LM3753 Datasheet, PDF (25/38 Pages) National Semiconductor (TI) – Scalable 2-Phase Synchronous Buck Controllers with Integrated FET Drivers and Linear Regulator Controller

The control loop is comprised of two parts. The first is the
power stage, which consists of the duty cycle modulator, cur-
rent sharing circuit, output filter and load. The second part is
the error amplifier, which is a voltage type operational ampli-
fier with a typical dc gain of 70 dB and a unity gain frequency
of 15 MHz. Figure 13 shows the power stage, error amplifier
and current sharing components.
FIGURE 13. Power Stage, Error Amplifier and Current Sharing
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The simplified power stage transfer function (also called the
control-to-output transfer function) for the LM3753/54 can be
written as:
Where:
With:
K_m is the dc modulator gain and R_i is the current-sharing gain.
K_FF is the input voltage feed-forward term, which is internally
set to a value of 0.232 V/V. The IAVE filter is accounted for
by H_a(s), which provides additional damping of the modulator
transfer function.
R_AV sets the gain of the current averaging amplifier. A fixed
value of 8 kΩ/phase must be used for proper scaling. Since
the effective resistance is in parallel, each LM3753/54 should
have a 4.02 kΩ 1% resistor at IAVE for 2-phase/controller op-
eration. C_AV sets the IAVE filter time constant of the current
sharing amplifier. For optimal performance of the current
sharing circuit, the IAVE filter is designed to settle to its final
value in five switching cycles. The optimal IAVE time constant
is defined as:
T = C_AV x R_AV
A value of C_AV = 1/(R_AV x f_SW) per phase must be used for the
optimal time constant. Each LM3753/54 should have a value
of two times the normalized single phase value of C_AV at IAVE
for 2-phase/controller operation. In this manner, the IAVE
time constant maintains a fixed value of T for any number of
phases.
Typical frequency response of the gain and the phase for the
power stage are shown in Figure 14 and Figure 15. It is de-
signed for V_IN = 12V, V_OUT = 1.2V, I_OUT = 25A per phase and
a switching frequency of 300 kHz. For 2-phase operation
R_AV = 4.02 kΩ and C_AV = 1000 pF. The power stage compo-
nent values per phase are:
L = 0.44 μH, R_L = 0.52 mΩ, C_O1 = 440 μF, R_C1 = 2.5 mΩ,
C_O2 = 44 μF, R_C2 = 1.5 mΩ, R_S = R_L = 0.52 mΩ and R_O =
V_OUT / I_OUT = 48 mΩ.
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