SP6120B Datasheet, PDF (17/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

two 0.04 square inches copper pad directly
under the package, without occupying addi-
tional board space, can increase the maximum
power from approximately 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 con-
ducts the current during the non-overlap time
when both MOSFETs are turned off. Unfortu-
nately, the body diode has high forward voltage
and reverse recovery problem. The reverse re-
covery of the body diode causes additional
switching noises when the diode turns off. The
Schottky diode alleviates these noises and addi-
tionally 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
SP6120B
Figure 18. The RC network connected to the COMP pin
provides a pole and a zero to control loop.
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 SP6120B
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 SP6120B is equipped with 3% window
comparator that takes over the control loop on
transient, the crossover frequency can be se-
lected 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 contrib-
uted 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 SP6120B.
R₁
=
1300V_OUT f_CO
V f _{IN P(LC)2}
f_Z(ESR)
Date: 5/25/04
SP6120B Low Voltage, AnyFET^TM, Synchronous, Buck Controller
17
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