LTC3815_15 Datasheet, PDF (25/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

LTC3815
Applications Information
R_C1 = A convenient resistor value ~1kΩ.
f = chosen crossover frequency
G = 10^(GAIN/20) (this converts GAIN in dB to G in absolute
gain)
Type 2 Loop:
k = tan^{⎛⎝⎜BOO2ST} + 45°⎞⎠⎟
C_C1
=
2π
•
f
•
1
G•
K
•
R_C1
( ) ^CC2 ^{= C}C1 ^{K2 − 1}
R_C2
=
2π
•
K
f•
C_C2
R_B
=
V_REF (R_C1)
V_OUT − V_REF
SV_IN
LTC3815
REF
PGLIM
R_REF = R1 + R2
3815 F11a
Figure 11a. PGOOD Window Set to ±10% Default
SV_IN
REF
R1
LTC3815
PGLIM
R2
3815 F11b
Figure 11b. PGOOD Window Set By R1/R2
Type 3 Loop:
Figure 11. PGOOD Window
k = tan^{⎛⎝⎜BOO4ST} + 45°⎞⎠⎟
C_C1
=
2π
•
f
1
•G
•
RC1
C_C2 = C_C1(K − 1₎
R_C2
=
2π
•
K
f•
C_C2
R_C3
=
R_C1
K−1
C3 =
1
2πf K • R_C3
R_B
=
V_REF (R_C1)
V_OUT − V_REF
Programming PGOOD Threshold and Filter Delay
The upper and lower Power Good threshold default to
±10% when the PGLIM pin is tied to V_IN (see Figure 11a).
However, if a narrower or wider window is desired, these
windows can be programmed to any desired value in the
range of 5% to 40%.
The PGOOD upper/lower threshold is programmed by
replacing the single resistor on the REF pin with a resistor
divider as follows (see Figure 11b):
Reference Voltage = 100µA • (R1+R2)
PGOOD Window = ±40% • R2/(R1+R2)
The PGOOD window is always centered on the DAC adjusted
reference voltage, thus the center will move to the new
reference voltage when margined up or down.
Output Voltage Programming
The output voltage is set by an external resistor according
to the following equation:
V_OUT = 100µA • R_REF,
where R_REF is the total resistance between REF pin and
SGND.
The falling PGOOD (power good to power bad) is filtered
and delayed by 16 clock cycles, thus giving the loop 16
switching cycles to recover from the power bad condition
before pulling the PGOOD pin low.
The rising PGOOD (power bad to power good) filter delay is
user programmable by a configuration resistor at the PGFD
pin and is programmable to one of 7 possible delays from
3815p
For more information www.linear.com/LTC3815
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