AS1335-BTDT-100 - Austriamicrosystems

AS1335

1.5A, 1.5MHz, Synchronous DC/DC Step-Down Converter

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 1 - 18

Datasheet

1 General Description

The AS1335 is a high-efficiency, constant-frequency

synchronous buck converter available in a fixed or an

adjustable output voltage version. The wide input volt-

age range (2.6V to 5.25V), the high output current (up to

1.5A) and minimal external component requirements

make the AS1335 perfect for any single Li-Ion battery-

powered application.

Typical supply current with no load is 400µA and

decreases to ≤1µA in shutdown mode. The highly effi-

cient duty cycle (100%) provides low dropout operation,

prolonging battery life in portable systems.

The device also offers a power-ok signal with a 215ms

delay, which can be reseted or delayed further via the

RSI pin.

An internal synchronous switch increases efficiency and

eliminates the need for an external Schottky diode. The

internally fixed switching frequency (1.5MHz) allows for

the use of small surface mount external components.

The AS1335 is available in a 10-pin TDFN 3x3mm pack-

age.

Figure 1. AS1335 - Typical Application Diagram

2 Key Features

! High Efficiency: Up to 96%

! Output Current: 1.5A

! Input Voltage Range: 2.6V to 5.25V

! Output Voltage Range: 0.6V to VIN

! Constant Frequency Operation: 1.5MHz

! No Schottky Diode Required

! Power OK with 215ms delay

! Low Dropout Operation: 100% Duty Cycle

! Low Quiescent Supply Current: 400µA

! Shutdo w n Mo de Sup p l y Current: ≤1µA

! Current Mode Operation for Excellent Line/Load

Transient Response

! Thermal Protection

! 10-pin TDFN 3x3mm Package

3 Applications

The device is ideal for mobile communication devices,

laptops and PDAs, ultra-low-power systems, threshold

detectors/discriminators, telemetry and remote systems,

medical instruments, or any other space-limited applica-

tion with low power-consumption requirements.

AS1335

COUT

22µF

CIN

22µF

VIN

2.6V to 5.25V VOUT

1.0V, 1.5A

2.2µH

GND

VIN SW

POK FB

GND

PGND

RSI

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 2 - 18

AS1335

Datasheet - Pin o u t

4 Pinout

Pin Assignments

Figure 2. Pin Assignments (Top View)

Pin Descriptions

Table 1. Pin Descriptions

Number Pin Name Description

1VIN

Positive Supply Voltage. This pin must be closely decoupled to PGND with a ≥ 22µF

ceramic capacitor.

2NC

Not Connected.

3EN

Enable Input. Driving this pin above 1.4V enables the device. Driving this pin below 0.3V

puts the device in shutdown mode. In shutdown mode all functions are disabled, drawing

≤1µA supply current.

Note: This pin should not be left floating.

4POK

Power-OK Output. Open-drain output with 215ms delay . Connect a 100kΩ pull-up resistor

to VOUT or pin VIN for logic levels. Leave this pin unconnected if the Power-OK feature is

not used.

LOW Signal: Out of regulation

HIGH signal: Within Regulation (after 215ms delay)

5GND

Analog Ground.

6RSI

Reset Input for POK. This input resets the 215ms timer of the POK signal.

As long as RSI is low the POK signal will work as described above.

A high input to RSI will reset the 215ms POK timer and delay the signal as long as RSI

stays high. A RSI low-to-high transition restarts the 215ms counter as long as the output

voltage is within regulation.

Note: Do not leave this pin floating.

7FB

Feedback Pin. Feedback input to the gm error amplifier. Connect a resistor divider tap to

this pin. The output can be adjusted from 0.6V to 5.25 V by VOUT = 0.6V[1 +(R1/R2)].

If the fixed output voltage version is used, connect this pin to VOUT.

8GND

Analog Ground. GND and PGND should only have one point connection.

9PGND

Power-Ground. Connect all power grounds to this pin.

10 SW Switch Node Co nnectio n to Ind uctor. This pin connects to the drains of the internal main

and synchronous power MOSFET switches.

11 Exposed Pad. The exposed pad must be connected to PGND. Ensure a good connection

to the PCB to achieve optimal thermal performance.

AS1335

POK

3EN

1VIN

7FB

9PGND

10 SW

8GND

5GND 6RSI

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 3 - 18

AS1335

Datasheet - Abs ol ut e M ax im um Ra ting s

5 Absolute Maximum Ratings

Stresses beyond those li st ed in Table 2 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 Electri cal Chara cter-

istics on page 4 is not implied. Exposure to absolute maximum rating conditions for extend ed periods may affect

device reliability.

Table 2. Absolute Maximum Ratings

Parameter Min Max Units Comments

VIN to GND -0.3 6 V

SW to GND -0.3 VIN + 0.3 V

EN, FB to GND -0.3 VIN V

P-Channel Switch Source Current (DC) 1.5 A

N-Channel Switch Source Current (DC) 1.5 A

Peak SW Sink and Source Current 3 A

Thermal Resistance ΘJA 36.7 ºC/W on PCB

Latch-Up -100 100 mA @85°C, JEDEC 78

Electrostatic Discharge 2kVHBM MIL-Std. 883E 3015.7 methods

Operating Temperature Range -40 +85 ºC

St orage Temperature Range -65 +150 ºC

Junction Temperature 125 ºC

Package Body Temperature +260 ºC

The reflow peak soldering temperature (body

temperature) specified is in accordance with

IPC/JEDEC J-STD-020D “Moisture/Reflow

Sensitivity Classification for Non-Hermetic

Solid State Surface Mount Devices”.

The lead finish for Pb-free leaded packages

is matte tin (100% Sn).

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 4 - 18

AS1335

Datasheet - Ele c t r i c a l C h a r a c t e r i s t i c s

6 Electrical Characteristics

VIN = EN = 3.6V, VOUT = VIN-0.5V, TAMB = -40°C to +85°C,

typ. values @ T

AMB

= +25ºC

(unless otherwise specified).

Table 3. Electrical Characteristics

Symbol Parameter Conditions Min Typ Max Units

VIN Input Voltage Range 2.6 5.25 V

IQ Quiescent Supply

Current1Normal Operation; V FB = 0.5V or VOUT =

90% of regula ted output voltage,

ILOAD = 0 A 300 400 µA

IOUT Output Current RMS 1.5 A

ISHDN Shutdown Current Shutdown Mode; VEN = 0V,

VIN = 4.2V 0.1 1 µA

Regulation

VOUT

Regulated Output

Voltage

fixed VOUT 0.975 1.0 1.025 V

adjustable VOUT 0.6 VIN -

0.5V V

VFB Regulated Feedback

Voltage2,3 TAMB = +25°C 0.5880 0.6 0.6120 V

TAMB = -40°C to +85°C 0.5850 0.6 0.6150

IFB Feedback Current3-30 +30 nA

ΔVLNR Reference Voltage

Line Regulation VIN = 2.6V to 5.25V 100 mV

ΔVLOADREG Output Voltage

Load Regulation ILOAD = 0A to 1.5A 100 mA

DC-DC Switches

IPK Peak Inductor Current

VIN = 3V, VFB = 0.5V or VOUT = 90% of

regulated output voltage,

Duty Cycle < 35% 2.4 A

RPFET P-Channel FET RDS(ON) ILSW = 100mA 0.4 Ω

RNFET N-Channel FET RDS(ON) ILSW = -100mA 0.35 Ω

ILSW SW Leakage VEN = 0V, VSW = 0V or 5V,

VIN = 5V -1 0.01 +1 µA

Enable

VIH Logic Input Thre sh ol d Input Hig h 1.4 V

VIL Input Low 0.4

IEN EN Leakage Current VIN = 3.6V, VEN = 0V to 3.6V -1 0.01 +1 µA

Power-OK Output

VPOK

Power Good Low

Voltage Threshold Rising 89.5 92 94.5 %

VOUT

Falling 85 88 91

Power Good High

Voltage Threshold Rising 108.2 110.7 113.2 %

VOUT

Falling 104 107 110

tDELAY POK Delay Time 150 215 275 ms

VOL POK Output Voltage

Low ISINK = 1mA, VFB = 0.7V 0.3 V

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 5 - 18

AS1335

Datasheet - Ele c t r i c a l C h a r a c t e r i s t i c s

Note: All limits are guaranteed. The parameters with min and max values are guaranteed with production tests or

SQC (Statistical Quality Control) methods.

IPOK POK Output Leakage

Current VPOK = VIN = 3.6V 0.01 1 µA

Oscillator

fOSC Oscillator Frequency VFB = 0.6V or VOUT = 100% of regulated

output voltage 1.2 1.5 1.8 MHz

Thermal Shutdown

Thermal Shutdown 150 °C

Thermal Shutdown

Hysteresis 25 °C

1. The dynamic supply current is higher due to the gate charge delivered at the switching frequency. The Quies-

cent Current is measured while the DC-DC Converter is not switching.

2. The device is tested in a proprietary test mode where VFB is connected to the output of the DC/DC converter.

3. Only valid for the adjustable version;

Table 3. Electrical Characteristics

Symbol Parameter Conditions Min Typ Max Units

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AS1335

Datasheet - Ty p i c a l O p e r a t i n g C h a r a c t e r i s t i c s

7 Typical Operating Characteristics

VOUT = 1.0V, IOUT = 100mA, TAMB = +25°C (unl ess otherwise specified).

Figure 3. Efficiency vs. Output Current, VOUT = 1.0V Figure 4. Efficiency vs. Output Current, VOUT = 1.5V

100

10 100 1000 10000

Output Current (mA )

Efficiency (%)

Vin = 5.5V

Vi n = 4.0V

Vin = 3.5V

Vi n = 3.0V

Vin = 2.5V

100

10 100 1000 10000

Output Current (mA )

Efficiency (%)

Vin = 5.5V

Vin = 5.0V

Vin = 4.0V

Vin = 3.6V

Vin = 2.6V

Figure 5. Efficiency vs. Output Current, VOUT = 2.5V Figure 6. Efficiency vs. Output Current, VOUT = 3.0V

100

10 100 1000 10000

Output Current (mA )

Efficiency (%)

Vin = 5.5V

Vin = 5.0V

Vin = 4.0V

Vin = 3.6V

100

10 100 1000 10000

Output Current (mA )

Efficiency (%)

Vin = 5.5V

Vin = 5.0V

Vin = 4.0V

Vin = 3.6V

Figure 7. Efficiency vs. Output Current, VOUT = 3.5V Figure 8. Efficiency vs. Input Voltage, VOUT = 1.0V

100

10 100 1000 10000

Output Current (mA )

Efficiency (%)

Vin = 5.5V

Vin = 5.0V

Vin = 4.5V

Vin = 4.0V 40

100

2.5 3.5 4.5 5.5

Input Volt age (V)

Efficiency (%)

Iout = 100m A

Iout = 300mA

Iout = 700mA

Iout = 1000mA

Iout = 1500mA

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 7 - 18

AS1335

Datasheet - Ty p i c a l O p e r a t i n g C h a r a c t e r i s t i c s

Figure 9. Efficiency vs. Input Voltage, VOUT = 3.5V Figure 10. Load Regulation, VOUT = 1.0V

100

2.6 3 3.4 3.8 4.2 4.6 5

Input Volt age (V)

Efficiency (%)

Iout = 400m A

Iout = 600m A

Iout = 800m A

Iout = 950m A

0.95

0.97

0.99

1.01

1.03

1.05

10 100 1000 10000

Output Current (mA )

Out p ut Voltage (V )

Vin = 5.5V

Vin = 5.0V

Vin = 4.5V

Vin = 3.5V

Vin = 2.5V

Figure 11. Load Regulation, VOUT = 1.5V Figure 12. Line Regulation, VOUT vs. VIN;

1.3

1.35

1.4

1.45

1.5

1.55

1.6

1.65

1.7

10 100 1000 10000

Output Current (mA )

Out p ut Voltage (V )

Vin = 5.5V

Vin = 5.0V

Vin = 3.6V 0.9

0.92

0.94

0.96

0.98

1.02

2.5 3 3.5 4 4.5 5 5.5

Input Volt age (V)

Out p ut Voltage (V )

Iout = 100m A

Iout = 300mA

Iout = 700mA

Iout = 1000mA

Iout = 1500mA

Figure 13. Load Step 40mA to 500mA; VIN = 4V Figure 14. Load Step 40mA to 1A; VIN = 4V

VOUT IOUT

100mV/Div

100µs/Div

200mA/Div

100µs/Div

VOUT IOUT

50mV/Div 500mA/Div

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 8 - 18

AS1335

Datasheet - Ty p i c a l O p e r a t i n g C h a r a c t e r i s t i c s

Figure 15. Shutdown Response; VIN = 3.4V Figure 16. Startup Response; VIN = 3.4V

200µs/Div

20mA/Div 2V/Div

1V/Div

IIN

VOUT

20µs/Div

500mA/Div 2V/Div

1V/Div

IIN

VOUT

Figure 17. Line Transient Response;

VIN = 3.5V to 4.5V, IOUT = 500mA

100µs/Div

100mV/Div 500mV/Div

VIN

VOUT

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 9 - 18

AS1335

Datasheet - D et a i l e d De s c r i p t i o n

8 Detailed Description

The AS1335 is a high-efficiency buck converter that uses a constant-frequency current-mode architecture. The device

contains two internal MOSFET switches and is available with a user-adjustable output voltage.

Figure 18. AS1335 - Block Diagram

Main Control Loop

During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch.

This switch is turned off when the current comparator (ICOMP) resets the RS latch. The peak inductor current (IPK) at

which ICOMP resets the RS latch, is controlled by the error amplifier. When ILOAD increases, VFB decreases slightly

relative to the internal 0.6V reference, causing the error amplifier’ s output voltage to increase until the average inductor

current matches the new load current.

When the top MOSFET is off, the bottom MOSFET is turned on until the inductor current starts to reverse as indicated

by the current reversal comparator (IRCMP), or the next clock cycle begins. The over-voltage detection comparator

(OVDET) guards against transient overshoots >7.8% by turning the main switch off and keeping it off until the transient

is removed.

AS1335

OSC

Frequency

Shift

0.6V

Reference

–

Error

Amp

Shutdown

Ramp

Compensator

0.6V

OSCN

Digital

Logic

0.6V +

ΔVOVL

0.6V -

ΔVOVL

–

OVDET

–

ICOMP

–

IRCMP

Anti-

Shoot

Through

Main

VIN

GND

Power-OK

Compare

Logic

RSI POK

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 10 - 18

AS1335

Datasheet - D et a i l e d De s c r i p t i o n

Short-Circuit Protection

This frequency reduction ensures that the inductor current has more time to decay, thus preventing runaway condi-

tions. fOSC will progressively increase to 1.5MHz when VOUT > 0V or VFB > 0V.

Dropout Operation

The AS1335 is working with a low input-to-output voltage difference by operating at 100% duty cycle. In this state, the

PMOS is always on. This is particularly useful in battery-powere d applications with a 3.3V output.

The AS1335 allows the output to follow the input batte ry voltage as it drops below the regulation voltage. The quies-

cent current in this state rises minimally to only 400µA (max), which aids in extending battery life. This dropout (100%

duty-cycle) operation achieves long battery life by taking full advantage of the entire battery range.

The input voltage requires maintaining regulation and is a function of the output voltage and the load. The difference

between the minimum input voltage and the output voltage is ca lled the dropout voltage. The dropout voltage is there-

fore a function of the on-resistance of the internal PMOS (RDS(ON)PMOS) and the inductor resistance (DCR) and this is

proportional to the load current.

Note: At low VIN values, the RDS(ON) of the P-channel switch increases (see Electrical Characteristics on page 4).

Therefore, power dissipation should be taken in consideration.

Shutdown

Connecting EN to GND or logic low places the AS1335 in shutdown mode and reduces the supply current to 0.1µA. In

shutdown the control circuitry and the internal NMOS and PMOS turn off and SW becomes high impedance discon-

necting the input from the output. The output capacitance and load current determine the voltage decay rate. For nor-

mal operation connect EN to VIN or logic high.

Note: Pin EN should not be left floating.

Power-OK Functionality

The AS1335’s power-ok circuitry offers a 215ms delayed power-ok signal. As lo ng as the output voltage is outside of

the power-ok regulation window the POK pin drives an open-drain low signal. As soon as the output voltage is within

the regulation window, the internal open-drain MOSFET is turned off and the POK pin can be externally pulled to high.

The output of the power-ok signal is delayed by 215ms.

RSI Signal

With the RSI signal the internal power-ok timer can be reseted or delayed. As long as the input to RSI is high the POK

signal remains low, regardless of the output voltage condition.

Thermal Shutdown

Due to its high-efficiency design, the AS1335 will not dissipate much heat in most applications. However, in applica-

tions where the AS1335 is running at high ambient temperature, uses a low supply voltage, and runs with high duty

cycles (such as in dropout) the heat dissipated may exceed the maximum junction temperature of the device.

As soon as the junction temperature reaches approximately 150ºC the AS1335 goes in thermal shutdown. In this mode

the internal PMOS & NMOS switch are turned off. The device will power up again, as soon as the temperature falls

below +125°C again.

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 11 - 18

AS1335

Datasheet - App l i c a t i o n I n f o r m a t i o n

9 Application Information

The AS1335 is perfect for mobile communications equipment, LED matrix displays, bar-graph displays, instrument-

panel meters, dot matrix displays, set-top boxes, white goods, professional audio equipment, medical equipment,

industrial controllers to name a few applications.

Adjustable Output Voltage

For the fixed outp u t vol tage (VOUT=1.0V) connect pin FB to VOUT (see Figure 19). For the adjustable output voltage

version connect a voltage divider to pin FB (see Figure 20).

The voltage divider from VOUT to GND programs the output voltage from 0.6V to 5.25V via pin FB as:

VOUT = 0.6V(1 + (R1/R2)) (EQ 1)

Figure 19. AS1335 - Step-Down Converter, Single Li-Ion to 1.0V / 1.5A fixed Output

Figure 20. AS1335 - Step-Down Converter, Single Li-Ion to 3.3V adjustable Output

AS1335-100

COUT

100µF

CIN

22µF

VIN

2.7V to 4.2V VOUT

1.0V, 1.5A

2.2µH

GND

VIN SW

POK FB

GND

PGND

RSI

100kΩ

AS1335-AD

COUT

100µF

CIN

22µF

VIN

3.35V to 5.25V VOUT

3.3V

2.2µH

GND

VIN SW

POK

GND

PGND

RSI

100kΩ

680kΩ

150kΩ

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 12 - 18

AS1335

Datasheet - App l i c a t i o n I n f o r m a t i o n

External Component Selection

Inductor Selection

For most applications the value of the external inductor should be in the range of 2.2µH to 4.7µH as the inductor value

has a direct effect on the ripple current. The selected inductor must be rated for its DC resistance and saturation cur-

rent. The inductor ripple current (ΔIL) decreases with high er inductance and increases with higher VIN or VOUT.

In Equation (EQ 2) the maximum inductor current in PWM mode under static load conditions is calculated. The satura-

tion current of the inductor should be rated higher than the maximum inductor current as calculated with Equation (EQ

3). This is recommended because th e inductor current will rise above the calculated value during heavy load tran-

sients.

f = Switching Frequency (1.5 MHz typical)

L = Inductor Value

ILmax = Maximum Inductor current

ΔIL = Peak to Peak inductor ripple current

The recommended starting point for setting ripple current is ΔIL = 600mA (40% of 1.5A).

The DC current rating of the inductor should be at least equal to the maximum load current plus ha lf the ripple current

to prevent core saturation. Thus, a 1.8A rated inductor should be sufficient for most applications (1.5A + 300mA).

Note: For highest efficiency, a low DC-resistance inductor is recommended.

Accepting larger values of ripple current allows the use of low inductance values, but results in higher output voltage

ripple, greater core losses, and lower output current capability.

The total losses of the coil have a strong impact on the efficiency of the DC/DC conversion and consist of both the

losses in the DC resistance and the following frequency-dependent components:

1. The losses in the core material (magnetic hysteresis loss, especially at high switchin g frequencies).

2. Additional losses in the conductor from the skin effect (current displacement at high frequencies).

3. Magnetic field losses of the neighboring windings (proximity effect).

4. Radiation losses.

Output Capacitor Selection

The advanced fast-response voltage mode control scheme of the AS1335 allows the use of tin y ceramic capacitors.

Because of their lowest output voltage ripple low ESR ceramic capacitors are recommended. X7R or X5R dielectric

output capacitor are recommended.

At high load currents, the device operates in PWM mode and th e RMS ripple current is calculated as:

While operating in PWM mode the overall output voltage ripple is the sum of the voltage spike caused by the output

capacitor ESR plus the voltage ripple caused by charging and discharging the output capacitor:

(EQ 2)

ΔILVOUT

1VOUT

VIN

-------------

–

Lf×

-----------------------

×=

(EQ 3)

ILMAX IOUTMAX

ΔIL

--------

(EQ 4)

IRMSCOUT VOUT

1VOUT

VIN

-------------

–

Lf×

-----------------------1

23×

----------------

××=

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AS1335

Datasheet - App l i c a t i o n I n f o r m a t i o n

Higher value, low cost ceramic capacitors are available in very small case sizes, and their high ripple current, high volt-

age rating, and low ESR make them ideal for switching regulator applications. Because the AS1335 control loop is not

dependant on the output capacitor ESR for stable operation, ceramic capacitors can be used to achieve very low out-

put ripple and accommodate small circuit size.

At light loads, the converter operates in powersave mode and the output voltage ripple is in direct relation to the output

capacitor and inductor value used. Larger output capacitor and inductor values minimize the voltage ripple in power-

save mode and tighten DC output accuracy in powersave mode.

Input Capacitor Selection

In continuous mode, the source current of the PMOS is a square wave of the duty cycle VOUT/VIN. To prevent large

voltage transients while minimizing the interference with other circuits caused by high input voltage spikes, a low ESR

input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given as:

where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple current,

IMAX = ILIM - ΔIL/2

This formula has a maximum at VIN = 2VOUT where IRMS = IOUT/2. This simple worst-case condition is commonly used

for design because even significant deviations only provid e negligible affects.

The input capacitor can be increased without any limit for better input voltage filtering. Take care when using small

ceramic input capacitors. When a small ceramic capacitor is used at the input, and the power is being supplied through

long wires, such as from a wall adapter , a load step at the output, or VIN step on the input, can induce ringing at the VIN

pin. This ringing can then couple to the output and be mistaken as loop instability, or could even damage th e part by

exceeding the maximum ratings.

Ceramic Input and Output Capacitors

When choosing ceramic capacitors for CIN and COUT, th e X5R or X7R dielectric formulations are recommended.

These dielectrics have the best temperature and voltage characteristics for a given value and size. Y5V and Z5U

dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequen-

cies and therefore should not be used.

Because ceramic capacitors lose a lot of their initial capacitance at their maximum rated voltage, it is recommended

that either a higher input capacity or a capacitance with a higher rated voltage is used.

Table 4. Recommended External Components

Name Part Number Value Rating Type Size Manufacturer

COUT T520B107M006ATE040 100µF 6.3V Tantal B

(3.5x2.8x1.9mm) Kemet

www.kemet.com

CIN, COUT GRM21BR60J226ME39 22µF 6.3V X5R 0805 Murata

www.murata.com

LMOS6020-222ML 2.2µH 3.26A 35mΩ6.8x6.0x2.4mm Coilcraft

www.coilcraft.com

MOS6020-472ML 4.7µH 1.82A 50mΩ6.8x6.0x2.4mm

(EQ 5)

ΔVOUT VOUT

1VOUT

VIN

-------------

–

Lf×

-----------------------1

8COUT

×f×

------------------------------- ESR+

⎝⎠

⎛⎞

××=

(EQ 6)

IRMS IMAX VOUT VIN VOUT

–()×

VIN

----------------------------------------------------------

×=

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AS1335

Datasheet - App l i c a t i o n I n f o r m a t i o n

Efficiency

The efficiency of a switching regulator is equivalent to:

Efficiency = (POUT/PIN)x100% (EQ 7)

For optimum design, an analysis of the AS1335 is needed to determine efficiency limitations and to determine design

changes for improved efficiency. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...) (EQ 8)

Where:

L1, L2, L3, etc. are the individual losses as a percentage of input power .

Althought all dissipative elements in the circuit produce losses, those four main sources should be considered for effi-

ciency calculation:

Input Voltage Quiescent Current Losses

The VIN current is the DC supply current given in the electrical characteristics which excludes MOSFET driver and con-

trol currents. V IN current results in a small (<0.1%) loss that increases with VIN, even at no load. The VIN quiescent cur-

rent loss dominates the efficiency loss at very low load currents.

I²R Losses

Most of the efficiency loss at medium to high load currents are attributed to I²R loss, and are calculated from the resis-

tances of the internal switches (RSW) and the external inductor (RL). In continuous mode, the average output current

flowing through inductor L is split between the internal switches. Therefore, the series resistance looking into the SW

pin is a function of both NMOS & PMOS RDS(ON) as well as the the duty cycle (DC) and can be calculated as follows:

RSW = (RDS(ON)PMOS)(DC) + (RDS(ON)NMOS)(1 – DC) (EQ 9)

The RDS(ON) for both MOSFETs can be obtained from the Electrical Characteristics on page 4. Th us, to obtain I²R

losses calculate as follows: I²R losses = IOUT²(RSW + RL)(EQ 10)

Switching Losses

The switching current is the sum of the control currents and the MOSFET driver. The MOSFET driver current results

from switching the gate capacitance of the power MOSFETs. If a MOSFET gate is switched from low to high to low

again, a packet of charge dQ moves from VIN to ground. The resulting dQ/dt is a current out of VIN that is typically

much larger than the DC bias current. In continuous mode:

IGC = f(QPMOS + QNMOS)(EQ 11)

Where: QPMOS and QNMOS are th e gate charges of the internal MOSFET switches.

The losses of the gate charges are proportional to VIN and thus their effects will be more visible at higher supply volt-

ages.

Other Losses

Basic losses in the design of a system should also be considere d. Internal batte ry resistances and coppe r trace can

account for additional efficiency degradations in battery operated systems. By making sure that CIN has adequate

charge storage and very low ESR at the given switching frequency, the internal battery and fuse resistance losses can

be minimized. CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total

additional loss.

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AS1335

Datasheet - App l i c a t i o n I n f o r m a t i o n

Checking Transient Response

The main loop response can be evaluated by examining the load transient response. Switching regulators normall y

take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an

amount equivalent to: VDROP = ΔILOAD x ESR (EQ 12)

Where:

ESR is the eff ective series resistance of COUT.

ΔILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts

to return VOUT to its steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing that

would indicate a stability problem.

Layout Considerations

The AS1335 requires proper layout and design techniques for optimum performance.

! The power traces (GND, SW, and VIN) should be kept as short, direct, and wide as is practical.

! Pin FB should be connected directly to the Output Voltage.

! The positive plate of CIN should be connected as close to VIN as is practical since CIN provides the AC current to

the internal power MOSFETs.

! Switching node SW should be kept far away from the sensitive FB node.

! The negative plates of CIN and COUT should be kept as close to each other as is practical. A starpoint to Ground is

recommended.

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AS1335

Datasheet - Pac ka ge Dr aw in gs an d Mar ki ng s

10 Package Drawings and Markings

The device is available in an 10-pin TDFN 3x3mm package.

Figure 21. 10-pin TDFN 3x3mm Package

Notes:

1. Figure 21 is shown for illustration only.

2. All dimensions are in millimeters; angles in degrees.

3. Dimensioning and tolerancing conform to ASME Y14.5 M-1994.

4. N is the total number of terminals.

5. The terminal #1 identifier and terminal numbering convention shall conform to JEDEC 95-1, SPP-012. Det ails of ter-

minal #1 identifier are optional, but must be located with in the zone indicated. The terminal #1 identifier may be either

a mold or marked feature.

6. Dimension b applies to metallized terminal and is measured between 0.15mm and 0.30mm from the terminal tip.

7. ND refers to the maximum number of terminals on side D.

8. Unilateral coplanarity zone app lies to the exposed heat sink slug as well as the terminals.

Symbol Min Typ Max Notes

A 0.70 0.75 0.80 1, 2

A1 0.00 0.02 0.05 1, 2

A3 0.20 REF 1, 2

L1 0.03 0.15 1 , 2

L2 0.13 1 , 2

aaa0.151, 2

bbb0.101, 2

ccc 0.10 1, 2

ddd0.051, 2

eee0.081, 2

ggg0.101, 2

Symbol Min Typ Max Notes

D BSC 3.00 1, 2

E BSC 3.00 1, 2

D2 2.20 2.70 1, 2

E2 1.40 1.75 1 , 2

L 0.30 0.40 0.50 1, 2

θ0º 14º 1, 2

K0.20 1, 2

b 0.18 0.25 0.30 1, 2, 5

e0.50

N101, 2

ND 5 1, 2, 5

SEE

DETAIL B

PIN 1 INDEX AREA

(D/2 xE/2)

BTM VIEW

N-1N

ddd

D2/2

(D/2 xE/2)

TOP VIEW

aaa C

PIN 1 INDEX AREA

ccc C

SIDE VIEW

(ND-1) X e

0.08 C

C A B

E2/2

SEATING

PLANE

DETAIL B

Datum A or B

Terminal Tip

ODD TERMINAL SIDE

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AS1335

Datasheet - O r d e r i n g I n f o r m a t i o n

11 Ordering Information

The device is available as th e following standard versions.

Note: All products are RoHS compliant and Pb-free.

Buy our products or get free samples online at ICdirect: http://www.austriamicrosystems.com/ICdirect

For further information and requests, please contact us mailto:sales@austriamicrosystems.com

or find your local distributor at http://www.austriamicrosystems.com/distributor

Table 5. Ordering Information

Ordering Code Marking Description Delivery Form Package

AS1335-BTDT-100 ASSI 1.5A, 1.5MHz, Synchronous DC/DC Step-Down

Converter, fixed VOUT = 1.0V Tape and Reel 10-pin TDFN

3x3mm

AS1335-BTDT-AD ASSC 1.5A, 1.5MHz, Synchronous DC/DC S tep-Down

Converter, user-adjustable Output Voltage Tape and Reel 10-pin TDFN

3x3mm

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335 Revision 1.03 18 - 18

AS1335

Datasheet

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