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850kHz 1A Synchronous Buck DC / DC Converter General Description

AAT1152

850kHz 1A Synchronous Buck DC / DC Converter General Description
AAT1152
Features
SwitchRegTM
Preliminary Information
Applications
· · · · · Computer Peripherals Set Top Boxes Network Cards Cable / DSL Modems High efficiency conversion from 5V or 3.3V supply
Typical Application
INPUT
AAT1152
4.1µH LX
ENABLE 100 VCC OUTPUT SGND 0.1µF PGND 47µF
850kHz 1A Synchronous Buck DC / DC Converter Pin Descriptions
AAT1152
Symbol
FB SGND EN VCC VP LX POK PGND
Function
Feedback input pin Signal Ground Converter enable pin Small Signal Filtered Bias Supply Input supply for converter power stage Inductor connection pin Power Good indicator. Open-drain output is low when VOUT falls out of regulation. Power ground return for output stage
Pin Configuration
MSOP-8
FB SGND EN VCC
PGND POK LX VP
850kHz 1A Synchronous Buck DC / DC Converter Absolute Maximum Ratings
Symbol
VCC, VP VLX VFB VEN, VPOK TJ TLEAD VESD
AAT1152
6 -0.3 to VP+0.3 -0.3 to VCC+0.3 -0.3 to 6 -40 to 150 300 3000
Description
VCC, VP to GND LX to GND FB to GND POK, EN to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec) ESD Rating 1 - HBM
Units
Note: Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum rating should be applied at any one time. Note 1: Human body model is a 100pF capacitor discharged through a 1.5K resistor into each pin.
Thermal Characteristics
Symbol
Description
Maximum Thermal Resistance (MSOP-8) Maximum Power Dissipation (MSOP-8) 2
Value
Units
Note 2: Mounted on a demo board.
Recommended Operating Conditions
Symbol
Description
Ambient Temperature Range
Rating
-40 to +85
Units
850kHz 1A Synchronous Buck DC / DC Converter Electrical Characteristics
VIN VOUT ILIM IQ VOUT (VOUTVIN) VOUT / VOUT FOSC RDSON(H) RDSON(L) VEN(H) VEN(L) IEN VUVLO VUVLO(hys) TSD THYS ISHDN VTH(POK) RPOK
AAT1152
Description
Operation Voltage DC Output Voltage Tolerance Current Limit Quiescent Supply Current Load Regulation Line Regulation Oscillator frequency High-side Switch On-resistance Low-side Switch On-resistance Enable input high voltage Enable input low voltage Enable Pin Leakage Current Undervoltage Lockout Undervoltage Lockout Hysteresis Over Temp Shutdown Threshold Over Temp Shutdown Hysteresis Shutdown current Power Good Threshold Power Good Pull-Down On-Resistance
Conditions
Units
850kHz 1A Synchronous Buck DC / DC Converter Typical Characteristics
High Side RDS(ON) vs. Temperature
AAT1152
Low Side RDS(ON) vs. Temperature
3.6V 2.7V RDS(ON) (m)
RDS(ON) (m)
3.6V 2.7V 5.5V 4.2V
Temperature (°C)
RDS(ON) vs. Input Voltage
Enable Threshold vs. Input Voltage
Enable Threshold (V)
High Side RDS(ON) (m)
VEN(H)
Low Side
VEN(L)
Input Voltage (V)
Oscillator Frequency Variation vs. Supply Voltage
Supply Voltage (V)
Temperature (°C)
850kHz 1A Synchronous Buck DC / DC Converter Typical Characteristics
AAT1152
Temperature (°C)
Input Voltage (V)
IOUT (mA)
Output Current (A)
Gain (dB)
100µF
Phase
47µF 69µF
Phase (degrees)
-16 47µF
69µF 100µF
Input Voltage (V)
Frequency (kHz)
850kHz 1A Synchronous Buck DC / DC Converter Typical Characteristics
AAT1152
Operating Current (µA)
Input Current (mA)
Temperature (°C)
Switching Waveform
VOUT 50mV / div V(LX) 2V / div Inductor Current 500mA / div
Transient Response
IL 500mA / div
Output Ripple 1.5V, No Load
Output Ripple 1.5V, 1A Load
500nsec / div
850kHz 1A Synchronous Buck DC / DC Converter Typical Characteristics
Output Ripple 3.3V, No Load
AAT1152
Output Ripple 3.3V, 1A Load
850kHz 1A Synchronous Buck DC / DC Converter Functional Block Diagram
AAT1152
1.0V REF
OP. AMP
LOGIC OSC
DL Temp. Sensing Power Good
Applications Information
850 kHz 1 Amp DC-DC Synchronous Buck Converter Control Loop
The AAT1152 is a peak current mode buck converter. The inner, wide bandwidth loop controls the peak current of the output inductor. The output inductor current is sensed through the P-Channel MOSFET (high side) and is also used for short circuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain stability. The loop appears as a voltage programmed current source in parallel with the output capacitor. The voltage error amplifier output programs the current loop for the necessary inductor current to
force a constant output voltage for all load and line conditions. The feedback resistive divider is internal, dividing the output voltage to the error amplifier reference voltage of 1.0V. The error amplifier does not have a large DC gain typical of most error amplifiers. This eliminates the need for external compensation components while still providing sufficient DC loop gain for load regulation. The crossover frequency and phase margin are set by the output capacitor value only.
Soft-Start / Enable
Soft start increases the inductor current limit point in discrete steps when the input voltage or enable input is applied. It limits the current surge seen at the input and eliminates output voltage overshoot. The enable input, when pulled low, forces the AAT1152 into a low power non-switching state. The total input current during shutdown is less that 1µA.
850kHz 1A Synchronous Buck DC / DC Converter
AAT1152
Enable 2V / div
200µsec / div
Figure 1: Inrush Limit
Power and Signal Source
Separate small signal ground and power supply pins isolate the internal control circuitry from the noise associated with the output MOSFET switching. The low pass filter R1 and C3 in schematic figures 3 and 4 filters the noise associated with the power switching.
Current Limit and Over-temperature protection
For overload conditions the peak input current is limited. Figure 2 displays the VI current limit characteristics. As load impedance decreases and the output voltage falls closer to zero, more power is dissipated internally, raising the device temperature. Thermal protection completely disables switching when internal dissipation becomes excessive, protecting the device from damage. The junction over-temperature threshold is 140°C with 15°C of hysteresis.
Current Limit Characteristic
VOUT (V)
IOUT (A)
Figure 2.
850kHz 1A Synchronous Buck DC / DC Converter
Power Good
AAT1152
Inductor
Input Capacitor
The primary function of the input capacitor is to provide a low impedance loop for the edges of pulsed current drawn by the AAT1152. A low ESR / ESL ceramic capacitor is ideal for this function. To minimize the stray inductance the capacitor should be placed as close as possible to the IC. This keeps the high frequency content of the input current localized, minimizing radiated and conducted EMI while facilitating optimum performance of the AAT1152. Ceramic X5R or X7R capacitors are ideal for this function. The size required will vary depending on the load, output voltage and input voltage source impedance characteristics. A typical value is around 10µF. The input capacitor RMS current varies with the input voltage and the output voltage. The equation for the maximum RMS current in the input capacitor is:
The input capacitor RMS ripple current reaches a maximum when VIN is two times the output voltage where it is approximately one half of the load current. Losses associated with the input ceramic capacitor are typically minimal and not an issue. The proper placement of the input capacitor can be seen in the reference design layout in figures 5 and 6.
850kHz 1A Synchronous Buck DC / DC Converter
AAT1152
Vin+ 3.3V
Sgnd Pgnd
VC1 Murata 10µF 6.3V X5R GRM42-6X 5R106K6.3 C2 MuRata 100µF 6.3V GRM43-2 X5R 107M 100µF 6.3V L1 Sumida CDRH4D28-2R 7µH
Figure 3: 3.3V to 1.25V converter
Output Capacitor
Since there are no external compensation components, the output capacitor has a strong effect on loop stability. Larger output capacitance will reduce the crossover frequency with greater phase margin. For the 1.5V 1A design using the 4.1 µH inductor, a 47µF capacitor provides a stable loop with 35 degrees of phase margin at a crossover frequency of 100 kHz. Doubling the capacitance to 100µF reduces the crossover frequency to half while increasing the phase margin to 60 degrees. In addition to assisting stability, the output capacitor limits the output ripple and provides holdup during large load transitions. A 100µF X5R or X7R ceramic capacitor provides sufficient bulk capacitance to
stabilize the output during large load transitions and has ESR and ESL characteristics necessary for low output ripple. The output capacitor rms ripple current is given by:
1 2 3 (VOUT + VFWD) (VIN - VOUT) L F VIN
For a ceramic capacitor the dissipation due to the RMS current of the capacitor is not a concern. Tantalum capacitors, with sufficiently low ESR to meet output voltage ripple requirements, also have an RMS current rating much greater than that actually seen in this application.
850kHz 1A Synchronous Buck DC / DC Converter
R5 100k U1 AAT1152-1.5 Vp EN Vcc EN FB Pok LX LX C2 100µF L1 4.1µH Pok
AAT1152
Vin+ 2.7V-5.5V R1 100 R2 C1 10µF 100k C3 0.1µF
Vo+ 1.5V 1A
Sgnd Pgnd
VC1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3 C2 MuRata 100µF 6.3V GRM43-2 X5R 107M 100µF 6.3V L1 Sumida CDRH5D 18-4R 1µH
1.5V Efficiency vs. IOUT
Iout (mA)
Figure 4: Lithium-Ion to 1.5V Output Converter
Figure 5: AAT1152 Layout Top Layer
Figure 6: AAT1152 Layout Bottom Layer
850kHz 1A Synchronous Buck DC / DC Converter
Adjustable Output
For applications requiring an output other than the fixed outputs available, the 1V version can be programmed externally. Resistors R3 and R4 of figure 3 force the output to regulate higher than 1 Volt. R4 should be 100 times less than the internal 1 MegOhm resistance of the FB pin. Once R4 is selected R3 can be calculated. For a 1.25V output with R4 set to 10.0k, R3 is 2.55k.
AAT1152
Layout Considerations
Figures 5 and 6 display the suggested PCB layout for the AAT1152. The most critical aspect of the layout is the placement of the input capacitor C1. For proper operation C1 must be placed as close as possible to the AAT1152.
Thermal Calculations
There are two types of losses associated with the AAT1152 output switching MOSFET, switching losses and conduction losses. The conduction losses are associated with the Rds(on) characteristics of the output switching device. At full load, assuming continuous conduction mode (CCM), a simplified form of the total losses is:
IO2 (RDSON(H) VO + RDSON(L) (VIN - VO)) + tsw F IO VIN + IQ VIN VIN
Once the total losses have been determined the junction temperature can be derived from the JA for the MSOP-8 package.
Design Example
Maximum Input Capacitor Ripple:
850kHz 1A Synchronous Buck DC / DC Converter
Inductor Selection:
AAT1152
Select Sumida inductor CDRH5D18 4.1µH 57m 2.0 mm height.
Output Capacitor Dissipation:
AAT1152 Dissipation:
(0.14 · 1.5V + 0.145 · (3.6V - 1.5V)) 3.6V
850kHz 1A Synchronous Buck DC / DC Converter
Table 1: Surface Mount Inductors
Manufacturer
TaiyoYuden Toko Sumida Sumida MuRata MuRata
AAT1152
Part Number
NPO5DB4R7M A914BYW-3R5M-D52LC CDRH5D28-4R2 CDRH5D18-4R1 LQH55DN4R7M03 LQH66SN4R7M03
Value
4.7µH 3.5µH 4.2µH 4.1µH 4.7µH 4.7µH
Max DC Current
1.4A 1.34A 2.2A 1.95A 2.7A 2.2A
Shielded Shielded Shielded Shielded Non-shielded Shielded
Table 2: Surface Mount Capacitors
Manufacturer
TDK MuRata MuRata MuRata MuRata
Part Number
C4532X5ROJ107M GRM43-2 X5R 107M 6.3 GRM43-2 X5R 476K 6.3 GRM40 X5R 106K 6.3 GRM42-6 X5R 106K 6.3
Value
100µF 100µF 47µF 10µF 10µF
Voltage
6.3V 6.3V 6.3V 6.3V 6.3V
Temp. Co.
X5R X5R X5R X5R X5R
850kHz 1A Synchronous Buck DC / DC Converter Ordering Information
Output Voltage 1.0V 1.1V 1.2V 1.5V 1.8V 2.0V 2.5V 3.0V 3.3V Package MSOP-8 MSOP-8 MSOP-8 MSOP-8 MSOP-8 MSOP-8 MSOP-8 MSOP-8 MSOP-8 Marking Part Number (Tape and Reel) AAT1152IKS-1.0-T1 AAT1152IKS-1.1-T1 AAT1152IKS-1.2-T1 AAT1152IKS-1.5-T1 AAT1152IKS-1.8-T1 AAT1152IKS-2.0-T1 AAT1152IKS-2.5-T1 AAT1152IKS-3.0-T1 AAT1152IKS-3.3-T1
AAT1152
Package Information
MSOP-8
All dimensions in millimeters.
GAUGE PLANE
850kHz 1A Synchronous Buck DC / DC Converter
AAT1152
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 18