E910.27A View Datasheet(PDF) - ELMOS Semiconductor AG
Part Name
Description
MFG CO.
'E910.27A' PDF : 50 Pages View PDF
- Preliminary -
E910.27A
7.8.1 Operating Modes
Hysteretic control does not require an internal oscillator. Switching frequency depends on external compo-
nents and operating modes. Operating frequency reduces at light loads resulting in excellent efficiency com-
pared to other architectures.
When delivering low output currents, the E910.27 operates in discontinuous conduction mode (DCM).
In case of delivering medium to high output currents, the E910.27 operates in PWM continuous conducted
mode (CCM). In this mode, current always flows through the choke and never ramps to zero. The pulse fre-
quency modulator (PFM) adjusts the internal P-channel MOS transistor duty cycle to maintain regulation
without exceeding the peak switching current set by the external current sense resistor (RS).
7.8.2 100% Duty Cycle and Dropout
The E910.27 operates with a duty cycle up to 100%. This feature extends the input voltage range by turning
the internal P-channel MOS transistor on continuously when the supply voltage (VS) approaches the output
voltage. This services the load when conventional switching regulators with less than 100% duty cycle would
fail. Dropout voltage is defined as the difference between the input (VINPUT) and output (VOUTPUT) volta-
ges when the input is low enough for the output to drop out of regulation. Dropout voltage depends on the
internal P-channel MOS drain to source on resistance, current sense resistor (RS), and choke (LCK) series
resistance and is proportional to the load current:
( ) VDROP = IOUT ⋅ RRDSON + RRS + RLCK
[3]
7.8.3 Discontinuous Conduction Mode
The E910.27 operates in discontinuous conduction mode (DCM) at light load current. In discontinuous con-
duction mode, current through the choke (LCK) starts at zero and ramps up to the peak as high as the vol-
tage comparator (VCMP) limit , then ramps down to zero. Next cycle starts when the VFB voltage reaches
the reference voltage. Until then, the choke current remains zero. Switching frequency is lower and switching
losses reduce. The switching frequency depends on input voltage, output capacitor and output load. The
switch waveform (Pin SWD) can exhibit ringing, which occurs at resonant frequency of the choke and stray
capacitance, due to residual energy stored in the core when the commutation diode (DSW) turns off.
7.8.4 Continuous Mode PFM Control Behaviour
The E910.27 PFM controller has an unique regulation scheme that allows PWM operation at medium and
high output current with automatic switching to pulse skipping mode at lower currents to improve light load
efficiency.
Under medium and heavy load operation, the choke (LCK) current is continuos and the SMPS part operates
in PWM mode. Current always flows through the choke and never ramps down to zero. In this condition, the
switching frequency is set by either the minimum on-time (TONM), or the minimum off-time (TOFFM),
depending on the duty cycle. The duty cycle is:
( ) Duty ≈
VOUTPUT + Vd DSW + IOUTMAX ⋅ RDCCHOKE
VINPUT + Vd DSW − IOUTMAX ⋅ RS + RDSON + RDCCHOKE
[4]
If the duty cycle is less than 33%, the minimum on-time (TONM) controls the switching frequency. The ope-
rating frequency is approximately:
ELMOS Semiconductor AG
Specification 22 / 50
03SP0357E.00 05.09.2006
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