SA57254-30GW Datasheet, PDF (11/18 Pages) NXP Semiconductors – CMOS switching regulator (PWM controlled)

Philips Semiconductors
CMOS switching regulator (PWM controlled)
Product data
SA57254-XX
Period 1: power switch on-time
During this period, a simple circuit loop is formed when the power
switch is on. The input voltage source is connected directly across
the boost inductor (L₀). A current ramp is exhibited whose slope is
described by:
IL_(on)
+
V_IN
L₀
Eqn. (1)
Energy is then stored within the core material of the inductor and is
described by:
E_sto + 0.5L₀ I_peak²
Eqn. (2)
This current ramp continues until the controller turns off the power
switch.
Period 2: inductor discharge period
The instant the power switch turns off, the current flowing through
the inductor forces the voltage at its output node (switched node) to
rise quickly above the input voltage (spike). This voltage is then
clamped when it exceeds the device’s output voltage and the output
rectifier becomes forward biased. The inductor empties its stored
energy in the form of a linearly decreasing current ramp whose
slope is dictated by:
I_L(off)
[
V_IN
* V_OUT
L₀
Eqn. (3)
The stored energy is transferred to the output capacitor. This output
current continues until the magnetic core is completely emptied of its
stored energy or the power switch turns back on.
Period 3: inductor empty state
DISCONTINUOUS MODE—This period as displayed in Figure 25 occurs
in the discontinuous–mode of operation of a boost converter. It is
identified by a period of “ringing” following the output period
(period 2). The inductor has been completely emptied of its stored
energy and the switched node returns to the level of the input
voltage. Ringing is seen at this node because a resonant circuit is
formed by the inductance of L₀ and any parasitic inductances and
capacitances connected to that node. This ringing has very little
energy and can easily be eliminated by a small passive snubber.
CONTINUOUS MODE—If the inductor is not completely emptied of its
stored energy before the power switch turns on again, the converter
is operating in the continuous mode. A small amount of residual flux
(energy) remains in the inductor core and the current waveform
jumps to an initial value when the power switch is again turned-on.
This mode offers some advantages over the discontinuous-mode,
because the peak current seen by the power switch is lower. In low
voltage applications, the inductor can store more energy with lower
peak currents.
The continuous mode waveforms can be seen in Figure 26.
2003 Nov 11
SPIKE
+V_IN
ENERGY BEING
STORED IN
INDUCTOR
ENERGY BEING
TRANSFERRED
TO OUTPUT
+V_OUT
0
V_IN
L₀
I_peak
(V_IN – V_OUT)
L₀
RESIDUAL FLUX
Figure 26. Continuous mode waveforms.
11
SL01465