CS1601 Datasheet, PDF (12/16 Pages) Cirrus Logic – Digital PFC Controller for Electronic Ballasts

CS1601
table below depicts approximate values for R3 and R4 for a
range of boost-to-auxiliary inductor turns ratio, N.
N
~R3
~R4
9
46 k
1.75 k
10
42 k
1.75 k
11
37.5 k
1.75 k
12
35.5 k
1.75 k
13
32 k
1.75 k
14
29.5 k
1.75 k
15
27.5 k
1.75 k
Table 1. Aux Inductor Turns Ratio vs. R3 and R4
Resistors R3 and R4 were calculated using V_{link =} 460V and
C_p = 10pF.
Equation 6 is used to calculate the cut-off frequency defined
by the RC circuit at the ZCD pin.
f_c = 1  2R3  R4C_p
[Eq.6]
where:
f_c
The cut-off frequency, f_c, needs to be 10x the ringing
frequency.
C_p
Capacitance at the ZCD pin
5.8 Brownout Protection
The CS1601 brownout detection circuit monitors the peak of
the V_rect input voltage and disables the PWM switching when
it drops below a pre-determined threshold. Hysteresis and
minimum detection time are provided to avoid brownout
detection during short input transients. When brownout is
detected, the CS1601 enters standby mode. On recovery from
brownout, it re-enters normal operating mode.
Current I_AC is proportional to the AC input voltage V_rect, where
V_rect = R_IACxI_ACand R_IAC = R1+R2 in Figure 19 on page 11.
The digitized current applied to the IAC pin is monitored by the
brownout protection algorithm. When V_rect drops below the
brownout detection threshold, the CS1601 triggers a timer.
The IC asserts the brownout protection and stops the gate-
drive switching only if the timer exceeds 56ms. This is the
equivalent of 7 rectified line cycles at 60Hz.
During the brownout state, the device continues monitoring
the input line voltage. The device exits the brownout state
when I_AC exceeds the brownout upper threshold for at least
56ms. Typical values for the lower (I_BP(lower)) and upper
(I_BP(upper)) brownout thresholds are 31.6A and 39.6A,
respectively.
The overpower protection may activate prior to brownout
protection, depending on the load.
Brownout
Thresholds
Upper
Lower
TBrownout
56 ms
56 ms
Start
Timer
Enter Standby
Start Timer
Exit Standby
Figure 22. Brownout Sequence
The maximum response time of the brownout protection
feature occurs at light-load conditions. It is calculated by
Equation 7.
T_Brownout = 8 ms + ⁸--₅---^m--_V---^s- 128 V – V_{BPth} + 56 ms
= 8 + ⁸⁵-- 128 – 94.8 + 56 = 117ms
[Eq.7]
where:
V_BP(th) Brownout threshold voltage, V_{BP(th) =} I_BP(lower)xR_IAC
5.9 Overvoltage Protection
The overvoltage protection (OVP) will trigger immediately and
stop the gate drive when the current into the IFB pin (I_OVP)
exceeds 105% of the reference current (I_ref) value. The IC
resumes gate drive switching when the measured current at IFB
drops below I_OVP – I_OVP(Hy). Equation 8 is used to calculate the
OVP threshold (V_OVP).
V_OVP = R_IFB  I_OVP + V_DD
[Eq.8]
5.10 Overcurrent Protection
To limit boost inductor current through the FET and to prevent
boost inductor saturation conditions, the CS1601 incorporates
a cycle-by-cycle peak inductor current limit circuit using an
external shunt resistor to ‘sense’ the FET source current
accurately. The overcurrent protection (OCP) circuit is
designed to monitor the current when the active switch is
turned on. The OCP circuit is enabled after the leading-edge
blanking time (t_LEB). The shunt voltage is compared to a
reference voltage, V_cs(th), to determine whether an
overcurrent condition exists. The OCP circuit triggers
immediately, allowing the OCP algorithm to turn off the gate
driver.
The overcurrent protection circuit is also designed to monitor
for a catastrophic overcurrent occurrence by sensing sudden
and abnormal operating currents. A second OCP threshold,
V_cs(clamp), determines whether a severe overcurrent condition
exists. This immediately turns off the gate drive and the
system enters a restart mode. The CS1601 inhibits all
switching operations for approximately 1.6ms then attempts to
restart normal operation.
12
DS931PP6