SC189CEVB-2 datasheet(16/23 Pages) SEMTECH

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SC189

Detailed Description

The SC189 is a synchronous step-down pulse width

modulated (PWM) voltage mode DC-DC regulator

operating at 2.5MHz fixed-frequency. The switching

frequency is chosen to minimize the size of the external

inductor and capacitors while maintaining high

efficiency.

Operation

During

normal

operation,

the

internal

high-

side PMOS device is activated on each ris-

ing edge of the internal oscillator.

The voltage

feedback loop uses an internal feedback resistor divider.

The period is set by the on board oscillator when in PWM

mode at average to high loads. The device has an internal

low-side synchronous NMOS device and does not require a

SchottkydiodeontheLXpin.Thedeviceoperatesasabuck

converter in PWM mode with a fixed frequency of 2.5MHz.

Output Voltage Selection

The SC189 is designed for fixed output voltage. There

are some options for preset output voltage shown in

Table 1. If the voltage desired is not shown in the Table

1, it can be programmed via an external resistor di-

vider. There will be typical 1uA current flowing into the

VOUT pin. The typical schematic of adjustable output

voltage option from the part with standard 1.0V, the

SC189A, is shown in Figure 1. The C

FF is needed for main-

tain the performance of the transient response. The

proper value of C

FF can be calculated by the equation

(

)

(

)

(

]

[

]

[

−

⋅

Ω

−

OSTD

OUT

,where the V

OSTD is the standard voltage shown in Table 1.

To simplify the design, it is recommended to program the

desired output voltage from standard 1.0V as shown in

Figure 1 with a proper C

FF calculated from the equation

shown above. For programming the output voltage from

other standard voltage, the R

FB1, RFB2 and CFF need to be

adjusted to meet the equation shown above.

Maximum Power Dissiption of SOT23-5 Package

The maximum power dissiption for junction tempera-

ture of less than 125°C on SOT23-5 package is shown

in figure 2. The curve is drawn based on the Θ

JA of

90°C/W which is measured in free convection, mounted

on 10mm x 10mm, 2 layer FR4 PCB shown in figure 8

with copper of 1oz for each layer. The maximum power

dissiption may limit the maximum output current over

temperature. The figure 3 and figure 4 show the typi-

cal maximum output current for T

J ≤ 125°C over tem-

perature of V

IN=5.0V and VIN=3.3V, respectively. If using

inductor with higher loss (i.e., chip inductor), due to

the higher board temperature, the Θ

JA will be a little bit

higher.

Protection Features

The SC189 provides the following protection features:

Thermal Shutdown

Current Limit

Over-Voltage Protection

Soft-Start Operation

Thermal Shutdown

The device has a thermal shutdown feature to protect

the SC189 if the junction temperature exceeds 160°C.

During thermal shutdown, the on-chip power devices

are disabled with the LX output floating. When the die

temperature drops by 10°C, the part will initiate a soft

start recovery to normal operation.

Current Limit

The internal PMOS power device in the switching stage is

protected by current limit feature. If the output is loaded

above the PMOS current limit for 32 consecutive cycles,

the SC189 enters foldback current limit mode and the

output current is limited to the current limit holding

current (I

CL_HOLD) of a few hundred milliampere. Under

these conditions the output voltage will be the product

of I

CL_HOLD and the load resistance. The

current limit

holding current (I

CL_HOLD) will be decreased when output

voltage is increased. The load presented must fall below

•

Applications Information

GND

OUT

VOUT

Enable

SC189A

VIN

FB1

FB2

10k



Note: (1) R

EN is optional.

(2) R

FB2=10k and CFF=10nF for standard design.

)

(

OUT

−

Schematic of Adjustable V

OUT from SC189A (Std VOUT=1.0V)

Figure 1 — Typical schematic for adjustable output

voltage option from standard 1.0V of SC189A

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