SC1480ITSTRT datasheet(7/22 Pages) SEMTECH

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 2006 Semtech Corp.

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SC1480

POWER MANAGEMENT

Applications Information

+5V Bias Supply

The SC1480 requires an external +5V bias supply in ad-

dition to the battery. If stand-alone capability is required,

the +5V supply can be generated with an external linear

regulator. There are two inputs for the external +5V bias

supply, VCCA & VDDP. The VCCA input powers the analog

section of the SC1480 while the VDDP input provides

power to the upper and lower gate drivers. VCCA will need

to be decoupled from the +5V supply through a 10 Ohm

resistor and the addition of a filter capacitor from VCCA

to ground. VCCA and VDDP must be separate in order to

utilize the low power S3 state of the SC1480. The bat-

tery input VIN and the +5V input VCCA can be tied to-

gether if the input voltage is fixed from +4.5V to +5.5V;

however, as before, VCCA will need to be decoupled from

the +5V supply through a 10 Ohm resistor and the addi-

tion of a filter capacitor from VCCA to ground.

Pseudo-Fixed Frequency Constant On-Time PWM

Controller

The PWM control architecture consists of a constant-on-

time, pseudo fixed frequency PWM controller, (Figure 1).

The output ripple voltage developed across the output

filter capacitors ESR provides the PWM ramp signal elimi-

nating the need for a current sense resistor. The high-

side switch on-time is determined by a one-shot whose

period is directly proportional to output voltage and in-

versely proportional to input voltage. A second one-shot

sets the minimum off-time to 400ns typically.

On-Time One-Shot (T

ON)

The on-time one-shot comparator has two inputs. One

input looks at the output voltage, while the other input

samples the input voltage and converts it to a current.

This input proportional current is used to charge an in-

ternal on-time capacitor. The TON time is the time re-

quired for the voltage on the capacitor to charge from

zero volts to VOUT, thereby making the on-time of the

high-side switch directly proportional to output voltage

and inversely proportional to input voltage. This imple-

mentation results in a nearly constant switching frequency

without the need of a clock generator.

)

(

OUT

TON









•

−

TON is a resistor connected from the input supply to the

TON pin. The graph on page 19 shows the relationship

between R

TON and switching frequency.

Reference I/O

The reference input can be generated off of a 2.5V or

VDDQ supply by a simple resistive divider. Resistors less

than 100k Ohms should be used and a small filter ca-

pacitor from the reference input to ground of 0.1uF will

remove any ripple voltage present on the input. The in-

put has a common mode range of the REFIN threshold

to 2.5V.

The voltage on the reference input passes through a unity

gain buffer prior to being sent to REFOUT. This reference

output has a class A output stage with 3mA of sourcing

capacity. It has a pull-down impedance of 50k. The out-

put will require a small RC filter of 10 Ohms and 1µF to

maintain stability.

Shutdown, Suspend to RAM and Run Mode

The SC1480 has three modes of operation: shutdown,

suspend to RAM, and run mode. All three modes must

have VCCA connected at all times. Shutdown mode is

controlled by REFIN. When REFIN is below 0.8V, the ref-

erence buffer will be off and the SMPS is disabled. In

this mode the bias current of the device will be less than

10µA.

Suspend to RAM, or S3 State, is controlled by REFIN and

VDDP. With REFIN is above 0.8V and VDDP is low (below

3V), the device will output the reference voltage onto

REFOUT, but the SMPS is disabled. In this mode the bias

current is approximately 125µA.

Run mode is activated by maintaining REFIN above 0.8V

and VDDP above 3V. In this mode the reference and SMPS

are active.

Current Limit Circuit

Current limiting of the SC1480 can be accomplished in

two ways. First, the device can implement on-state re-

sistance of the low-side MOSFET as the current sensing

element (RDS

ON sensing). Second, the device can accept

a resistive element in the low-side source (R

SENSE, resistor

sensing). The second method offers greater accuracy of

the current limit threshold over RDS

ON sensing, at the

added expense of a sense resistor and associated effi-

ciency loss.

Whether RDS

ON sensing or RSENSE resistor sensing is used,

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