Diodes AP2145/ AP2155 - Manual (Page 9)

AP2145/ AP2155
Document number: DS32031 Rev. 3 - 2
9 of 13
www.diodes.com
March 2013
© Diodes Incorporated
AP2145/ AP2155
Application Information
Power Supply Considerations
A 0.1- μF to 1-μF X7R or X5R ceramic bypass capacitor between IN and GND, close to the device, is recommended. Placing a high-value
electrolytic capacitor on the input and output pin(s) is recommended when the output load is heavy. This precaution reduces power-supply
transients that may cause ringing on the input. Additionally, bypassing the output with a 0.01- μF to 0.1-μF ceramic capacitor improves the immunity
of the device to short-circuit transients.
Over-Current and Short Circuit Protection
An internal sensing FET is employed to check for over-current conditions. Unlike current-sense resistors, sense FETs do not increase the series
resistance of the current path. When an overcurrent condition is detected, the device maintains a constant output current and reduces the output
voltage accordingly. Complete shutdown occurs only if the fault stays long enough to activate thermal limiting.
Three possible overload conditions can occur. In the first condition, the output has been shorted to GND before the device is enabled or before V
IN
has been applied. The AP2145/AP2155 senses the short circuit and immediately clamps output current to a certain safe level namely I LIMIT .
In the second condition, an output short or an overload occurs while the device is enabled. At the instance the overload occurs, higher current may
flow for a very short period of time before the current limit function can react. After the current limit function has tripped (reached the over-current
trip threshold), the device switches into current limiting mode and the current is clamped at I
LIMIT
.
In the third condition, the load has been gradually increased beyond the recommended operating current. The current is permitted to rise until the
current-limit threshold (I
TRIG
) is reached or until the thermal limit of the device is exceeded. The AP2145/AP2155 is capable of delivering current up
to the current-limit threshold without damaging the device. Once the threshold has been reached, the device switches into its current limiting mode
and is set at I
LIMIT
.
To protect against short circuit to GND at extremely low temperature (< -30°C), a minimum 120- μF electrolytic capacitor on the output pin is
recommended. A correct capacitor type with capacitor voltage rating and temperature characteristics must be properly chosen so that capacitance
value does not drop too low at the extremely low temperature operation. A recommended capacitor should have temperature characteristics of less
than 10% variation of capacitance change when operated at extremely low temp. Our recommended aluminum electrolytic capacitor type is
Panasonic FC series.
FLG Response
When an over-current or over-temperature shutdown condition is encountered, the FLG open-drain output goes active low after a nominal 7-ms
deglitch timeout. The FLG output remains low until both over-current and over-temperature conditions are removed. Connecting a heavy capacitive
load to the output of the device can cause a momentary over-current condition, which does not trigger the FLG due to the 7-ms deglitch timeout.
The AP2145/AP2155 is designed to eliminate false over-current reporting without the need of external components to remove unwanted pulses .
Power Dissipation and Junction Temperature
The low on-resistance of the internal MOSFET allows the small surface-mount packages to pass large current. Using the maximum operating
ambient temperature (T
A
) and R
DS(ON)
, the power dissipation can be calculated by:
P D = R DS(ON) × I 2
Finally, calculate the junction temperature:
T J = P D x R JA + T A
Where:
T A = Ambient temperature
C
R JA = Thermal resistance
P
D
= Total power dissipation
Thermal Protection
Thermal protection prevents the IC from damage when heavy-overload or short-circuit faults are present for extended periods of time. The
AP2145/AP2155 implements a thermal sensing to monitor the operating junction temperature of the power distribution switch. Once the die
temperature rises to approximately 140°C due to excessive power dissipation in an over-current or short-circuit condition the internal thermal sense
circuitry turns the power switch off, thus preventing the power switch from damage. Hysteresis is built into the thermal sense circuit allowing the
device to cool down approximately 25°C before the switch turns back on. The switch continues to cycle in this manner until the load fault or input
power is removed. The FLG open-drain output is asserted when an over-temperature shutdown or over-current occurs with 7-ms deglitch.