PI354X-00 Datasheet, PDF (28/39 Pages) –

Thermal De-rating
Thermal de-rating curves are provided that are based on
component temperature changes versus load current, input
voltage and air flow. It is recommended to use these curves as a
guideline for proper thermal de-rating. These curves represent the
entire system and are inclusive to both the Picor regulator and the
external inductor. Maximum thermal operation is limited by either
the MOSFETs or inductor depending upon line and
load conditions.
Thermal measurements were made using a standard
PI354x-00 Evaluation board which is 2.5 x 4 inches in area and
uses 4-layer, 2oz copper. Thermal measurements were made on
the three main power devices, the two internal MOSFETs and the
external inductor, with air flows of 0, 200, and 400LFM.
Small Signal Model - Constant Voltage Mode
The PI354x-00 product family is a variable frequency CCM/DCM
ZVS Buck Regulator. The small signal model for this powertrain
is that of a voltage controlled current source which has a
trans‑conductance that varies depending on the operating mode.
When the converter is operating at its normal frequency, it is in
discontinuous mode. As the load increases to the point at which
the boundary between discontinuous and continuous modes is
reached, the powertrain changes frequency to remain in critical
conduction mode. This mode of operation allows the PI354x-00
product family to have a very simple compensation scheme, as
the control to output transfer function always has a slope of
-1. In addition, when critical conduction is reached, the voltage
controlled current source becomes nearly ideal with a high output
equivalent resistance.
V_EAO
g_MOD
+
V_OUT
r_EQ
C_OUT
R_LOAD
Figure 54 — PI354x-00 Small Signal Model Control-Output
The control to output transfer function of the PI354x-00 product
family is defined as the gain from the output of the error
amplifier, through the modulator and to the output voltage. The
transfer function equation is shown in Equation (4), where g_MOD is
assumed to be 7S, r_EQ = 0.4Ω, C_OUT = 600µF and R_LOAD = 1Ω:
G_CO(s) =
1
R_LOAD
+
g_MOD
1
r_EQ
+ s (C_OUT )
(4)
PI354x-00
The Control-Output transfer function (also known as the small
signal modulator gain) has a single pole response determined
by the parallel combination of R_LOAD and r_EQ and the output
capacitor C_OUT. Equation (5) determines the frequency of the
modulator pole:
F_{P_MOD} =
2•π •
1
R_LOAD • r_EQ
R_LOAD + r_EQ
• C_OUT
(5)
Figure 55 depicts the small signal response of the modulator
when perturbing EAO and measuring the differential gain and
phase from EAO to V_OUT.
20
0
20
40
60
1
0
Gain - dBV
Phase-Degrees
20
40
60
80
100
10
100
1000
10000
100000
Frequency- Hz
Figure 55 — PI354x-00 Control-Output Gain/Phase Example
Error Amplifier
The small signal model of the error amplifier and compensator
is shown in Figure 56. The error amplifier is a trans-conductance
amplifier (TCA). The transfer function is shown in Equation (6),
where in this example R1 = 2.3kΩ, R2 = 1kΩ, GM_EAO = 5.1mS,
R_OUT = 1Meg, C_HF = 56pF, Ccomp = 4.7nF and R_ZI = 5kΩ. Here
it is important to note that the external components are Ccomp,
R1 and R2. The other components are internal to each specific
model. See the data tables section “Soft Start, Tracking And Error
Amplifier” for details.
V_OUT
GM_EAO
+
R1
C_HF
R_ZI
C_COMP
R_OUT
V_EAO
R2
Figure 56 — PI354x-00 Error Amplifier Model
Cool-Power^® ZVS Switching Regulators
Page 28 of 39
Rev 1.6
03/2017
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