ISL78200_14 Datasheet, PDF (16/22 Pages) Intersil Corporation – 2.5A Regulator with Integrated High-Side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78200
the boost input power source is turned off. At boost mode
applications where there is no external control signal to
enable/disable the IC, an external input UVLO circuit must be
implemented for the start-up and shutdown sequence.
PFM is not available in boost mode.
Oscillator and Synchronization
The oscillator has a default frequency of 500kHz with the FS pin
connected to VCC, 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k = ¹----⁴----⁵---⁰----⁰---⁰--_F---–_S----_-¹--_k-⁶--_H---^--_z-^F--_--^S----^--^k---^H-----^z----^
(EQ. 9)
The SYNC pin is bi-directional and it outputs the IC’s default or
programmed local clock signal when it’s free running. The IC
locks to an external clock injected to SYNC pin (external clock
frequency recommended to be 10% higher than the free running
frequency). The delay from the rising edge of the external clock
signal to the PHASE rising edge is half of the free running switching
period pulse 220ns, (0.5Tsw+220ns). The maximum external clock
frequency is recommended to be 1.6 of the free running frequency.
When the part enters PFM pulse skipping mode, the
synchronization function is shut off and also no clock signal
output in SYNC pin.
With the SYNC pins simply connected together, multiple
ISL78200s can be synchronized. The slave ICs automatically
have 180° phase shift respect to the master IC.
1200
1000
800
600
400
200
0
0
500
1000
1500
2000
2500
F_S (kHz)
FIGURE 30. R_FS vs FREQUENCY
Fault Protection
Overcurrent Protection
The overcurrent function protects against any overload condition
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 default at 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 the ILIMIT pin to
ground. Use Equation 10 to calculate the resistor.
R_LIM
=
----------³---⁰----⁰---⁰----⁰---⁰-----------
I_OCA + 0.018
(EQ. 10)
Note that with the lower R_LIM, I_OC1 is higher. Considering the OC
programming circuit tolerances over temperature range -40°C to
+105°C, 71.5k is the lowest resistor value recommended to be
used for R_LIM to achieve highest OC threshold. With 71.5k R_LIM,
the OC limit is 4.18A (TYP). Resistor lower than 71.5k would
result in default 3.6A OC1 threshold.
370
320
270
220
170
120
70
0.0
1.0
2.0
3.0
4.0
5.0
6.0
I_OC1 (A)
FIGURE 31. R_LIM vs I_OC1
The second current protection threshold, I_OC2, is 15% higher
than I_OC1 mentioned above. At the instant the high-side MOSFET
current reaches I_OC2, the PWM shuts off after a 2 cycle delay and
the IC enters hiccup mode. In hiccup mode, the PWM is disabled
for a dummy soft-start duration equal to 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 ISL78200. When
overcurrent limiting, the switching frequency is reduced to
proportional to the output voltage in order to keep the inductor
current under the limit threshold during overload condition. 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 drivers shut down immediately and
won’t be allowed 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 ISL78200 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 Selections
ISL78200 iSim model available in internet can be used to
simulate the operating behaviors to assist the design.
16
FN7641.2
December 24, 2013