41 Overview BuckSwitching Converters 2 Synchronous NonSynchronous ExternalFET Controllers InternalFET Regulators LM3102 LM22676 LM3489 LM2747 Vin S D L C Vout Io Ic ID: 662660
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Slide1
Buck Regulator Architectures
4.1 OverviewSlide2
Buck-Switching Converters
2
Synchronous
Non-Synchronous
(External-FET) Controllers
(Internal-FET) Regulators
LM3102
LM22676
LM3489
LM2747Slide3
Vin
S
D
L
C
+
-
Vout
Io
Ic
Id
Is
IL
Δ
I
L, Ton
=
Δ
I
L, Toff
Δ
I
L, Ton
= (Vin – Vout ) * Ton / L
Δ
I
L, Toff
= -Vout * Toff
/ L
Vout = D*Vin
D = Ton / (Toff +Ton)
Non-Synchronous Buck Converters
3Slide4
Buck Topology:
Current and Voltage Waveforms
4
Input current
Low side switch or diode
Output and Inductor
SwitchSlide5
Light-Load Operation:
CCM and DCM
5
t
Inductor current reverses polarity at light loads
t
Inductor current drops to zero before the end of the cycle: “Discontinuous Conduction Mode” (DCM)
Full Synchronous
Mode. Stays in Continuous Conduction Mode (CCM)
Diode or
Diode Emulation
D = V
OUT
/V
IN
D
≠
V
OUT
/V
INSlide6
Cross Conduction with
Synchronous Buck
Direct Connection between V
IN
and GroundHigh – Side and Low – Side must not be in ON state at the same time
A time in which both MOSFET are Turned OFF is required: DEAD - TIME
6Slide7
DEAD – TIME
FIXED DEAD – TIME
Fixed time between Turn-OFF and Turn-ON
No flexibility in MOSFET choice
ADAPTATIVE DEAD – TIMEHigh-Side turns ON only if LS is OFF and vice versaFull flexibility in MOSFET choiceIt is necessary to detect the Turn-OFF of both MOSFET
7Slide8
Control Mode
Voltage Mode Control (VMC)
Current Mode Control (CMC)
Peak Current Mode Control (PCMC)
Valley Current Mode Control (VCMC)Average Current Mode Control (ACMC)Hysteretic Mode Control (HMC)
8Slide9
Voltage Mode Control
Advantages and Disadvantages
Advantages
Stable modulation/less sensitive to noise
Single feedback pathCan work over a wide range of duty cyclesDisadvantagesLoop gain proportional to V
INLC double pole often drives Type III compensationCCM and DCM differences - a compensation challengeSlow response to input voltage changes
Current limiting must be done separately9Slide10
Current Mode Control
Advantages and Disadvantages
Advantages
Power plant gain offers a single-pole roll-off
Line rejectionCycle-by-cycle current limiting protectionCurrent sharingDisadvantages
NoiseMinimum ON-timeSense resistor
10Slide11
Hysteretic Mode Control
Advantages and Disadvantages
Advantages
Ultra fast transient response (preferred to use in power hungry load)
No phase compensation required; In other words, Hysteretic Mode is a kind of large signal controlDisadvantagesNoise Jitter susceptibleVery layout sensitive
Large switching frequency variation; Minimum ripple requirement11Slide12
Thank you!
12