SP6120 Datasheet, PDF (17/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

APPLICATIONS INFORMATION: Continued
thermal improvement can be achieved in the maxi-
mum power dissipation through the proper design
of copper mounting pads on the circuit board. For
example, in a SO-8 package, placing two 0.04
square inches copper pad directly under the pack-
age, without occupying additional board space,
can increase the maximum power from approxi-
mately 1 to 1.2W. For DPAK package, enlarging
the tap mounting pad to 1 square inches reduces
the R_ΘJA from 96°C/W to 40°C/W.
Schottky Diode Selection
When paralleled with the bottom MOSFET, an
optional Schottky diode can improve efficiency
and reduce noises. Without this Schottky diode,
the body diode of the bottom MOSFET conducts
the current during the non-overlap time when both
MOSFETs are turned off. Unfortunately, the body
diode has high forward voltage and reverse recov-
ery problem. The reverse recovery of the body
diode causes additional switching noises when the
diode turns off. The Schottky diode alleviates
these noises and additionally improves efficiency
thanks to its low forward voltage. The reverse
voltage across the diode is equal to input voltage,
and the diode must be able to handle the peak
current equal to the maximum load current.
The power dissipation of the Schottky diode is
determined by
P_DIODE = 2V_FI_OUTT_NOLF_S
where
T_NOL = non-overlap time between GH and GL.
V_F = forward voltage of the Schottky diode.
COMP
®
R1
C2
C1
SP6120
Loop Compensation Design
The goal of loop compensation is to manipulate
loop frequency response such that its gain crosses
over 0db at a slope of –20db/dec. The SP6120
has a transconductance error amplifier and re-
quires the compensation network to be con-
nected between the COMP pin and ground, as
shown in Figure 18.
The first step of compensation design is to pick
the loop crossover frequency. High crossover
frequency is desirable for fast transient response,
but often jeopardize the system stability. Since
the SP6120 is equipped with 3% window com-
parator that takes over the control loop on tran-
sient, the crossover frequency can be selected
primarily to the satisfaction of system stability.
Crossover frequency should be higher than the
ESR zero but less than 1/5 of the switching
frequency. The ESR zero is contributed by the
ESR associated with the output capacitors and
can be determined by:
f_Z(ESR)
=
1
2πC_OUTR_ESR
Crossover frequency of 20kHz is a sound first
try if low ESR tantalum capacitors or poscaps
are used at the output. The next step is to calcu-
late the complex conjugate poles contributed by
the LC output filter,
1
f_P(LC) = _{2π√ LCOUT}
The open loop gain of the whole system can be
divided into the gain of the error amplifier,
PWM modulator, buck converter, and feedback
resistor divider. In order to crossover at the
selected frequency fco, the gain of the error
amplifier has to compensate for the attenuation
caused by the rest of the loop at this frequency.
In the RC network shown in Figure 18, the
product of R1 and the error amplifier
transconductance determines this gain. There-
fore, R1 can be determined from the following
equation that takes into account the typical error
amplifier transconductance, reference voltage
and PWM ramp built into the SP6120.
Figure 18. The RC network connected to the COMP pin
provides a pole and a zero to control loop.
R₁
=
1300V_OUT f_CO
V f _{IN P(LC)2}
f_Z(ESR)
Date: 1/21/05
SP6120 Low Voltage, AnyFET^TM, Synchronous, Buck Controller
© Copyright 2005 Sipex Corporation
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