SP6120B Datasheet, PDF (18/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

In Figure 18, R1 and C1 provides a zero f_Z1
which needs to be placed at or below f_P(LC). If f_Z1
is made equal to f_P(LC) for convenience, the
value of C1 can be calculated as
C₁
=
1
2πf_P(LC)R₁
The optional C2 generates a pole f_P1 with R1 to
cut down high frequency noise for reliable op-
eration. This pole should be placed one decade
higher than the crossover frequency to avoid
erosion of phase margin. Therefore, the value of
the C2 can be derived from
C₂
=
1
20πf_COR₁
Figure 19 illustrates the overall loop frequency
response and frequency of each pole and zero.
To fine-tune the compensation, it is necessary to
physically measure the frequency response us-
ing a network analyzer.
Gain
-20db/dec
-40db/dec
Loop
-20db/dec
f
-20db/dec
Error Amplifier
-20db/dec
f
f_Z1 f_P(LC) f_Z(ESR) f_CO f_P1
Figure 19. Frequency response of a stable system and its
error amplifier.
Current Sense
The SP6120B allows sensing current using the
inductor, PCB trace or current-sense resistor.
Inductor-sense utilizes the voltage drop across
the ESR of the inductor, while PCB trace and
current-sense resistor introduce additional re-
sistance in series with the inductor. The resis-
tance of the sense element determines the
overcurrent protection threshold as follows,
I_LIM = ^43mV
R_SEN
R_SEN = current-sense resistance which can be
implemented as ESR of the inductor, trace or
discrete resistor.
The maximum power dissipation on the current-
sense element is:
P_{S EN}
= I R2
OUT( max) SEN
For the inductor-sense scheme shown in the
application circuit, R_S and C_S are used to repli-
cate the signal across the ESR of the inductor. R_S
and C_S can be looked at as a low pass filter
whose output represents the DC differential
voltage between the switch node and the output.
At steady state, this voltage happens to be the
output current times the ESR of the inductor. In
addition, if the following relationship is satisfied,
L
ESR
= R_SC_S
the output of the RsCs filter represents the exact
voltage across the ESR, including the ripple.
Since the SP6120B’s hiccup overcurrent pro-
tection scheme is intended to safeguard sus-
tained overload conditions, the DC portion of
the current signal is more of interest. Therefore,
designing the R_SC_S time constant higher than L/
ESR provides reliable current sense against any
premature triggering due to noise or any tran-
sient conditions. Pick Rs between 10k and 100k,
and Cs can be determined by:
C_S
=
2
L
ESR
1
R_S
Here the time constant of R_SC_S is twice the value
of L/ESR.
In some applications, it may be desirable to
extend the current sense capability of a given
R_SEN element (usually the inductor ESR) be-
Date: 5/25/04
SP6120B Low Voltage, AnyFET^TM, Synchronous, Buck Controller
18
© Copyright 2004 Sipex Corporation