SP6659_06 Datasheet, PDF (8/11 Pages) Sipex Corporation – 1.5MHz, 600mA Synchronous Buck Regulator

THEORY OF OPERATION
In the Idle Mode, the inductor current may
reach zero Amps or reverse on each pulse.
The PWM control loop will automatically
skip pulses to maintain output regulation.
The bottom MOSFET is turned off by the
current reversal comparator, I_ZERO, and the
switch voltage will ring. This is the discon-
tinuous mode of operation, and is normal
behavior for the switching regulator.
_______________________ DROPOUT
OPERATION
When the input voltage decreases toward
the value of the output voltage, the SP6659
allows the main switch to remain on for more
than one switching cycle and increases the
duty cycle until it reaches 100%.
The duty cycle D of a step-down converter is
defined as:
D = T_ON X ƒ_OSC X 100%
≈ VOUT _{X 100%}
V_IN
where T_ON is the main switch on time, and
ƒ is the oscillator frequency (1.5Mhz).
OSC
The output voltage then is the input voltage
minus the voltage drop across the main
switch and the inductor. At low input supply
voltages, the R_DSON of the P-Channel
MOSFET increases, and the efficiency of
the converter decreases. Caution must be
exercised to ensure that the heat dissipated
does not exceed the maximum junction tem-
perature of the IC.
________________________ MAXIMUM
LOAD CURRENT
The SP6659 will operate with an input sup-
ply voltage as low as 2.5V, however, the
maximum load current decreases at lower
inputs due to a large IR drop on the main
switch and synchronous rectifier. The slope
compensation signal reduces the peak in-
ductor current as a function of the duty cycle
to prevent sub-harmonic oscillations at duty
cycles greater than 50%. Conversely, the
current limit increases as the duty cycle
decreases.
_______________________ INDUCTOR
SELECTION
For most designs, the SP6659 operates with
inductors of 1μH to 4.7μH. Low inductance
values are physically smaller but require
faster switching, which can result in some
efficiency loss. The inductor value can be
derived from the following equation:
L = VOUT X (VINMax - VOUT)
V_INMax X ΔI_L X
ƒ
OSC
Where ΔI_L is the inductor Ripple Current.
Large value inductors lower ripple current
and small value inductors result in higher
ripple current. Choose inductors at ripple
current of approximately 35% of the maxi-
mum load current 600mA, or ~ 210mA.
For output voltages above 2.0V, when light-
load efficiency is important, the minimum
recommended inductor is 2.2μH. For opti-
mum voltage-positioning load transients,
Date: 9/25/06 Rev C
SP6659 1.5MHz, 600mA Synchronous Buck Regulator
8
© 2006 Sipex Corporation