Transcript Load/Line Regulation (LDO)

Linear LDO Regulator
03/07/2011
Linear Regulator (LDO)
Application circuit
LDO
I IN
VIN
IO
R
VOUT
(regulated)
(unregulated)
C IN
COUT
GND
RL
IQ
1. VO < V IN
2. I O = I IN - I Q » I IN
Block digram
VIN
VOUT
series-pass
element
R2
Error amplifier
Vo = Vref  (1 
VFB
R1
reference
voltage
GND
VREF
R2
)
R1
Why use LDO and concerns
Why use:
• Easy design
• Simple structure and low cost
• Ripple-free and less noisy
• Fast transient response
Concerns:
• Poor efficiency
• Output voltage needs to be lower than input voltage
Key Specifications (LDO)
 Lower Dropout Voltage(Vdo)
 Lower Quiescent Current(Ground Current)
 Line and Load Regulation
 Transient Response
 PSRR(Power Supply Rejection Ratio)
 Current Limiting and Thermal Protection
 Output Noise Voltage
Dropout voltage (LDO)
The minimum input-to-output different voltage is required to maintain the
output voltage within specified voltage.
e.g., Vin_min = 2.8V(typical Vo)+ 0.3V(Vdo) =3.1V
higher efficiency for smaller Vin-Vo
 longer battery life. Battery discharges down to Vbat=Vo + Vdo

V Dropout = V IN - VOUT
VIN
VOUT
OUT
IN
GND
Quiescent Current (LDO)
Quiescent current (Iq):
Lower Iq, higher efficiency. Especially important under standby mode
Quiescent current I q = I in - I O
I in
Io
IN
VIN
OUT
GND
VOUT
Iq
Load/Line Regulation (LDO)
Load regulation:
VO
LR(%) =
IO
Line regulation:
VR(%) =
ΔVO
ΔVIN
Transient Response (LDO)
Transient load response:
Vtr ,max =
Vtr ,max =
IN
OUT
Io,max
C o  Cb
Io,max
Co  Cb
t 1  VESR
t 1  VESR
Vo
Vin
Cin
LDO
esr
GND
Cb
Load
Cout
PSRR (LDO)
Power rejection ratio:
Specified in db under 100, 1k, or 10k Hz .
-66dB at 10kHz
PSRR = 20 log
VOUT
VIN
Stability

LDO require proper Cout esr value to create a zero for enough phase
margin to maintain control loop stability.
 Usually stable region of ESR is provided on the datasheets.
Region of Stable COUT ESR vs. Load Current
100
Unstable Range
Vin
Cin
OUT
10
Vo
LDO
esr
GND
Cout
Cb
RL
COUT ESR (Ω)
IN
VIN=4.3V
CIN=COUT=1μF
1
Stable Range
0.1
0.01
Unstable Range
0.001
0
50
100
150
200
Load Current (mA)
250
300
Low output noise (LDO)
Output noise voltage:
Mainly caused by internal bandgap reference voltage.
RF(Wifi, Wimax, MP) applications require LDO with low noise voltage <100uVrms to avoid
noise voltage interfere with RF signals..
VIN=3.3V
VOUT=1.8V/10mA
θn
Output noise voltage =28.4uVrms
Efficiency and thermal issue (LDO)
Efficiency:
Efficiency ( ) =
I o Vo
V
(%) » o (%)
(I o  Iq )Vi
Vi
Power dissipation(Pd):
device power dissipation should below package power rating
Pd = (Vi –Vo) * Io
JA : thermal resistance from junction to ambient
JC = thermal resistance from junction(die temperature) to case
Thermal concern:
typical package power rating below is for reference
(please refer to datasheets JA,data to calculate for each LDO)
SOT23/25/26: Pw = 0.35W
SOT89: Pw = 0.5W
SOT223: Pw = 0.8W
TO252: Pw = 1.2W
TO263: Pw = 2W
SOP8: Pw = 0.6W
Q&A