DESCRIPTION
The CX8508 is a monolithic synchronous buck regulator. The device integrates 100mΩMOSFETS that provide 2A continuous load cur-rent over a wide operating input voltage of 4.75V to 25V. Current mode control provides fast transient respon and cycle-by-cycle cur-rent limit.
An adjustable soft-start prevents inrush current at turn-on. In shutdown mode, the supply cur-rent drops below 1 μA.
This device, available in an 8-pin SOIC pack-age, provides a very compact system solution with minimal reliance on external components. FEATURES
z 2A Output Current
z Wide 4.75V to 25V Operating Input Range z Integrated
100mΩ Power MOSFET Switches z Output Adjustable from 0.925V to 20V
affablez Up to 95% Efficiency
z Programmable
Soft-Start
z Stable with Low ESR Ceramic Output Capaci-tors
z Fixed 340KHz Frequency
z Cycle-by-Cycle Over Current Protection
z Input Under Voltage Lockout
z Thermally Enhanced 8-Pin SOIC Package APPLICATIONS
z Distributed Power
Systems
z Networking
Systems
z Set-top
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Box
z LCD
TV/Monitor
z Notebook or Mini-Book
PACKAGE REFERENCE
Part number Package Temperature
CX8508 SOP8–20° C to +85° C
ABSOLUTE MAXIMUM RATINGS (1) Supply Voltage (V IN).......................-0.3V to 26V Switch Volta
汤尼语言中心ge (V SW)...................–1V to V IN + 0.3V Bootstrap Voltage (V BS) .. .Vsw-0.3V to V SW + 6V Enable/UVLO Voltage (V EN)............–0.3V to +6V Comp Voltage (V COMP) .....................–0.3V to +6V Feedback Voltage (V FB) ..................–0.3V to +6V Junction Temperature ........................... +150°C Lead Temperature ................................. +260°C –55°C to + 150°C Recommended Operating Conditions(2) Input Voltage (V IN) ....................... 4.75V to 25V Output Voltage (V SW) ..................... 0.925 to 20V –20°C to +85°C
Thermal Resistance(3)θJA θJC
<.50.... 10... °C/W Notes:statusmonitor
1) Exceeding the ratings may damage the device.
2) The device is not guaranteed to function outside of its
operating conditions.
3) Measured on approximately 1” square of 1 oz copper.
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ELECTRICAL CHARACTERISTICS
V IN = 12V, T A = +25°C, unless otherwi noted.
Parameter Condition Min Typ Max Units Shutdown Supply Current V EN ≤ 0.3V0.3 3.0µA Supply Current V EN ≥ 2.6V, V FB = 1.0V 1.3 1.5mA Feedback Voltage 4.75V ≤ V I N≤ 25V0.9000.9250.950V Feedback Overvoltage Threshold 1.1 V
Error Amplifier Voltage 480 V/V Error Amplifier Transconductance ΔI C = ±1 0μA800 µA/V High-Side Switch-On Resistance100mΩ
Low-Side Switch-On Resistance100mΩHigh-Side Switch Leakage V EN = 0V, V SW = 0V110µA
2.5 A Upper Switch Current Limit 2.0
Lower Switch Current Limit0.9A
COMP to Current Sen Transconduc-
4.0 А/V tance
Oscillator Frequency300340380KHz Short Circuit Frequency V FB = 0V 150 KHz Maximum Duty Cycle V FB = 0.8V90% Minimum On Time 120 nS
EN Shutdown Threshold Voltage V EN Rising 1.1 1.5 2.0V
EN Shutdown Threshold Voltage Hys-
200 mV terisis
EN Lockout Threshold Voltage 2.2 2.5 2.7V
EN Lockout Hysterisis210mV
无语的英文Input UVLO Threshold Rising V IN Rising 3.80 4.20 4.40V
Input UVLO Threshold Hysteresis210mV
Soft-start Current V SS = 0V 6.5 µAtwisty
Soft-start Period C SS = 0.1μF15 ms Thermal Shutdown4160°C
Note:
4) Guaranteed by design
APPLICATION
PIN FUNCTIONS
Pin #
Name
Description
1 BS
High-Side Gate Drive Boost Input. BS supplies the drive for the high-side N-Channel
MOSFET switch. Connect a 0.01 μF or greater capacitor from SW to BS to power the high side switch.
2 IN
Power Input. IN supplies the power to the IC, as well as the step-down converter switches. Drive IN with a 4.75V to 25V power source. Bypass IN to GND with a suitably large capacitor to eliminate noi on the input to the IC. See Input Capacitor .
3 SW Power Switching Output. SW is the switching node that supplies power to the output. Connect the output LC filter from SW to the output load. Note that a capacitor is required
from SW to BS to power the high-side switch. 4 GND Ground.
5 FB Feedback Input. FB ns the output voltage to regulate that voltage. Drive FB with a resistive voltage divider from the output voltage. The feedback threshold is 0.925V. See
Setting the Output Voltage . 6 COMP Compensation Node. COMP is ud to compensate the regulation control loop.
Connect a ries RC network from COMP to GND to compensate the regulation control
loop. In some cas, an additional capacitor from COMP to GND is required. Seesubquently
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Compensation Components. 7 EN Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to
turn on the regulator, drive it low to turn it off. Pull up with 100k Ω resistor for automatic
t t
8 SS
Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor from SS to GND to t the soft-start period. A 0.1μF capacitor ts the soft-start period to 15ms. To disable the soft-start feature, leave SS unconnected.
2A 2A 2A
CX8508
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１５
44.2
5V
OPERATION
FUNCTIONAL DESCRIPTION
The CX8508 is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages from 4.75V to 25V down to an output voltage as low as 0.925V, and supplies up to 2A of load current. The CX8508 us current-mode control to regulate the output voltage. The output voltage is measured at FB through a resistive voltage di-vider and amplified through the internal trans- conductance error amplifier. The voltage at the COMP pin is compared to the switch current measured internally to control the output voltage.
The converter us internal N-Channel MOSFET switches to step-down the input voltage to the regu-lated output voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor connected between SW and BS is needed to drive the high side gate. The boost capaci-tor is charged from the internal 5V rail when SW is low.
When the CX8508 FB pin exceeds 20% of the nominal regulation voltage of 0.925V, the over volt-age comparator is tripped and the COMP pin and the SS pin are discharged to GND, forcing the high-side switch off.
Figure 1 – Functional Block Diagram
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110/340k
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is t using a resistive volt-
age divider from the output voltage to FB (e Typical Application circuit on page 1). The volt-age divider divides the output voltage down by the ratio:
Where V FB is the feedback voltage and V
OUT is the output voltage.
Thus the output voltage is:
R2 can be as high as 100k Ω, but a typical value is 10k
punch
Ω. Using the typical value for R2, R1 is determined by:
For example, for a 3.3V output voltage, R2 is 10k Ω, and R1 is 26.1k Ω
. Table 1 lists recom-mended resistance values of R1 and R2 for standard output voltages.
Where V OUT is the output voltage, V IN is the input volt-age, f S is the switching frequency, and ΔI L is the peak-
to-peak inductor ripple current.
Choo an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by:
Where I LOAD is the load current.
The choice of which style inductor to u mainly de-pends on the price vs. size requirements and any EMI requirements.
Optional Schottky Diode
During the transition between high-side switch and low-side switch, the body diode of the lowside power MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schot-
tky diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 2 lists example Schottky diodes and their Manufacturers.
Inductor
The inductor is required to supply constant cur-rent to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher ries resistance, and/or lower saturation current. A good rule for determining the induc-tance to u is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by:
cah
Input Capacitor
The input current to the step-down converter is dis-continuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. U low ESR ca-pacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choo X5R or X7R di-electrics when using ceramic capacitors. Since the input capacitor absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by:
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