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參數(shù)資料
| 型號： | L6996D |
| 廠商： | STMICROELECTRONICS |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO24 |
| 封裝： | MO-153AD, TSSOP-24 |
| 文件頁數(shù)： | 4/26頁 |
| 文件大?。?/td> | 272K |
| 代理商： | L6996D |
L6996
12/26
has an average value equal to Io. The intention is to find the output average value called
Vo. It is important to
remember that the loop is closed a valley of the ripple, in this conditions the inputs of PWM comparator must be
equal, so the VFB+ =VFB-. Suppose R4=0 and R3=open.
Considering this and watching the figure 14 it can be written two equations at the VFB+ and VFB- node:
Eq 4
Rsense Io = Vc
Eq 5
Imposing Eq4=Eq5 it can be found the VOVALLEY value:
Eq 6
Vovalley = Vprog + Rs (1 + R1/R2) Io
Form Eq6 it can be noted the active drop effect due to R1, R2 resistors; it can be also noted the output average
value is different from the VPROG value, the error is due to the valley control, and it is equal to half of the ESR
voltage ripple.
To reduce the error of the average output voltage we can change the VPROG value using resistors. In fact con-
sidering the R3 resistor we can make a Thevenin equivalent:
Eq 7
Vprogeq = Vprog R3/(R3 + R2)
Eq 8
Req = R3//R2
How it can be seen the VPROGEQ is less the VPROG and so we can reduce the average output error. Remember
that the R1, R2 and RSENSE are selected in base at the Voltage Positioning needs.
The R4 resistor can be used to set also a positive offset at zero load. Considering the PWM comparator inputs:
Eq 9
Vo = VFB+ +R4 5A
Respect to a traditional PWM controller, that has an internal oscillator setting the switching frequency, in a hys-
teretic system the frequency can change with some parameters (input voltage, output current). In L6996 is im-
plemented the voltage feed-forward circuit that allows constant switching frequency during steady-sate
operation with the input voltage variation. There are many factors affecting switching frequency accuracy in
steady-state operation. Some of these are internal as dead times, which depend on high side MOSFET driver.
Others related to the external components as high side MOSFET gate charge and gate resistance, voltage
drops on supply and ground rails, low side and high side RDSON and inductor parasitic resistance.
During a positive load transient, (the output current increases), the converter switches at its maximum frequency
(the period is TON+TOFFmin) to recover the output voltage drop. During a negative load transient, (the output
current decreases), the device stops to switch (high side MOSFET remains off).
1.3 Transition from PWM to PFM
To achieve high efficiency at light load conditions, PFM mode is provided. The PFM mode differs from the PWM
mode essentially for the off section; the on section is the same. In PFM after a turn-on cycle the system turns-
on the low side MOSFET, until the current reaches the zero A value, when the zero-crossing comparator turns
off the low side MOSFET. In this way the energy stored in the output capacitor will not flow to ground, through
the low side MOSFET, but it will flow to the load. In PWM mode, after a turn on cycle, the system keeps the low
side MOSFET on until the next turn-on cycle, so the energy stored in the output capacitor will flow through the
low side MOSFET to ground. The PFM mode is naturally implemented in hysteretic controller, in fact in PFM
mode the system reads the output voltage with a comparator and then turns on the high side MOSFET when
the output voltage goes down a reference value. The device works in discontinuous mode at light load and in
V
oval l ey
V
prog
–
() R1
R1
R2
+
----------------------------------------------------------- -
Vc
=
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