration, a resistor
is added in series with the Schottky diode between the
negative output and the D pin of the LT3483/LT3483A. The
purpose of this resistor is to smooth/reduce the current
spike in the flying capacitor when the switch turns on. A
10Ω resistor works well for a Li⁺ to –8V application, and
the impact to converter efficiency is less than 3%. The
resistor values recommended in the applications circuits
also limit the switch current during a short-circuit condi-
tion at the output.
Inverting Flyback
The inverting flyback regulator, shown in the – 5V application
circuit, uses a coupled inductor and is an excellent choice
where the magnitude of the output is less than or equal
6
to the supply voltage. The inverting flyback also performs
well in a step-up/invert application, but it occupies more
board space compared with the inverting charge pump.
Also, the maximum |V_OUT| using the flyback is less than
can be obtained with the charge pump—it is reduced from
38V by the magnitudes of V_IN and ringing at the switch
node. Under a short-circuit condition at the output, a pro-
prietary technique limits the switch current and prevents
damage to the LT3483/LT3483A even with supply voltage
as high as 16V. As an option, a 0.47µF capacitor may be
added between terminals D and SW of LT3483/LT3483A
to suppress ringing at SW.
Inductor Selection
Several recommended inductors that work well with the
LT3483/LT3483A are listed in Table 1, although there are
many other manufacturers and devices that can be used.
Consult each manufacturer for more detailed information
and for their entire selection of related parts. Many differ-
ent sizes and shapes are available. For inverting charge
pump regulators with input and output voltages below
7V, a 4.7µH or 6.8µH inductor is usually the best choice.
For flyback regulators or for inverting charge pump
regulators where the input or output voltage is greater
than 7V, a 10µH inductor is usually the best choice. A
larger value inductor can be used to slightly increase the
available output current, but limit it to around twice the
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