RT9728B Datasheet, PDF (10/14 Pages) Richtek Technology Corporation – Power Switch with Adjustable Current Limit

RT9728B
Applications Information
The RT9728B is a single P-MOSFET power switch with
an active-high/low enable input, optimized for self-powered
and bus-powered Universal Serial Bus (USB) applications.
The switch's low R_DS(ON) meets USB voltage drop
requirements and a flag output is available to indicate fault
conditions to the local USB controller.
Current Limit and Short-Circuit Protection
When a heavy load or short-circuit situation occurs while
the switch is enabled, a large transient current may flow
through the device. The RT9728B includes current-limit
circuitry to prevent the MOSFET switch and the hub
downstream ports from damage due to large transient
current. The RT9728B provides an adjustable current limit
threshold between 120mA and 1.8A (typ) via an external
resistor, R_ILIM, whose resistance is between 15kΩ and
232kΩ. However, if the ILIM pin is connected to V_IN, the
current limit threshold will be 75mA (typ). Once the current
limit threshold is exceeded, the device enters constant-
current mode until either thermal shutdown occurs or the
fault is removed. Figure 1 shows the curve of current limit
value vs. R_ILIM resistor. The recommended R_ILIM resistor
selection is shown in Table 1.
Current Limit Threshold vs. R_ILIM
2000
1750
1500
1250
1000
750
500
250
0
0 30 60 90 120 150 180 210 240
R_ILIM (kΩ)
Figure 1. Current Limit Threshold vs. R_ILIM
Desired Nominal
Current Limit (mA)
75
120
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
Table 1. Recommended R_ILIM Resistor Selection
Ideal Resistor Closet 1%
(kΩ)
Resistor (kΩ)
Actual Limits (Include R Tolerance)
IOS Min (mA) IOS Nom (mA) IOS Max (mA)
Short ILIM to VIN
50.0
75.0
100.0
226.1
226.0
101.3
120.0
142.1
134.0
133.0
173.7
201.5
233.9
88.5
88.7
262.1
299.4
342.3
65.9
66.5
351.1
396.7
448.7
52.5
52.3
443.9
501.6
562.4
43.5
43.2
535.1
604.6
674.1
37.2
37.4
616.0
696.0
776.0
32.4
32.4
708.7
800.8
892.9
28.7
28.7
797.8
901.5
1005.2
25.8
26.1
875.4
989.1
1102.8
23.4
23.2
982.1
1109.7
1237.3
21.4
21.5
1057.9
1195.4
1332.9
19.7
19.6
1158.0
1308.5
1459.0
18.5
18.7
1225.7
1385.0
1544.3
17.3
17.4
1317.3
1488.5
1659.7
16.2
16.2
1414.8
1598.7
1782.6
15.2
15.0
1528.1
1726.7
1925.3
14.4
14.3
1602.9
1811.2
2019.5
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
is a registered trademark of Richtek Technology Corporation.
DS9728B-00 August 2013