ISL78235 Datasheet, PDF (16/20 Pages) Intersil Corporation – 5A Automotive Synchronous Buck Regulator

ISL78235
Tie SS to SGND for internal soft-start (1ms typical). Connect a
capacitor from SS to SGND to adjust the soft-start time. This
capacitor, along with an internal 2.1µA current source, sets the
soft-start interval of the converter, t_SS as shown by Equation 2.
C_SSF = 3.1  t_SSs
(EQ. 2)
C_SS must be less than 33nF to insure proper soft-start reset after
fault condition.
Enable
The Enable (EN) input allows the user to control the turning on or
off of the regulator for purposes such as power-up sequencing or
minimizing power dissipation when the output is not needed.
When the regulator is enabled, there is typically a 600µs delay
for waking up the bandgap reference and then the soft start-up
begins.
Discharge Mode (Soft-stop)
When a transition to shutdown mode occurs, (EN low or fault
condition) or the V_IN UVLO is set, the output is discharged to GND
through an internal 100Ω switch on the PHASE pin.
Power MOSFETs
The power MOSFETs are optimized for highest efficiency. The
ON-resistance for the P-FET is typically 35mΩ and the
ON-resistance for the N-FET is typically 11mΩ.
100% Duty Cycle
The ISL78235 features 100% duty cycle operation to maximize
the battery operation life and provide very low dropout down to
the minimum operating voltage. When the battery voltage drops
to a level that the ISL78235 can no longer maintain the
regulation at the output, the regulator completely turns on the
P-FET. The maximum dropout voltage under the 100% duty cycle
operation is the product of the load current and the
ON-resistance of the P-FET.
Thermal Shutdown
The ISL78235 has built-in over-temperature thermal protection.
When the internal temperature reaches +150°C, the regulator
completely shuts down. As the temperature drops to +125°C, the
ISL78235 resumes operation after a soft-start cycle.
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operation, the ISL78235
typically uses a 0.33µH to 0.78µH output inductor. Higher or
lower inductor values can be used to optimize the total converter
system performance. For example, for higher output voltage 3.3V
application, in order to decrease the inductor current ripple and
output voltage ripple, the output inductor value can be increased.
It is recommended to set the ripple inductor current to
approximately 30% of the maximum output current for optimized
performance. The inductor ripple current can be expressed as
shown in Equation 3:
I = ^V-----^O-----_L^-----^{}--¹--_f--_S-–---_W--^V--V-------I--^O--^N------^{}-
(EQ. 3)
The inductor’s saturation current rating needs to be at larger than
the positive peak current limit specified in the electrical
specification table. The ISL78235 has a typical peak current limit
of 7.5A. The inductor saturation current needs to be over 7.5A for
proper operation.
The ISL78235 uses an internal compensation network for
regulator stability and the output capacitor value is dependent on
the output voltage. The recommended ceramic capacitors are
low ESR X7R rated or better. The recommended minimum output
capacitor values are shown in Table 1 on page 4.
Table 1, shows the minimum output capacitor value is given for
the different output voltages to make sure that the whole
converter system is stable. Additional output capacitance should
be added for better performance in applications where high load
transient or low output ripple is required. It is recommended to
check the system level performance along with the simulation
model.
Output Voltage Selection
The output voltage of the regulator is programmed with an external
resistor divider that is used to scale the output voltage relative to the
internal reference voltage (0.6V) and fed back to the inverting input
of the error amplifier FB pin (see Figure 46).
VOUT
VIN
16 15 14 13
VIN ₁
PGND
12
VDD ₂
ISL78235
PGND
11
PG ₃
10 ^SGND
R2
SYNC ₄
9 ^FB
5
6
7
8
R3
FIGURE 46. PROGRAMMING OUTPUT VOLTAGE WITH R₂ AND R₃
The output voltage programming resistor R₂ (from VOUT to FB)
will depend on the value chosen for the feedback resistor and the
desired output voltage of the regulator. The value for the
feedback resistor, R₃ (from FB to GND), is typically between
10kΩ and 100kΩ. R₂ is chosen as shown in Equation 4. Where
VFB = 0.6V and V_O is the output voltage.
R₂ = R_{3V}---^V--_F--^O--_B-- – 1_
(EQ. 4)
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FN8713.2
July 1, 2015