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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
June 2007
FAN5308 800mA High-Efficiency Step-Down DC-DC Converter
Features
96% efficiency, synchronous operation Adjustable output voltage options from 0.8V to VIN 2.5V to 5.5V input voltage range Up to 800mA output current Fixed-frequency 1.3MHz PWM operation High-efficiency, power-save mode 100% duty cycle low-dropout operation Soft-start Output over-voltage protection Dynamic output voltage positioning 25A quiescent current Thermal shutdown and short-circuit protection Pb-free 3x3mm 6-lead MLP package
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Description
Designed for use in battery-powered applications, the FAN5308 is a high-efficiency, low-noise synchronous PWM current mode and pulse skip (power-save) mode DC-DC converter. It can provide up to 800mA of output current over an input range from 2.5V to 5.5V. The output voltage can be externally adjusted over a range of 0.8V to 5.5V by means of an external voltage divider. At moderate and light loads, pulse skipping modulation is used. Dynamic voltage positioning is applied and the output voltage is shifted 0.8% above nominal value for increased headroom during load transients. At higher loads, the system automatically switches over to current mode PWM control, operating at 1.3MHz. A current mode control loop with fast transient response ensures excellent line and load regulation. To achieve high efficiency and ensure long battery life, the quiescent current is reduced to 25A in power-save mode, and the supply current drops below 1A in shut-down mode. The FAN5308 is available in a 3x3mm 6-lead MLP package.
Applications
Pocket PCs, PDAs Cell phones Battery-powered portable devices Digital cameras Hard disk drives Set-top boxes Point-of-load power Notebook computers Communications equipment
Ordering Information
Product Number
FAN5308
Output Voltage
Adjustable
Package Type
3x3mm 6-Lead MLP
Order Code
FAN5308MPX
Typical Application
VIN CIN 10F EN
3
SW
1 2 6
3.3H NC FB R2 10K R1 5K
VOUT 1.2V (800mA) COUT 2 x 10F
PGND
P1 (AGND)
5 4
Figure 1. Typical Application
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2 www.fairchildsemi.com
FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Pin Configuration
VIN PGND EN
1 2 3
6
SW NC FB
P1 (AGND)
5 4
Figure 2. Pin Assignment for 3x3mm 6-Lead MLP
Pin Description
Pin #
P1 1 2 3 4 5 6
Name
AGND VIN PGND EN FB NC SW Supply Voltage Input.
Description
Analog Ground. P1 must be soldered to the PCB ground. Power Ground. This pin is connected to the internal MOSFET switches. This pin must be externally connected to AGND. Enable Input. Logic high enables the chip and logic low disables the chip, reducing the supply current to less than 1A. Do not float this pin. Feedback Input. Adjustable voltage option, connect this pin to the resistor divider. No Connection Pin. Switching Node. This pin is connected to the internal MOSFET switches.
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Absolute Maximum Ratings
Stresses exceeding the absolute maximum ratings may damage the device. The device may not function or be operable above the recommended operating conditions and stressing the parts to these levels is not recommended. In addition, extended exposure to stresses above the recommended operating conditions may affect device reliability. The absolute maximum ratings are stress ratings only.
Parameter
VIN Voltage On Any Other Pin Lead Soldering Temperature (10 seconds) Junction Temperature Storage Temperature Thermal Resistance, Junction-to-Case (JC), 3x3mm 6-lead MLP(1) Electrostatic Discharge Protection (ESD) Level(2) HBM CDM
Min.
-0.3 -0.3
Max.
7.0 VIN 260 150
Unit
V V C C C C/W kV
-65 4 1
150 8
Notes: 1. Junction-to-ambient thermal resistance, JA, is a strong function of PCB material, board thickness, thickness and number of copper planes, number of via used, diameter of via used, available copper surface, and attached heat sink characteristics. 2. Using Mil Std. 883E, method 3015.7 (Human Body Model) and EIA/JESD22C101-A (Charged Device Model).
Recommended Operating Conditions
The Recommended Operating Conditions table defines the conditions for actual device operation. Recommended operating conditions are specified to ensure optimal performance to the datasheet specifications. Fairchild does not recommend exceeding them or designing to absolute maximum ratings.
Parameter
Supply Voltage Range Output Voltage Range, Adjustable Version Output Current Inductor(3) Input Capacitor
(3) (3)
Min.
2.5 0.8
Typ.
Max.
5.5 VIN 800
Unit
V V mA H F F
3.3 10 2 x 10 -40 -40 +85 +125
Output Capacitor
Operating Ambient Temperature Range Operating Junction Temperature Range Note: 3. Refer to the Applications section for details.
C C
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Electrical Characteristics
VIN = VOUT + 0.6V (minimum 2.5V) to 5.5V, IOUT = 350mA, VOUT =1.2V, EN = VIN, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = 25C.
Symbol
VIN IQ ISD VUVLO VENH VENL IEN
Parameter
Input Voltage Quiescent Current Shutdown Supply Current Under-Voltage Lockout Threshold Enable High Input Voltage Enable Low Input Voltage EN Input Bias Current PMOS On Resistance
Conditions
0 mA IOUT 800 mA IOUT = 0mA, Device is not switching IOUT = 0mA, Device is switching(4) EN = GND VIN Rising Hysteresis R2 =10k R2 =100k
Min.
2.5
Typ.
20 50 25 0.1
Max.
5.5 35
Units
V A A A
1.0 2.3
A V mV V
1.9 1.3
2.1 150
0.4 EN = VIN or GND VIN = VGS = 5.5V VIN = VGS = 2.5V VIN = VGS = 5.5V VIN = VGS = 2.5V 2.5V < VIN < 5.5V 1300 1000 0.01 250 300 200 250 1500 1300 0.1 0.1 0.16 0.15 0.8 0mA IOUT 800mA PWM Mode Only 350mA IOUT 800mA Rising Temperature Hysteresis -3 150 20 800 +3 0.10 350 400 300 350 2000 1500 1 1
V A m m mA KHz A A %/V % V % C C s
RDS-ON NMOS On Resistance ILIM fOSC Ilkg_(N) Ilkg_(P) P-channel Current Limit Oscillator Frequency
N-Channel Leakage Current VDS = 5.5V P-Channel Leakage Current VDS = 5.5V Line Regulation Load Regulation IOUT = 10mA 350mA IOUT 800mA
VREF
Reference Voltage Output DC Voltage Accuracy(5) Over-Temperature Protection
tST
Start-Up Time
IOUT = 800mA, COUT = 20F
Notes: 4. Refer to the Application section for details. 5. For output voltages 1.2V, a 40F output capacitor value is required to achieve a maximum output accuracy of 3% while operating in power-save mode (PFM mode).
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Typical Performance Characteristics
TA = 25C, CIN = 10F, COUT = 20F, L = 3.3H, R2 = 10k, unless otherwise noted.
100 95 90 100 95 90 85 80 75 70 65 60 VIN = 5.5V 55 50 45 40 35 0.1 1
V OUT = 3.3V VIN = 3.9V R 2 = 100k
Efficiency (%)
85 80 75 70 65 60 1 10 100 1000
VIN = 5V VIN = 3.6V V OUT = 3V V OUT = 3.3V VIN = 3.6V VOUT = 1.2V
Efficiency (%)
10
100
1000
Load Current (mA)
Load Current (mA)
Figure 3. Efficiency vs. Load Current
Figure 4. Efficiency vs. Load Current
100
V OUT = 1.2V
1.214 1.212 1.210 1.208 1.206 1.204 1.202 1.200 1.198 1.196 1.194 1.192 0 200 400 600 800 1000
VIN = 5V
90
R 2 = 100k
70
V IN = 5.5V
60 50 40 30 0.1
V IN = 3. 6V
VIN = 2.5V
1
10
100
1000
Load Current (mA)
Output Voltage (V)
80
Efficiency (%)
Load Current (mA)
Figure 5. Efficiency vs. Load Current
Figure 6. Output Voltage vs. Load Current
80
1400
Oscillator Frequency (kHz)
70
V OUT = 1. 2V
1380 1360 1340 1320 1300 1280 1260 1240 1220 1200 -40 -20 0 20
VIN = 2.5V V IN = 3.6V VIN = 5.5V
Quiescent Current (A)
60 50 40 30 20 10 0 2.5 3.0 3.5 4.0 4.5
R 2 = 100k R 2 = 10k
5.0
5.5
40
60
80
100
Input Voltage (V)
Temperature (C)
Figure 7. Quiescent Current vs. Input Voltage
Figure 8. Frequency vs. Temperature
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Typical Performance Characteristics (Continued) TA = 25C, CIN = 10F, COUT = 20F, L = 3.3H, R2 = 10k, unless otherwise noted.
SW Node Voltage (2V/div)
Inductor Output Current Voltage (200mA/div) (5mV/div)
Time (1s/div)
Inductor Current (200mA/div)
Output SW Node Voltage Voltage (20mV/div) (2V/div)
Time (5s/div)
Figure 9. PWM Mode
Figure 10. Power-Save Mode
Inductor Load Current Output Current Step Voltage (50mV/div) (500mA/div)
100mA
600mA
Inductor Load Current Current Step (500mA/div)
600mA
VOUT = 1.2V
100mA
VOUT = 1.2V
Time (10s/div)
Output Voltage (50mV/div)
Time (10s/div)
Figure 11. Load Transient Response
Figure 12. Load Transient Response
Voltage at Enable Pin (5V/Div)
Inductor Output Current Voltage (500mV/div) (200mA/div)
Inductor Output Voltage at Current Voltage Enable Pin (400mA/div) (5V/Div) (500mV/div)
VOUT = 1.2V IOUT = 10mA
VOUT = 1.2V IOUT = 800mA
Time (100s/div)
Time (200s/div)
Figure 13. Start-Up Response
Figure 14. Start-Up Response
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Block Diagram
EN VIN
DIGITAL SOFT START IS REF PFM COMP
UNDER-VOLTAGE LOCKOUT
IS
CURRENT SENSE
FB ERROR AMP COMP 0.8V MOSFET DRIVER
LOGIC CONTROL
SW
GND
IS
OVERVOLTAGE COMP
OSC SLOPE COMPENSATION
REF
FB
NEG. LIMIT COMP
NEG. LIMIT SENSE
GND
Figure 15. Block Diagram
Detailed Operation Description
The FAN5308 is a step-down converter operating in a current-mode PFM/PWM architecture with a typical switching frequency of 1.3MHz. At moderate to heavy loads, the converter operates in pulse-width-modulation (PWM) mode. At light loads, the converter enters a power-save mode (PFM pulse skipping) to keep the efficiency high.
PFM (Power-Save) Mode
As the load current decreases and the inductor current reaches negative value, the converter enters pulse-frequency-modulation (PFM) mode. The transition point for the PFM mode is given by the equation:
1 - ( V OUT V IN ) I OUT = V OUT x ------------------------------------------2xLxf
EQ. 1
PWM Mode
In PWM mode, the device operates at a fixed frequency of 1.3MHz. At the beginning of each clock cycle, the Pchannel transistor is turned on. The inductor current ramps up and is monitored via an internal circuit. The Pchannel switch is turned off when the sensed current causes the PWM comparator to trip when the output voltage is in regulation or when the inductor current reaches the current limit (set internally to typically 1500mA). After a minimum dead time, the N-channel transistor is turned on and the inductor current ramps down. As the clock cycle is completed, the N-channel switch is turned off and the next clock cycle starts.
The typical output current, when the device enters PFM mode, is 150mA for input voltage of 3.6V and output voltage of 1.2V. In PFM mode, the device operates with a variable frequency and constant peak current, thus reducing the quiescent current to minimum. Consequently, the high efficiency is maintained at light loads. As soon as the output voltage falls below a threshold, set at 0.8% above the nominal value, the P-channel transistor is turned on and the inductor current ramps up. The P-channel switch turns off and the N-channel turns on as the peak inductor current is reached (typical 450mA).
(c)2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
The N-channel transistor is turned off before the inductor current becomes negative. At this time, the P-channel is switched on again, starting the next pulse. The converter continues these pulses until the high threshold (typical 1.6% above nominal value) is reached. A higher output voltage in PFM mode gives additional headroom for the voltage drop during a load transient from light to full load. The voltage overshoot during this load transient is also minimized due to active regulation during turn on of the N-channel rectifier switch. The device stays in sleep mode until the output voltage falls below the low threshold. The FAN5308 enters the PWM mode as soon as the output voltage can no longer be regulated in PFM with constant peak current.
UVLO and Soft Start
The reference and the circuit remain reset until the VIN crosses its UVLO threshold. The FAN5308 has an internal soft-start circuit that limits the inrush current during start-up. This prevents possible voltage drops of the input voltage and eliminates the output voltage overshoot. The soft-start is implemented as a digital circuit, increasing the switch current in four steps to the P-channel current limit (1500mA). Typical start-up time for a 20F output capacitor and a load current of 800mA is 800s.
Short-Circuit Protection
The switch peak current is limited cycle-by-cycle to a typical value of 1500mA. In the event of an output voltage short circuit, the device operates with a frequency of 400kHz and minimum duty cycle; therefore, the average input current is typically 200mA.
100% Duty Cycle Operation
As the input voltage approaches the output voltage and the duty cycle exceeds the typical 95%, the converter turns the P-channel transistor continuously on. In this mode, the output voltage is equal to the input voltage, minus the voltage drop across the P-channel transistor: VOUT = VIN - ILOAD x (RdsON + RL)
where: RdsON = P-channel switch on resistance ILOAD = Output current RL = Inductor DC resistance
EQ. 2
Thermal Shutdown
When the die temperature exceeds 150C, a reset occurs and remains in effect until the die cools to 130C. At that time, the circuit is allowed to restart.
(c)2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Applications Information
Setting the Output Voltage
The internal reference is 0.8V (typical). The output voltage is divided by a resistor divider, R1 and R2 to the FB pin. The output voltage is given by: R1 V OUT = V REF x 1 + ------- R2
where R1 + R2 < 800K.
EQ. 3
Capacitors Selection
For best performances, a low-ESR input capacitor is required. A ceramic capacitor of at least 10F, placed close to the VIN and AGND pins, is recommended. The output capacitor determines the output ripple and the transient response.
Capacitor Value
10F
Vendor
Taiyo Yuden TDK Murata
Part Number
JMK212BJ106MG JMK316BJ106KL C2012X5ROJ106K C3216X5ROJ106M GRM32ER61C106K
According to this equation, and assuming desired output voltage of 1.5096V, and given R2 = 10k, the calculated value of R1 is 8.87k. If quiescent current is a key design parameter, a higher value feedback resistor can be used (e.g. R2 = 100k) and a small bypass capacitor of 10pF is required in parallel with the upper resistor, as shown in Figure 16.
VIN CIN 10F EN
3
SW
1 2 6
3.3H NC FB R2 R1
PGND
P1 (AGND)
Cf 5K
VOUT 1.2V (800mA) COUT 2 x 10F
Table 2: Recommended Capacitors
5 4
PCB Layout Recommendations
The recommended PCB layout is shown in Figure 17. The inherently high peak currents and switching frequency of power supplies require careful PCB layout design.
10K
Figure 16. Setting the Output Voltage
Inductor Selection
The inductor parameters directly related to the device's performances are saturation current and DC resistance. The FAN5308 operates with a typical inductor value of 3.3H. The lower the DC resistance, the higher the efficiency. For saturation current, the inductor should be rated higher than the maximum load current plus half of the inductor ripple current. This is calculated as follows:
1 - ( V OUT V IN ) I L = V OUT x ----------------------------------------------Lxf where: IL = Inductor Ripple Current f = Switching Frequency L = Inductor Value
EQ. 4
Figure 17. Recommended PCB Layout Use wide traces for high-current paths and place the input capacitor, the inductor, and the output capacitor as close as possible to the integrated circuit terminals. To minimize voltage stress to the device resulting from everpresent switching spikes, use an input bypass capacitor with low ESR. Note that the peak amplitude of the switching spikes depends upon the load current; the higher the load current, the higher the switching spikes. The resistor divider that sets the output voltage should be routed away from the inductor to avoid RF coupling. The ground plane at the bottom side of the PCB acts as an electromagnetic shield to reduce EMI. For more board layout recommendations, download the Fairchild application note PCB Grounding System and FAN2001/FAN2011 High-Performance DC-DC Converters (AN-42036).
Inductor Value
3.3H 3.3H
Vendor
Panasonic Murata
Part Number
ELL6PM3R3N LQS66C3R3M04
Table 1: Recommended Inductors
(c)2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
Mechanical Dimensions
Dimensions are in millimeters unless otherwise noted.
Figure 18. 3x3mm 6-Lead MLP
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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FAN5308 -- 800mA High-Efficiency Step-Down DC-DC Converter
(c) 2005 Fairchild Semiconductor Corporation FAN5308 Rev. 1.0.2
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