AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Feature
‧ 4.75V to 16V Supply Voltage Operating
Range.
‧ 380KHz ±15% Fixed Switching Frequency.
‧ Voltage-Mode PWM Step-Down Regulator.
‧ Built-in Power MOSFET, Rds(on)≒100mΩ.
‧ 2A Output Current, Up to 91.9% Efficiency
‧ 1.215V±1.5% Internal Reference
‧ Shutdown Current less than 20uA.
‧ Thermal-Shutdown and Built-in Soft-Start.
‧ Cycle-by-Cycle Current-Limit Protection.
‧ Short Circuitry Protection.
‧ Under-Voltage Lockout Protection.
‧ Output Voltage Adjustable From 1.215V to
16V.
‧ SOP-8 Package.
Application
‧ Distributed Power Systems
‧ Battery Charger.
‧ Palmtop Computers, PDAs.
‧ DSL Modems
Description
AT1799 is a voltage-mode PWM step-down
DC-DC converter capable of driving up to 2A
loads over a wide input supply range with excellent
line and load regulation characteristics. High
efficiency is obtained through the use of a low
Rds(on) P-channel power switch.
It uses a fixed switching frequency up to 380KHz
thus allowing smaller sized components. The
output voltage level is user-
p
rogrammable via an
external resistive voltage divider. The built-in
soft-start can reduce inrush current on the input
source at turn on.
The device also has built-in cycle-by-cycle
current-limit, thermal shutdown, under-voltage
lockout and an ON/OFF logic-control that can
power down the regulator to a shutdown mode. In
shutdown mode the regulator draws less than 20uA
of supply current. When a short circuit is detected
on the output , the device immediately change
switching frequency to 42 KHz and duty cycle is
limited to 5% to protect IC from damage.
AT1799 is available in SOP-8 package.
Block Diagram
Aimtron reserves the right without notice to change this circuitry and specifications.
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1
AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Pin Configuration
Ordering Information
Part number Package Marking
AT1799S SOP-8 Date Code
AT1799S_GRE SOP-8,Green
,Date Code with one bottom line
: Date Code
*For more marking information, contact out sales representative directly
Pin Descriptions
Pin N0. Symbol I / O Description
1 IN P
Power Input. IN supplies power to the IC and Step-down converter
switch. IN must be bypassed with a low-ESR capacitor.
2 EN I
Enable input. EN is a digital input that turns the regulator on/off. Drive
EN high to turn on the regulator, drive it low to turn it off.
3 COMP O
Error Amplifier Output. A series RC network connected to this pin
compensates AT1799.
4 FB I
Feedback Input. Connect a resistive divider from switching output to
FB to ground.
5,6 GND P Ground.
7,8 SW O Power Switching Output.
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Absolute Maximum Ratings *1
Parameter Rated Value
unit
IN , EN voltage -0.3 to 16 V
SW voltage -1 to IN+1 V
FB, COMP voltage -0.3 to 6 V
Quiescent current 5 mA
Thermal Resistance (Junction to ambient θJA)*2 105 ℃/W
Junction Temperature (Tj) 150 ℃
Lead Temperature (Soldering 10 sec) 260 ℃
Storage Temperature -65~150 ℃
HBM 2 KV
ESD Susceptibility*3 MM 200 V
1. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the
device. These are stress ratings only, and functional operation of the device at these or any other
conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure
to absolute maximum rating conditions for extended periods may affect device reliability.
2. Measured on approximately 1” square of 1 oz. copper FR4 board.
3. Device are ESD sensitive. Handling precaution recommended. The Human Body model is a 100pF
capacitor discharged through a 1.5KΩ resistor into each pin.
Recommended Operation Conditions Values Parameter Symbol
Min. Typ. Max. Unit
Power supply voltage IN 4.75 12 16 V
Operating temperature Top -20 +25 +85 ℃
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Electrical Characteristics (IN=12V, EN=5V, Ta=+25℃, unless otherwise noted)
Parameter Symbol
Test Condition Min. Typ. Max. Units
Input operating voltage IN 4.75 12 16 V
Input under-voltage lockout
threshold
VUVLO IN Rising 3.8 4.2 4.6 V
UVLO hysteresis - 500 - mV
Quiescent Current Iin VFB=1.5V - 3.5 - mA
Shutdown Current ISHDN EN =GND - 20 50 uA
Oscillator Frequency fSW Ta=+25℃ - 380 440 KHz
Short-Circuit Frequency FB=0V - 42 - KHz
Feedback Regulation voltage VFB Ta=+25℃ 1.20 1.215 1.233 V
FB Input Bias Current IFB VFB=1.215V -50 - 50 nA
Current limit 2.5 2.75 - A
Maximum Duty Cycle DMAIN FB=1.0V - - 100 %
Minimum Duty Cycle DMIIN FB=1.5V 0 - - %
SW Switch On-Resistance RSW-DS(ON) ISW=2A - 100 130 mΩ
SW Leakage Current ISW IN=13.5V,VSW=GND
, EN=0V - 0.1 20
µA
Load Regulation 0 A<ISW< 2A 0.2 %
Line Regulation 4.75V < IN<16V - 1.0 - %/V
Error Amplifier Voltage Gain - 70 - dB
Error Amplifier Trans-conductance 500 800 1000
µA/V
Soft-Start Time - 10 - ms
EN Threshold Voltage Hysteresis 150mV - 1 - V
EN Pull Up Current EN=0V - 1 -
µA
Thermal Shutdown - 160 - ℃
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Typical characteristics
Efficiency vs. O utput C urrent and Voltage
70
75
80
85
90
95
0.1 0.2 0.3 0.5 0.7 1 1.3 1.5 1.7 2
O utput C urrent (A )
Efficiency (%)
5.0V
3.3V
VIN=10V
Reference Voltage vs Temperature
1.215
1.216
1.217
1.218
1.219
1.22
-20-100 1025407080
Temperature (OC)
VREF (V)
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Load Transient Response
VIN=12V, Vout=5.0V, L=22uH, Iout=500mA to 2.0A
Ch2:Vout, Ch4:Iout, Time:1ms/div
Line Transient Response
VIN=7V to 14V, Vout=5.0V, L=22uH, Iout=200mA
Ch2:Vout, Ch3:VIN, Time:0.5s/div
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Typical Application Circuit
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
General Description
The AT1799 features a PWM step-down converter operating with a fixed switching
frequency of 380 KHz and uses internal power MOSFET to provide maximum
efficiency. The output voltage of the converter can be set as low as 1.215V with
external resistive divider. When trans-conductance signal is higher than saw-tooth
wave, then PWM comparator will produce ON-time signal to set internal flip-flop,
which turns on the switching power MOSFET. The external inductor current ramps up
linearly, storing energy in a magnetic field. Once peak current of flowing through
switching power MOSFET over current-limited threshold or trans-conductance signal
level is lower than saw-tooth wave, the switching power MOSFET will turns off, the
flip-flop resets, and external schottky diode turns on. This forces the current through
the inductor to ramp back down, transferring the energy stored in the magnetic field to
the output capacitor and load. To add higher flexibility to the selection of external
component values, the device uses external loop compensation.
Enable Control
Digital logic of EN provides an electrical ON/OFF control of the power supply. Connecting
this pin to ground or to any voltage less than 1.0V will completely turn OFF the regulator. In
this state, current drain from the input supply is only 20µA, the internal reference, error
amplifier, comparators, and biasing circuitry turn off . When EN control is not required that it
should be left open circuited.
Soft-Start
AT1799 is built-in soft-start function. When power up, after input voltage is above the
under-voltage lockout threshold, then soft-start allows a gradual increase of COMP voltage to
get to a steady state operating point. Therefore, reducing input surge currents. During this
time, the COMP voltage increases and the output duty cycle is allowed to increase from zero
to the value required for regulation. The maximum load current is available after the soft-start
cycle is completed. When EN pin is taken low, the soft-start capacitor is discharge to ground.
Short-Circuit Protection
If feedback voltage of the regulator falls below 0.7V, the oscillator clock switched
operating frequency to 42KHz and duty cycle is limited to 5% to reduce input power
delivered to output. If the short circuitry is not removed, the protection circuitry will
activate sustainability. When short-circuit problem is to eliminate, oscillator clock will
switch back to 380KHz automatically.
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Cycle-by-Cycle Over-Current Protection
The AT1799 provides cycle-by-cycle over-current protection. Current limit is
accomplished using a separate dedicated comparator. If the current sense amplifier
output voltage is larger than current-limited threshold level, it will be immediately
turned off power MOS. The current-limit feature protects against a hard short or over
current fault at the output.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation in the AT1799. When the junction
temperature exceeds Tj=160 , a thermal sensor activates the thermal protection, which shuts ℃
down the IC, allowing the IC to cool. Once the device cools down by 15 , IC will ℃
automatically recover normal operation. For continuous operation , do not exceed the absolute
maximum junction-temperature rating of Tj=150 .℃
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Applications Information
External components of step-down converter can be designed by performing simple
calculations. It must be to follow regulation by the output voltage and the maximum
load current, as well as maximum and minimum input voltages. Begin by selecting an
inductor value. Once L is know, choose the diode and capacitors.
Inductor
Inductor selection depends on input voltage, output voltage, maximum current,
switching frequency and availability of inductor values. The following buck circuit
equations are useful in choosing the inductor values based on the application. Choose
an inductor that does not saturate under the maximum rating load conditions. The
magnitude of inductance is selected to maintain a peak to peak ripple current of 30%
of the maximum load current.
The peak inductor current is given by:
in
O
in
LAVG
L
LAVGLpeak
V
V
D
D
I
I
I
II
=
=
∆
+= 2
D is the MOSFET turn on ratio
where:
I△L is the inductor peak-to-peak current ripple and is decided by:
OSCL
Oin
OSC
Oin
LfI DVV
L
fD
LVV
I×∆
×
−
=⇒×
−
=∆ )(
fOSC is the switching frequency.
The inductor should be chosen to be able to handle this current and inductor saturation
current rating should be greater than IPEAK.
Diode selection
When the power switch turns off, the current through the inductor continues to flow.
The path for this current is through the diode connected between the switch output
and ground. This forward biased diode must has a minimum voltage drop and
recovery times. Schottky diode is recommended and it should be able to handle those
current. As usual, the reverse voltage rating of the diode should be at least 1.3 times
greater than the maximum input voltage, and current rating is greater than the
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
maximum load current.
Feedback Resistor Network
An external resistor divider is required to divide the output voltage down to the
nominal reference voltage. Current drawn by the resistor network should be limited to
maintain the overall converter efficiency. The maximum value of the resistor network
is limited by the feedback input bias current and the potential for noise being coupled
into the feedback pin. A resistor network in the order of 100kΩ is recommended. The
buck converter output voltage is determined by the following relationship:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛+×=
2
3
1R
R
VV REFO
where VREF =1.215V as specified.
Input capacitor
An input capacitor helps to provide additional current to the power supply as well as
smooth input voltage variations in high current switching regulators. When selecting
an input capacitor, a low ESR capacitor is required to keep the noise at the IC to a
minimum. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic
capacitors may also suffice. Choose an input capacitor who’s maximum voltage rating
is 1.3 times greater than the maximum input voltage, and who’s RMS current rating is
equal to one-half of the maximum dc load current. It may be necessary in some
designs to add a small valued ceramic type capacitor in parallel with the input
capacitor to prevent any ring.
Output Capacitor
The output capacitor acts to smooth the dc output voltage and also provides energy
storage. Selection of an output capacitor, with an associated equivalent series
resistance (ESR), impacts both the amount of output ripple voltage and stability of the
control loop. Low ESR capacitors are preferred to keep the output voltage ripple low.
The output voltage ripple is estimated to be:
2
8
)1(
OSCOUT
O
ripple fCL
VD
V×××
×−
=∆
For low ESR ceramic capacitors, the output ripple is dominated by the charging or
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
discharging of the output capacitor.
In the case of low-ESR electrolytic capacitors, Output ripple voltage drop caused by
the switching current through the ESR of the output capacitor. The output ripple is
estimated as:
ESRLRIPPLE RIV ×∆≅
Impacting frequency stability of the overall control loop, the output capacitance, in
conjunction with the inductor, creates a double pole inside the feedback loop. In
addition the capacitance and the ESR value create a zero. These frequency response
effects together with the internal frequency compensation circuitry of AT1799 modify
the gain and phase shift of the closed loop system.
Compensation
The step-down loop can be compensated by adjusting the external components
connected to the COMP pin. The COMP pin is connected to the output of the internal
trans-conductance error amplifier. The compensation capacitor adjusts the low
frequency gain , and the series resistor value adjusts the high frequency gain. A series
capacitor-resistor combination sets a pole-zero combination to control the
characteristics of the control system. The system has 2 poles and one zero of
importance, 2 poles consist of compensation capacitor and output capacitor. One zero
is produced by the compensation capacitor and the compensation resistor.
If electrolytic capacitor with relatively high ESR is used, the zero due to the
capacitance and ESR of the output capacitor can be compensated by a third pole set
by the compensation resistor and another compensation capacitor connected form
COMP pin to ground.
PCB layout guidelines
Careful printed circuit layout is extremely important to avoid causing parasitical
capacitance and line inductance. The following layout guidelines are recommended to
achieve optimum performance.
‧ Please the buck converter diode and inductor close to the SW pin and no via.
‧ Please ceramic bypass capacitors near the input pin.
‧ Locate all feedback resistive dividers as close to their respective feedback pins as
possible.
‧ Use wide traces and trace length is short as possible.
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AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
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13
AT1799
2A Step-Down Voltage Regulator
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Outline 8-pin SOP
INCHES MILLIMETERS
SYMBOL MIN MAX MIN MAX
NOTES
A 0.188 0.197 4.80 5.00 -
B 0.149 0.158 3.80 4.00 -
C 0.228 0.244 5.80 6.20 -
D 0.050 BSC 1.27 BSC -
E 0.013 0.020 0.33 0.51 -
F 0.004 0.010 0.10 0.25 -
H 0.053 0.069 1.35 1.75 -
J 0.011 0.019 0.28 0.48
K 0.007 0.010 0.19 0.25
M 0.016 0.050 0.40 1.27
e1 45O45O
α 00800080-
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