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* The output voltage is set by R2 and R3: V OUT = 1.21V • [1 + (R2/R3)].
* R T: R T is optional. The resistor range is from 130KΩto 1.5 MΩ. FT021H can work when keeping RT pin floating. The default frequency is 130KHZ.
* I LIM:FT021H can work when keeping ILIM pin floating, with 3A limit current in this situation.
P i n A s s i g n m e n t a n d D e s c r i p t i o n
A b s o l u t e M a x i m u m R a t i n g s(N o t e1)
Input Supply Voltage ....................................................................................................-0.3V ～ 35V FB, ILIM, . -0.3V ～ 6V SW Voltage ........................................................................................................-0.3V ～(VIN + 1V) Operating Temperature Range (Note 2).........................................................-40℃～ +85℃ Storage -65℃～ +150℃ Junction Temperature Range.. (150)
Lead Temperature (Soldering, 10 sec.) (265)
Note 1: Stresses beyond those listed Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.
Note 2: The FT021H is guaranteed to meet performance specifications from 0℃ to 70℃. Specifications over the –40℃to 85℃ operating temperature range are assured by design, characterization and correlation with statistical process controls.
PIN NAME DESCRIPTION
1 FB Feedback
2 RT Frequency
Setting
3 ILIM Current
路缘石滑模
Limit
4 VIN Input Supply Voltage
5, 6 SW Switch Node
7, 8 GND Ground
E l e c t r i c a l C h a r a c t e r i s t i c s
Operating Conditions: T A=25, V刺辊
℃IN=12V, R2=470k, R3=150k, unless otherwise specified.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V IN Operating Voltage Range 8 30 V
I Q Quiescent
=0A 10
15
20
mA
I SHDN Shutdown
Current 110 150 μA V UVLO Input UVLO Threshold 4.25 4.5 V
ΔV UVLO UVLO
Hysteresis 50 100
mV V FB Regulated
Voltage 1.188 1.21 1.236 V
I FB Feedback Pin Input Current0.05 μA
f OSC Oscillator Frequency Range Float RT Pin 130 kHz
f OSC_MAX Maximum Oscillator
Frequency
500 kHz
DC Max Duty Cycle 100 % I LIM_def Default Limit Current Float ILIM Pin 3.0 A
I LIM-TH Current Limit Sense Pin
Source Current 7 8.5 10 μA
R PFET R DS(ON) of P-Channel FET 40 mΩR NFET R DS(ON) of N-Channel FET 30 mΩT SD Thermal Shutdown Temperature
Rising 145 ℃
ΔT SD Thermal Shutdown
Hysteresis 30
℃
P i n F u n c t i o n s
FB (Pin 1): Feedback Pin. Receive the feedback voltage from an external resistive divider across the output. In the adjustable version, the output voltage is fixed. The Output voltage is set by R2 and R3:
V OUT = 1.21V • [1 + (R2/R3)].
RT (Pin 2): The internal oscillator is set with a single resistor between this pin and the GND pin.
ILIM (Pin 3): Monitors current through the high-side switch and triggers current limit operation if the inductor valley current exceeds a user defined value that is set by R LIM and the Sense current sourced out of this pin during operation.
VIN (Pin 4): Main Supply Pin. The FT021H operates from 8V to 30V unregulated input. It must be closely decoupled to GND, with a 100μF or greater ceramic capacitor to prevent large voltage spikes from appearing at the input.
SW (Pin 5, 6): Switch Node Connection to Inductor.
车载硬盘
GND (Pin 7, 8): Ground Pin. B l o c k D i a g r a m
A p p l i c a t i o n I n f o r m a t i o n
The FT021H operates by a constant frequency, current mode architecture. The output voltage is set by an external divider returned to the FB pin. An error amplifier compares the divided output voltage with a reference voltage of 1.21V and adjusts the peak inductor current accordingly.
During normal operation, the internal P-channel MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, resets the RS latch.While the P-channel MOSFET is off, the N-channel MOSFET is turned on until either the inductor current starts t
o reverse, as indicated by the current reversal comparator or the beginning of the next clock cycle.
Thermal Protection
The total power dissipation in FT021H is limited by a thermal protection circuit. When the device temperature rises to approximately 145℃, this circuit turns off the output, allowing the IC to cool.The thermal protection circuit can protect the device from being damaged by overheating in the event of fault conditions.Continuously running the FT021H into thermal shutdown degrades device reliability.
Current Limit
Current limit detection occurs during the on-time by monitoring the current through the high-side P-MOSFET. The current limit value is defined by R LIM. If during the on-time the current in the high-side P-MOSFET exceeds the user defined current limit value, the next on-time cycle is immediately terminated.Current sensing is achieved by comparing the voltage across the high-side FET with the voltage across the current limit set resistor R LIM. The current limit value rises when the set resistor R LIM rises.
Oscillator Frequency
The FT021H oscillator frequency is set by a single external resistor connected between the RT pin and the GND pin. The resistor should be located very close to the device and connected directly to the pins of the IC (RT and GND). An internal amplifier holds the RT pin at a fixed voltage typically 0.6V. The oscillator frequency rises when the resistor R T falls. To determine the timing resistance for a given switching frequency, use the equation below:
R T(kΩ)= 4800000/ (200*f OSC(kHz)-24000)
Setting Output Voltage
The output voltage is set with a resistor divider from the output node to the FB pin. It is recommended to use divider resistors with 1% tolerance or better.To improve efficiency at very light loads consider using larger value resistors. If the values are too high the regulator is more susceptible to noise and voltage errors from the FB input current are noticeable. For most applications, a resistor in the 10kΩ to 1MΩrange is suggested for R3. R2 is then given by:
R2 = R3 • [(V OUT / V REF) – 1]
where V REF is 1.21V.
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