LTC3612_15 Datasheet, PDF (19/30 Pages) Linear Technology – 3A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3612
Applications Information
AVP Mode
Fast load transient response, limited board space and low
cost are typical requirements of microprocessor power
supplies. A microprocessor has typical full load step with
very fast slew rate. The voltage at the microprocessor must
be held to about ±0.1V of nominal in spite of these load
current steps. Since the control loop cannot respond this
fast, the output capacitors must supply the load current
until the control loop can respond.
Normally, several capacitors in parallel are required to
meet microprocessor transient requirements. Capacitor
ESR and ESL primarily determine the amount of droop or
overshoot in the output voltage.
Consider the LTC3612 without AVP with a bank of tantalum
output capacitors. If a load step with very fast slew rate
occurs, the voltage excursion will be seen in both direc-
tions, for full load to minimum load transient and for the
minimum load to full load transient.
If the ITH pin is tied to SV_IN, the active voltage position-
ing (AVP) mode and internal compensation are selected.
AVP mode intentionally compromises load regulation by
reducing the gain of the feedback circuit, resulting in an
output voltage that varies with load current. When the load
current suddenly increases, the output voltage starts from
a level slightly higher than nominal so the output voltage
can droop more and stay within the specified voltage
range. When the load current suddenly decreases the
output voltage starts at a level lower than nominal so the
output voltage can have more overshoot and stay within
the specified voltage range (see Figures 3 and 4).
The benefit is a lower peak-to-peak output voltage deviation
for a given load step without having to increase the output
filter capacitance. Alternatively, the output voltage filter
capacitance can be reduced while maintaining the same
peak to peak transient response. Due to the reduced loop
gain in AVP mode, no external compensation is required.
DDR Mode
The LTC3612 can both source and sink current if the MODE
pin is configured to forced continuous mode.
Current sinking is typically limited to 1.5A, for 1MHz
frequency and a 1µH inductor, but can be lower at higher
frequencies and low output voltages. If higher ripple current
can be tolerated, smaller inductor values can increase the
sink current limit. See the Typical Performance Charac-
teristics curves for more information.
In addition, tying the DDR pin to SV_IN, lower external
reference voltage and tracking output voltage between
channels are possible. See the Output Voltage Tracking
and External Reference Input sections.
Soft-Start
The RUN pin provides a means to shut down the LTC3612.
Tying the RUN pin to SGND places the LTC3612 in a low
quiescent current shutdown state (I_Q < 1µA).
The LTC3612 is enabled by pulling the RUN pin high. How-
ever, the applied voltage must not exceed SV_IN. In some
applications, the RUN signal is generated within another
power domain and is driven high while the SV_IN and PV_IN
is still 0V. In this case, it’s required to limit the current into
V_OUT
200mV/DIV
I_L
1A/DIV
V_OUT
100mV/DIV
I_L
1A/DIV
V_IN = 3.3V
50µs/DIV
V_OUT = 1.8V
I_LOAD = 100mA TO 3A
V_MODE = 1.5V
COMPENSATION FIGURE 1
3612 F03
V_IN = 3.3V
50µs/DIV
V_OUT = 1.8V
I_LOAD = 100mA TO 3A
V_MODE = 1.5V
V_ITH = 3.3V
OUTPUT CAPACITOR VALUE FIGURE 1
3612 F04
Figure 3. Load Step Transient Forced Continuous Mode
(AVP Inactive)
Figure 4. Load Step Transient Forced Continuous Mode
with AVP Mode
3612fc
For more information www.linear.com/LTC3612
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