MP1410
2A Step Down
DC to DC Converter
MPS
R1 = 2π*C7*VOUT*fC / (GEA*GCS*VFB)
Determine if the second compensation
capacitor, C4 is required. It is required if the
ESR zero of the output capacitor happens at
less than four times the crossover frequency.
Or:
Putting in the know constants and setting the
crossover frequency to the desired 40kHz:
R1 ≈ 1.37x108 C7*VOUT
8π*C7*RESR*fC ≥ 1
The value of R1 is limited to 10Kꢀ to prevent
output overshoot at startup, so if the value
calculated for R1 is greater than 10Kꢀ, use
10Kꢀ. In this case, the actual crossover
frequency is less than the desired 40kHz, and
is calculated by:
where RESR is the equivalent series resistance
of the output capacitor.
If this is the case, then add the second
compensation resistor. Determine the value
by the equation:
fC = R1*GEA*GCS*VFB / (2π*C7*VOUT
)
C4 = C7*RESR(max) / R1
or
fC ≈ 2.92 / (C7 * VOUT
)
Where RESR(MAX) is the maximum ESR of the
output capacitor.
Choose the compensation capacitor to set the
zero to ¼ of the crossover frequency.
Determine the value by the following equation:
Example:
VOUT =3.3V
C5 = 2 / π*R1*fC ≈ 1.59x10-5 / R1
C7 = 22µF Ceramic (ESR = 10mꢀ)
R1 ≈ (1.37 x 108) (22 x 10-6)(3.3V) = 9.9Kꢀ
Use the nearest standard value of 10Kꢀ.
if R1 is less than 10Kꢀ, or if R1 = 10Kꢀ use
the following equation:
C5 = 4C7*VOUT / (R12*GEA*GCS*VFB)
C5 ≈ 1.59x10-5 / 10KΩ = 1.6nF. Use the
nearest standard value of 1.5nF.
C5 ≈ 2.2x10-5 C7 * VOUT
2π C7 RESR fC = .055 which is less than 1,
therefore no second compensation capacitor is
required.
MP1410 Rev 1.6_ 07/25/03
7
MPS [ MONOLITHIC POWER SYSTEMS ]
