RT7297C datasheet(11/15 Pages) RICHTEK

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RT7297C

DS7297C-01

September 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

OUT

I =

⎡⎤ ⎡

⎤

Δ× −

⎢⎥ ⎢

⎥

⎣⎦ ⎣

⎦

Under Voltage Protection

Hiccup Mode

For the RT7297CH, it provides Hiccup Mode Under Voltage

Protection (UVP). When the VFB voltage drops below 0.4V,

the UVP function will be triggered to shut down switching

operation. If the UVP condition remains for a period, the

RT7297CH will retry automatically. When the UVP

condition is removed, the converter will resume operation.

The UVP is disabled during soft-start period.

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output voltage

ripple. High frequency with small ripple current can achieve

the highest efficiency operation. However, it requires a

large inductor to achieve this goal.

For the ripple current selection, the value of

ΔIL= 0.24(IMAX)

will be a reasonable starting point. The largest ripple

current occurs at the highest VIN. To guarantee that the

ripple current stays below the specified maximum, the

inductor value should be chosen according to the following

equation :

Table 2. Suggested Inductors for Typical

Application Circuit

Component

Supplier

Series

Dimensions

(mm)

TDK

VLF10045

10 x 9.7 x 4.5

TDK

SLF12565

12.5 x 12.5 x 6.5

TAIYO

YUDEN

NR8040

8 x 8 x 4

OUT

L(MAX)

IN(MAX)

L =

⎡⎤ ⎡

⎤

×−

⎢⎥ ⎢

⎥

×Δ

⎣⎦ ⎣

⎦

The inductor's current rating (caused a 40

°C temperature

rising from 25

°C ambient) should be greater than the

maximum load current and its saturation current should

be greater than the short circuit peak current limit. Please

see Table 2 for the inductor selection reference.

Latch-Off Mode

For the RT7297CL, it provides Latch-Off Mode Under

Voltage Protection (UVP). When the FB voltage drops

below half of the feedback reference voltage, VFB, UVP

will be triggered and the RT7297CL will shut down in Latch-

Off Mode. In shutdown condition, the RT7297CL can be

reset by EN pin or power input VIN.

Figure 5. Hiccup Mode Under Voltage Protection

Figure 6. Latch-Off Mode Under Voltage Protection

Over Temperature Protection

The RT7297C features an Over Temperature Protection

(OTP) circuitry to prevent from overheating due to

excessive power dissipation. The OTP will shut down

switching operation when junction temperature exceeds

150

°C. Once the junction temperature cools down by

approximately 20

°C, the converter will resume operation.

To maintain continuous operation, the maximum junction

temperature should be lower than 125

°C.

Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current

ΔIL increases with higher VIN

and decreases with higher inductance.

Time (50ms/Div)

Hiccup Mode

VOUT

(2V/Div)

ILX

(2A/Div)

IOUT = Short

Time (250

μs/Div)

Latch-Off Mode

VOUT

(2V/Div)

ILX

(2A/Div)

IOUT = Short

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