LTC3815_15 Datasheet, PDF (27/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

LTC3815
Applications Information
Soft-Start
After the LTC3815 is turned on or power applied and
finishes its ~500µs initialization sequence, the chip enters
a soft start-up state. The type of soft startup behavior is
set by the TRACK/SS pin:
1. Tying TRACK/SS to V_IN selects the internal soft-start
circuit. This circuit ramps the output voltage to the final
value within 1ms.
2. If a longer soft-start period is desired, it can be set ex-
ternally with a capacitor on the TRACK/SS pin as shown
in Figure 10. The TRACK/SS pin reduces the value of
the internal reference at FB until TRACK/SS charges
above the REF pin voltage. The external soft-start dura-
tion can be calculated by using the following formula:
t_SS
=
V_REF • C_SS
5µA
3. The TRACK/SS pin can be used to track the output
voltage of another supply.
Regardless of either internal or external soft-start state,
the MODE pin is ignored and soft-start will always be in
discontinuous mode until SS voltage reaches the pro-
grammed V_OUT reference voltage for the first time.
TRACKING
Using the TRACK/SS or the REF pin, two types of tracking/
sequencing can be used for the LTC3815 (see Figure 12).
For ratiometric tracking, V_OUT will always track a ratio of
the input tracking voltage. In coincident tracking, V_OUT
will track a ratio of the input tracking voltage until V_OUT ≥
V_REF, then V_OUT is regulated to V_REF. Also, with ratiometric
tracking, the V_OUT can be adjusted with the margin com-
mands and MARGIN pin relative to the tracking voltage.
With coincident tracking, V_OUT can only be adjusted relative
to V_REF when V_MASTER ≥ V_REF.
For coincident tracking, be aware that the PGOOD window
is always centered around the DAC adjusted REF pin volt-
age, not the TRACK/SS pin voltage.
Ratiometric Tracking
To implement ratiometric tracking (Figure 13a and Figure
13b) the controlling voltage, V_MASTER is connected to the
REF pin. This source must be able to sink the 100µA I_REF
current. Using the REF pin allows V_OUT to be adjusted
with the margining commands and MARGIN pin. For
V_MASTER > V_SLAVE connect V_MASTER to the REF pin thru a
resistive divider. The relationship of V_MASTER to V_SLAVE is:
V_MASTER
=
V_SLAVE
•
⎛⎝⎜₁₊
R1⎞
R2 ⎠⎟
−
R1•
100µA
Note that the 100µA I_REF current requires V_MASTER > R1
• 100µA before V_SLAVE starts rising above 0V. Choose a
low value for R1 to minimize this offset.
For V_MASTER < V_SLAVE connect V_MASTER directly to REF
pin. The ratio of V_MASTER to V_SLAVE is now:
V_{MASTER =} R1
V_SLAVE R1+R2
Coincident Tracking
To implement coincident tracking with V_MASTER ≥ V_SLAVE
(Figure 13c) connect V_MASTER to the TRACK/SS pin directly
or with a resistive divider:
V_{MASTER =} R1
V_SLAVE R1+R2
V_OUT will follow V_MASTER (or a ratio of it as set by R1/R2)
until V_MASTER > V_REF at which time V_OUT is regulated to V_REF.
DDR Mode
The LTC3815 can both sink and source current if the MODE/
SYNC pin is set to forced continuous mode. Current sink-
ing is limited to –6A+ΔI_L/2. An external reference voltage
connected to the REF pin can be used to set the output
voltage. The output voltage can be margined by ±25% in
the same way as a resistor programmed reference.
For more information www.linear.com/LTC3815
3815p
27