MC34164-3 Datasheet, PDF (18/23 Pages) Freescale Semiconductor, Inc – 3.0 A 1.0 MHz Fully Integrated DDR Switch-Mode Power Supply

TYPICAL APPLICATIONS
PROTECTION AND DIAGNOSTIC FEATURES
In order to find the maximum allowed ESR,
The effects of the ESR is often neglected by the designers
and may present a hidden danger to the ultimate supply
stability. Poor quality capacitors have widely disparate ESR
value, which can make the closed loop response
inconsistent.
Io
Io_step
Current
response
F_SW
Gate
Driver
Ramp
Generator
PWM
Comparitor
V_REFOUT
Error
Amplifier
SW
VOUT
INV
C_X
COMP
L
R_S
C_S
R_F
C_F
R_O ^CO
34712
Figure 9. Type III Compensation Network
Consider the crossover frequency, F_CROSS, of the open loop
gain at one-sixth of the switching frequency, F_SW.
Then,
⇒
dt_I_rise
Worst case
assumption
Figure 8. Transient Parameters
TYPE III COMPENSATION NETWORK
Power supplies are desired to offer accurate and tight
regulation output voltages. To accomplish this requires a high
DC gain. But with high gain comes the possibility of instability.
The purpose of adding compensation to the internal error
amplifier is to counteract some of the gains and phases
contained in the control-to-output transfer function that could
jeopardized the stability of the power supply. The Type III
compensation network used for 34712 comprises two poles
(one integrator and one high frequency pole to cancel the
zero generated from the ESR of the output capacitor) and two
zeros to cancel the two poles generated from the LC filter as
shown in Figure 9.
where R_O is a user selected resistor. Knowing the LC
frequency, it can be obtained the values of R_F and C_S:
This gives as a result,
&
34712
18
Analog Integrated Circuit Device Data
Freescale Semiconductor