Datasheet

UNISONIC TECHNOLOGIES CO., LTD
UC3750
Preliminary
CMOS IC
600kHZ PWM/PFM STEP-DOWN
DC-DC CONTROLLER
„
DESCRIPTION
The UTC UC3750 is a high frequency, micropower, voltage
mode step-down DC-DC controller IC and is designed for battery
operated hand-held electronic products. It can provide up to 2.0A
loading current with conversion efficiency by connection of
appropriate external P-type MOSFET. It also can be applied to
operate for voltage regulation with minimum external components
and board space.
The UTC UC3750 can operate in the constant -frequency PWM
(Pulse Width Modulation) mode that provide low output ripple noise
in the normal operation and will automatically switch to PFM(Pulse
Frequency Modulation) mode at low output loads for higher
efficiency. Moreover, it is ideal for portable applications profited from
Chip Enable (CE) to reduce IC off-stage current and integrated
feedback resistor network.
The UTC UC3750 is suitable for applications, such as PDA,
camcorders and digital still camera, hand-held instrument, distributed
power system, computer peripheral, conversion from four NiMH or
NiCd or one Lithium-ion cells to 3.3V/1.8V.
„
FEATURES
* High efficiency: 92% (Typ.)
* Low quiescent bias current :50μA
* Low shutdown current:0.3μA (Typ.)
* Output voltage:1.8V~3.3V ±2.0%
* Low output voltage ripple:50mV (Typ.)
* Built-in soft-start (SS)
* Pulse Width Modulation (PWM)switching
frequency:600kHz
„
* Automatic PWM/PFM switchover under light load
condition
* Very low dropout operation, 100% max. duty cycle
* Chip enable(CE) pin with on-chip 150nA pull-up current
source
* Input voltage:2.45V- 5.5V
* Internal under-voltage lockout (UVLO) protection
* Low profile and minimum external components
ORDERING INFORMATION
Ordering Number
Lead Free
Halogen Free
UC3750L-xx-AF5-R
UC3750G-xx-AF5-R
Note: xx: Output Voltage, refer to Marking Information.
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Copyright © 2011 Unisonic Technologies Co., Ltd
Package
Packing
SOT-25
Tape Reel
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UC3750
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Preliminary
CMOS IC
MARKING INFORMATION
PACKAGE
VOLTAGE CODE
MARKING
4
5
27: 2.7V
SOT-25
33: 3.3V
Voltage Code
UFXXG
L: Lead Free
1
„
PIN CONFIGURATION
„
PIN DESCRIPTION
„
PIN NO.
PIN NAME
1
CE
2
GND
3
VOUT
4
5
EXT
VIN
G: Halogen Free
2
3
DESCRIPTION
Chip Enable pin, active high (internal pull-up current source). By connecting this pin to
GND, the switching operation of the controller will be stopped.
Ground Connection
Output voltage monitoring input. This pin must be connected to the regulated output
node as a feedback to on-chip control circuitry. VOUT is internally connected to the
on-chip voltage divider that determines the output voltage level.
Gate drive for external P-MOSFET.
Power supply input
BLOCK DIAGRAM
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UC3750
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Preliminary
CMOS IC
ABSOLUTE MAXIMUM RATING (TA=25°C, unless otherwise specified)
PARAMETER
SYMBOL
RATINGS
UNIT
Device Power Supply
VIN
-0.3 ~ 6.0
V
Input Voltage (CE)
VCE
-0.3 ~ 6.0
V
Output Voltage (VOUT)
VOUT
-0.3 ~ 6.0
V
Output Voltage (EXT)
VEXT
-0.3 ~ 6.0
V
Junction Temperature
TJ
+150
°C
Operating Temperature
TOPR
−40~+85
°C
Storage Temperature
TSTG
−55~+150
°C
Note: Absolute maximum ratings are those values beyond which the device could be permanently damaged.
Absolute maximum ratings are stress ratings only and functional device operation is not implied.
„
THERMAL DATA
PARAMETER
Junction to Ambient
„
SYMBOL
θJA
RATINGS
250
UNIT
°C/W
ELECTRICAL CHARACTERISTICS
(TA = 25°C (TYP. value), TA =-40°C ~ 85°C (MIN. and MAX. values), unless otherwise specified)
UC3750-27(2.7V)
PARAMETER
TOTAL DEVICE
Input Voltage
Output Voltage
Input Current into VOUT Pin
SYMBOL
VIN
VOUT
ILOAD = 0mA, TA = 25°C
MIN
2.45
2.646
II(VOUT)
Operating Current
IDD
Off-State Current
OSCILLATOR
Frequency
Maximum Duty Cycle
Switchover ON Time Threshold (Note)
Soft-Start Delay Time (Note)
Protection Delay Time
OUTPUT DRIVE (EXT PIN)
EXT “H” Output Current
EXT “L” Output Current
EXT “L-H” Rise Time
EXT “H-L” Fall Time
EXT “L-H” Rise Time
EXT “H-L” Fall Time
CE (CE PIN)
CE “H” Input Voltage
CE “L” Input Voltage
CE “H” Input Current
CE “L” Input Current
Under-Voltage Lockout
Under-Voltage Lockout Threshold
Under-Voltage Lockout Hysteresis
IOFF
VIN =5.0V, VCE =5.0 V
(No External Components)
VIN = 5.0V, VCE = 0V, TA = 25°C
MAX UNIT
2.7
2.5
5.50
2.754
4.0
V
V
μA
50
80
μA
0.3
0.5
μA
600
690
500
kHz
%
nS
mS
mS
Auto Restart
320
8.0
8.0
IEXTH
IEXTL
TR
TF
TR
TF
VEXT =VIN -0.4V
VEXT = 0.4 V
CLOAD = 1000pF,VIN = 5.0V
CLOAD = 1000pF, VIN = 5.0V
CLOAD = 5.0nF, VIN = 5.0V
CCLOAD = 5.0nF, VIN = 5.0V
-60
100
65
40
140
90
VCEH
VCEL
ICEH
ICEL
VUVLO
VUVLO(HYS)
510
100
167
TYP
FOSC
DMAX
TON
TSS
TPORT
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TEST CONDITIONS
mA
mA
nS
nS
nS
nS
1.3
VIN =VCE = 5.0V
VIN =5.0V, VCE =0V
-0.5
-0.5
0
0.15
1.60
2.20
50
0.3
0.5
0.5
2.40
V
V
μA
μA
V
mV
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Preliminary
CMOS IC
ELECTRICAL CHARACTERISTICS
UC3750-33(3.3V)
PARAMETER
TOTAL DEVICE
Input Voltage
Output Voltage
Input Current into VOUT Pin
Operating Current
SYMBOL
VIN
VOUT
TEST CONDITIONS
ILOAD = 0mA, TA = 25°C
II(VOUT)
IDD
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TYP
MAX UNIT
2.45 5.0V 5.50
3.234 3.3 3.366
2.5
4.0
V
V
μA
50
80
μA
0.3
0.5
μA
600
690
320
8.0
8.0
500
kHz
%
nS
mS
mS
VIN =5.0V, VCE =5.0 V
(No External Components)
VIN = 5.0V, VCE = 0V, TA = 25°C
Off-State Current
IOFF
OSCILLATOR
Frequency
FOSC
Maximum Duty Cycle
DMAX
Switchover ON Time Threshold (Note)
TON
Soft-Start Delay Time (Note)
TSS
Protection Delay Time
TPORT
Auto Restart
OUTPUT DRIVE (EXT)
EXT “H” Output Current
IEXTH
VEXT =VIN -0.4V
EXT “L” Output Current
IEXTL
VEXT = 0.4 V
EXT “L-H” Rise Time
TR
CLOAD = 1000pF,VIN = 5.0V
EXT “H-L” Fall Time
TF
CLOAD = 1000pF, VIN = 5.0V
EXT “L-H” Rise Time
TR
CLOAD = 5.0nF, VIN = 5.0V
EXT “H-L” Fall Time
TF
CCLOAD = 5.0nF, VIN = 5.0V
CE (CE)
CE “H” Input Voltage
VCEH
CE “L” Input Voltage
VCEL
CE “H” Input Current
ICEH
VIN =VCE =5.0V
CE “L” Input Current
ICEL
VIN =5.0V, VCE =0V
Under-Voltage Lockout
Under-Voltage Lockout Threshold
VUVLO
Under-Voltage Lockout Hysteresis
VUVLO(HYS)
Notes: 1. PWM/PFM Switchover ON Time Threshold min/max guaranteed by design only.
UNISONIC TECHNOLOGIES CO., LTD
MIN
510
100
167
-60
100
65
40
140
90
mA
mA
nS
nS
nS
nS
1.3
-0.5
-0.5
0
0.15
1.60
2.20
50
0.3
0.5
0.5
2.40
V
V
μA
μA
V
mV
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Preliminary
CMOS IC
DESCRIPTION INFORMATION
1. Detailed Operating Description
The UTC UC3750 is a step-down DC−DC controllers designed specially in portable applications powered by
battery cells. The UTC UC3750 combines the advantages of Pulse frequency modulation (PFM) and
Constant-frequency pulse width modulation (PWM) which can provide excellent efficiency with light loads and high
efficiency and low output voltage ripple at heavy loads respectively. This device can provide up to 2.0 A loading
current with appropriate external P-type MOSFET connected. Because of working at high switching frequency, it’ s
possible to use small size surface mount inductor and capacitors to reduce PCB area and provide better interference
handling for noise sensitive applications.
2. The Internal Oscillator
The UTC UC3750 needs an oscillator to govern the switching of a PWM control cycles. The UTC UC3750 has
an internal Fixed− frequency oscillator. The oscillator signal generates all the using timing signals. And the oscillator
frequency is set to 600 kHz ±15%.
3. Voltage Reference and Soft-Start
The UTC UC3750 has a built-in soft-start (SS) circuit that controls the ramping up of the internal reference
voltage during the power-up of the converter. This function effectively enables the output voltage rise gradually over
the specified soft-start time (8 ms typ.), which prevents the output voltage from overshooting during startup of the
converter.
The UTC UC3750 includes an internal high accuracy voltage reference. This reference voltage is connected to
the inverting input terminal of the error amplifier (EA), A1, which compared with portion of the output voltage that is
derived from an integrated voltage divider with precise trimming to give the required output voltage with ±2%
accuracy.
4. Voltage Mode Pulse width modulation (PWM) Control Scheme
The UTC UC3750 is working in constant-frequency pulse width modulation (PWM) voltage mode control. The
controller operates with the internal oscillator which generates the required ramp function to compare with the output
of the error amplifier (EA), A1. The error amplifier (EA) compares the internally divided-down output voltage with the
voltage reference to produce an error voltage at its output. This error voltage is compared with the ramp function to
generate the control pulse to drive the external power switch. On a cycle-by-cycle basis, the greater the error voltage
is greater; the switch is held on longer. Hence, corresponding corrective action will be made to keep the output
voltage within regulation. Constant-frequency PWM reduces output voltage ripple and noise, which is one of the
important characteristics for noise sensitive communication applications. The high switching frequency allows small
size surface mount components to improve layout compactness, reducing PC board area, eliminating audio and
emission interference.
5. Power-Saving Pulse-Frequency-Modulation (PFM) Control Scheme
The converter enters the discontinuous conduction mode (DCM) operation when the load is decreasing, which
means the inductor current will decrease to 0 before the next switching cycle starts. In DCM operation, the ON time
for each switching cycle will decrease significantly when the output current decreases. In order to maintain high
conversion efficiency even at light load conditions, the ON time for each switching cycle is closely monitored and for
any ON time is smaller than the preset value (320 ns), the switching pulse will be skipped. As a result, when the
loading current is small, the converter will be operating in a “Constant ON time (320 ns nominal), variable OFF time”
Pulse frequency modulation (PFM) mode. This innovative control scheme improves the conversion efficiency for the
system at light load and standby operating conditions hence extend the operating life of the battery.
6. Low Power Shutdown Mode
Once the CE pin is tied to GND the UTC UC3750 can’t work. In shutdown mode, the internal reference,
oscillator, control circuitry, driver and internal feedback voltage divider are turned off and the output voltage falls to 0
V. Because most of the internal circuits can’t work and the current paths are cut-off, the device consume rather small
in this condition.
7. Under-Voltage Lockout (UVLO)
The UTC UC3750 incorporated the UVLO to prevent operation of the P-channel MOSFET below rated input
voltage levels. The comparator will turn-off the control circuitry and shut the converter down, once the input supply
voltage drops below about 2.2 V.
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Preliminary
CMOS IC
DESCRIPTION INFORMATION(Cont.)
8. Inductor Value Calculation
Selecting the proper inductance must take an inductor’s physical size, transient respond and power conversion
requirements into consideration. The fact is that lower inductor value can make the cost, PC board space smaller
and can provide faster transient response while resulting in higher ripple current and core losses. The loading
current, IOUT = 0.5A and the inductor ripple current, IL−RIPPLE(P−P) is designed to be less than 40% of the load current,
i.e. 0.5A x 40% = 0.2A. The relationship between the inductor value and inductor ripple current is as followed:
L=
TON × ( VIN - RDS( ON) × IOUT - VOUT )
IL - RIPPLE(P - P)
Where RDS (ON) is the ON resistance of the external P-channel MOSFET.
9. P-Channel Power MOSFET Selection
The operation of UTC UC3750 must be used with an external P-Channel power MOSFET. The main keys for
the power MOSFET are the gate threshold, VGS, the “ON” resistance, RDS (ON) and its total gate charge, QT. Low input
voltage operation need a low gate threshold device that can work down to the minimum input voltage level. RDS (ON)
determines the conduction losses for each switching cycle, the lower the ON resistance, the higher and the efficiency
can be achieved. A power MOSFET with lower gate charge can give lower switching losses but the fast transient can
cause unwanted EMI to the system. Compromise is required during the design stage.
10. Flywheel Diode Selection
The flywheel diode is turned on and carries load current during the off time. The average diode current depends
on the P-Channel switch duty cycle. At high input voltages, the diode conducts most of the time. In case of VIN
approaches VOUT, the diode conducts only a small fraction of the cycle. While the output terminals are shorted, the
diode will subject to its highest stress. Under this condition, the diode must be able to safely handle the peak current
circulating in the loop. So, it is important to select a flywheel diode that can meet the diode peak current and average
power dissipation requirements. Under normal conditions, the average current conducted by the flywheel diode is
given by:
ID =
VIN - VOUT
× IOUT
VIN + VF
Where ID is the average diode current and VF is the forward diode voltage drop.
A fast switching diode must also be used to optimize efficiency. Schottky diodes are ideal for low forward drop
and fast switching times.
11. Input and Output Capacitor Selection (CIN and COUT)
In continuous mode operation, the source current of the P-Channel MOSFET is a square wave of duty cycle
(VOUT +VF)/VIN. We must select a low ESR input capacitor that can support the maximum RMS input current to
prevent large input voltage transients. The maximum RMS input current, can be estimated by the equation below:
1
IRMS(MAX )
VOUT ( VIN - VOUT ) 2
≈IOUT ×
VIN
IRMS (MAX) has a maximum value at VIN = 2VOUT, where IRMS (MAX) = IOUT/2. As a general practice, this simple
worst-case condition is used for design. Selecting of the output capacitor, the required effective series resistance
(ESR) of the capacitor decide the COUT and the capacitance will be adequate for filtering unless the ESR requirement
is met. The output voltage ripple, VRIPPLE is approximated by:
VRIPPLE ≈IL - RIPPLE(P -P) × (ESR+
1
)
4FOSCCOUT
Where FOSC is the switching frequency and ESR is the effective series resistance of the output capacitor. We
can see from equation 4 that the output voltage ripple contributed by two parts. For a better performance, Low ESR
tantalum capacitors should be used. The major contributor is the capacitor ESR for most. Ordinary
aluminum-electrolytic capacitors have high ESR and should be avoided while higher quality Low ESR
aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Surface-mount tantalum capacitors are
better and provide neat and compact solution for space sensitive applications.
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Preliminary
CMOS IC
DESCRIPTION INFORMATION(Cont.)
12. PCB Layout Recommendations
Good PCB layout is important in switching mode power conversion. Careful PCB layout can minimize ground
bounce, EMI noise and unwanted feedbacks that can affect the converter performance.
13. Grounding
The output power return ground, the input power return ground and the device power ground should be
connected at the Star-ground. Feedback signal path must be separated from the main current path and sensing
directly at the anode of the output capacitor. All high current running paths must be thick enough for current flowing
through and producing insignificant voltage drop along the path.
14. Components Placement
Power components including input capacitor, inductor and output capacitor must be placed as close as possible.
All connecting traces must be short, direct and thick. In order to avoid unwanted injection of noise into the feedback
path high current flowing and switching paths must be kept away from the feedback pin terminal.
15. Feedback Path
Feedback of the output voltage must be separated from the power path. The output voltage sensing trace to the
feedback pin should be connected to the output voltage directly at the anode of the output capacitor.
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Preliminary
CMOS IC
TYPICAL APPLICATION CIRCUIT
L
M
VIN
CIN
VOUT
COUT
5
4
VIN
EXT
SD
CE GND VOUT
1
2
3
CE
GND
GND
UTC assumes no responsibility for equipment failures that result from using products at values that
exceed, even momentarily, rated values (such as maximum ratings, operating condition ranges, or
other parameters) listed in products specifications of any and all UTC products described or contained
herein. UTC products are not designed for use in life support appliances, devices or systems where
malfunction of these products can be reasonably expected to result in personal injury. Reproduction in
whole or in part is prohibited without the prior written consent of the copyright owner. The information
presented in this document does not form part of any quotation or contract, is believed to be accurate
and reliable and may be changed without notice.
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