PI354X-00 Datasheet, PDF (30/39 Pages) –

V_IN
C_IN
VIN
PGND _PI354X
VDR
SYNCO
SYNCI
PWRGD
EN
TESTx
SGND
VS1
VOUT
VSN
VSP
VDIFF
LGH
EAIN
EAO
COMP
TRK
L1
R1
R2
C_OUT
RC
V_OUT
R_LGH
R_SHUNT
Figure 59 — Lighting Configuration Using CC Mode
When using the CC mode, it is important to set R1 and R2
appropriately to avoid voltage loop interaction with the current
loop. In this case, the voltage setting at the EAIN pin should be
set so that the error between it and the 1V reference is sufficient
to force the EAO to be open loop and source current always.
When not using the LGH amplifier, the LGH pin should be
connected to SGND.
The LGH amplifier is able to sink more current than the error
amplifier can source, thus avoiding arbitration issues when
transitioning back and forth from LGH mode to voltage mode.
The equation for setting the source current for EAO is shown
in Equation (11).
( ) I_EAO = V_EAIN – V_REF • G_MEA > 400µA
(11)
LGH Amplifier Small Signal Model
A small signal model of the LGH amplifier is shown in Figure 60.
V_LGH
R_ZI
C_INT
+
E_INT
GM_LGH
+
+
E_LS
400µA
l_EAO
C_HF
R_ZI
C_COMP
R_OUT
V_EAO
Figure 60 — LGH Amplifier Small Signal Model
The LGH amplifier consists of three distinct stages. The first
is a wide bandwidth integrator stage, followed by a fixed
gain level shift circuit. Finally, the level shift circuit drives a
trans‑conductance (TCA) amplifier with an open collector sink
only output stage. Since the LGH output is internally connected
to the output of the voltage error amplifier, the compensation
components show up in the model and are used by both stages,
depending on which one is in use. Only one stage should be in
use at a time. When using LGH or if the LGH input rises above
Cool-Power^® ZVS Switching Regulators
Page 30 of 39
Rev 1.6
03/2017
PI354x-00
the internal reference, the voltage error amplifier acts as a 400µA
current source pull up for the EAO pin.
Figure 61 shows a small signal model of the modulator gain
when using the application circuit shown in Figure 59 with two
3.4V high current LED’s in series. R_LED is the series combination
of the AC resistance of each LED, which is 0.2Ω. R_SHUNT is used
to sense the current through the LED string. It has a value of
50mΩ in this case. The other component values were defined
earlier and remain the same values. Equation (12) defines the
transfer function of the modulator and Equation (13) defines
the pole of transfer function. The transfer function of the LGH
amplifier is defined in Equation (14). The open loop gain of E_INT is
2500 and E_LS = 4.4.
V_EAO
g_MOD
+
V_OUT
R_LED
r_EQ
C_OUT
R_SHUNT
V_LGH
Figure 61 — Lighting Application Modulator Gain Model
Figure 62 is the Bode plot of the G_LED(s) transfer function, which
in LGH mode is what needs to be compensated for by the LGH
amplifier and compensator. This transfer function defines the
gain and phase from the error amplifier output (EAO) to the
current shunt R_SHUNT. Figure 65 is a plot of the transfer function
G_{LGH_EAO}(s), which defines the gain and phase from the LGH
pin (voltage across current sensing R_SHUNT) to EAO. As shown in
Equation (14), the output is dependent on the integrator stage
and the following trans-conductance stage. Figures 63 and 64
show the two individual sections that make up Equation (14)
which produces G_{LGH_EAO}(s).
0
20
40
60
80
1
0
Gain - dBV
Phase-Degrees
20
40
60
80
100
10
100
1000
10000
100000
Frequency- Hz
Figure 62 — G_LED(s) Gain/Phase Plot
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