ISL78205 Datasheet, PDF (13/16 Pages) Intersil Corporation – 2.5A Buck Controller with Integrated High-Side MOSFET

ISL78205
Oscillator and Synchronization
The oscillator has a default frequency of 500kHz with the FS pin
connected to VCC, or ground, or floating. The frequency can be
programmed to any frequency between 200kHz and 2.2MHz with
a resistor from the FS pin to GND.
R_FS[kΩ]
=
1_----4_----5_---0_----0_---0_{-----–-----}1_---6_-----⋅_---F_----S_----[_--k_---H_-----z_----]
FS[kHz]
(EQ. 2)
1200
1000
800
600
400
200
0
0
500
1000
1500
2000
2500
F_S (kHz)
FIGURE 16. R_FS vs FREQUENCY
With the SYNC pins simply connected together, multiple
ISL78205s can be synchronized. The slave ICs automatically
have 180 degree phase shift with respect to the master IC.
With an external square pulse waveform (with frequency 10%
higher than the local frequency, 10% to 90% duty cycle and pulse
width higher than 150ns) on the SYNC pin, the ISL78205 will
synchronize its switching frequency to the fundamental
frequency of the input waveform. The internal oscillator
synchronizes with the leading edge of the input signal. The rising
edge of UGATE PWM is delayed by 180 degrees from the leading
edge of the external clock signal.
Fault Protection
Overcurrent Protection
The overcurrent function protects against any overload conditions
and output shorts at worst case, by monitoring the current
flowing through the upper MOSFET.
There are 2 current limiting thresholds. The first one, I_OC1, is to
limit the high-side MOSFET peak current cycle-by-cycle. The
current limit threshold is set to a default of 3.6A with the ILIMIT
pin connected to GND or VCC, or left open. The current limit
threshold can also be programmed by a resistor R_LIM at ILIMIT
pin to ground. Use Equation 3 to calculate the resistor.
R_LIM
=
----------³---⁰----⁰---⁰----⁰---⁰-----------
I_OC[A] + 0.018
(EQ. 3)
Note that with the lower R_LIM, IOC1 is higher. IOC1 reaches its
maximum 5.4A with R_LIM at 54.9k (TYP). With R_LIM lower than
54.9k (TYP), the OC limit goes to its default value of 3.6A (TYP).
370
320
270
220
170
120
70
0
1
2
3
4
5
6
I_OC1 (A)
FIGURE 17. R_LIM vs IOC1
The second current protection threshold, I_OC2, is 15% higher
than I_OC1 mentioned above. Upon the instant that the high-side
MOSFET current reaches I_OC2, the PWM shuts off after 2 cycle
delay and the IC enters hiccup mode. In hiccup mode, the PWM
is disabled for dummy soft-start duration equaling 5 regular
soft-start periods. After this dummy soft-start cycle, the true
soft-start cycle is attempted again. The I_OC2 offers a robust and
reliable protection against worst case conditions.
The frequency fold back is implemented for the ISL78205. When
overcurrent limiting, the switching frequency is reduced to be
proportional to the output voltage in order to keep the inductor
current under limit threshold during overload conditions. The low
limit of frequency under frequency foldback is 40kHz.
Overvoltage Protection
If the voltage detected on the FB pin is over 110% of reference,
the high-side and low-side driver shuts down immediately and
will not be allowed to turn on until the FB voltage falls down to
0.8V. When the FB voltage drops to 0.8V, the drivers are released
on. If the 120% overvoltage threshold is reached, the high-side
and low-side driver shut down immediately and the IC is latched
off. The IC has to be reset for restart.
Thermal Protection
The ISL78205 PWM will be disabled if the junction temperature
reaches +155°C. A +15°C hysteresis insures that the device will
not restart until the junction temperature drops below +140°C.
Component Selection
Output Capacitors
Output capacitors are required to filter the inductor current and
supply the load transient current.
All ceramic output capacitors are achievable with this IC. Also, in
applications the aluminum electrolytic type capacitor provides
better load transient and longer holdup time for the load. When
low cost, high ESR aluminum capacitors are used at the output.
Ceramic capacitors (2.2µF to 10µF) are recommended to handle
the ripple current and reduce the total equivalent ESR effectively.
13
FN7926.0
September 22, 2011