ISL78213_15 Datasheet, PDF (14/16 Pages) Intersil Corporation – 3A Low Quiescent Current, High Efficiency Synchronous Buck Regulator

ISL78213
PWM
PFM
CLOCK
I_L
0
8 CYCLES
NOMINAL +1.5%
PFM CURRENT LIMIT
LOAD CURRENTT
V_OUT
NOMINAL
FIGURE 37. SKIP MODE OPERATION WAVEFORMS
PG
During power-up, the open-drain power-good output holds low for
about 1ms after V_OUT reaches the regulation voltage. The PG
output also serves as a 1ms delayed power-good signal when the
pull-up resistor R₁ is installed.
UVLO
When the input voltage is below the undervoltage lock out (UVLO)
threshold, the regulator is disabled.
Soft Start-up
The soft start-up reduces the inrush current during the start-up. The
soft-start block outputs a ramp reference to the input of the error
amplifier. This voltage ramp limits the inductor current as well as the
output voltage speed so that the output voltage rises in a controlled
fashion. When VFB is less than 0.2V at the beginning of the soft-start,
the switching frequency is reduced to 1/3 of the nominal value so that
the output can start up smoothly at light load condition. During
soft-start, the IC operates in the Skip mode to support prebiased
output condition.
Enable
The enable (EN) input allows the user to control turning on or off
the regulator for purposes such as power-up sequencing. 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 or the V_IN UVLO is
set, the outputs discharge to GND through an internal 100Ω
switch.
Power MOSFETs
The power MOSFETs are optimized for best efficiency. The
ON-resistance for the P-MOSFET is typically 50mΩ and the
ON-resistance for the N-MOSFET is typically 50mΩ.
100% Duty Cycle
The ISL78213 features 100% duty cycle operation to maximize
the battery life. When the battery voltage drops to a level that the
ISL78213 can no longer maintain the regulation at the output,
the regulator completely turns on the P-MOSFET. The maximum
dropout voltage under the 100% duty-cycle operation is the
product of the load current and the ON-resistance of the
P-MOSFET.
Thermal Shutdown
The ISL78213 has built-in thermal protection. When the internal
temperature reaches +140°C, the regulator is completely shut
down. As the temperature drops to +115°C, the ISL78213
resumes operation by stepping through the soft-start.
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operations, ISL78213
typically uses a 1.5µH output inductor. The higher or lower
inductor value can be used to optimize the total converter system
performance. For example, for higher output voltage 3.3V
applications, 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
approximately 30% of the maximum output current for optimized
performance. The inductor ripple current can be expressed as
shown in Equation 1:
I = ^V-----^O-----^---_L^{}---¹--_---_f^–--_S---^V-V--------^I--^O--^N-----^{}--
(EQ. 1)
The inductor’s saturation current rating needs to be at least
larger than the peak current. The ISL78213 protects the typical
peak current 4.8A. The saturation current needs be over 5.5A for
maximum output current application.
ISL78213 uses an internal compensation network and the
output capacitor value is dependent on the output voltage. The
ceramic capacitor is recommended to be X5R or X7R. The
recommended X5R or X7R minimum output capacitor values are
shown in Table 1. In Table 1, the minimum output capacitor value
is given for the different output voltage to make sure that the
whole converter system is stable. Additional output capacitance
should be added for better performances 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.
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FN7550.3
May 12, 2015