MIC2876 Datasheet, PDF (13/26 Pages) Microchip Technology – 4.8A ISW, Synchronous Boost Regulator with Bi-Directional Load Disconnect

5.0 APPLICATION INFORMATION
5.1 General Description
The MIC2876 is a 2 MHz, current-mode, PWM,
synchronous boost converter with an operating input
voltage range of 2.5V to 5.5V. At light load, the
converter enters pulse-skipping mode to maintain high
efficiency over a wide range of load current. The
maximum peak current in the boost switch is limited to
4.8A (typical).
5.2 Bi-Directional Output Disconnect
The power stage of the MIC2876 consists of a NMOS
transistor as the main switch and a PMOS transistor as
the synchronous rectifier. A control circuit turns off the
back gate diode of the PMOS to isolate the output from
the input supply when the chip is disabled (V_EN = 0V).
An “always on” maximum supply selector switches the
cathode of the backgate diode to either the IN or the
OUT (whichever of the two has the higher voltage). As
a result, the output of the MIC2876 is bi-directionally
isolated from the input as long as the device is
disabled. The maximum supply selector and hence the
output disconnect function requires only 0.3V at the IN
pin to operate.
5.3 Integrated Anti-Ringing Switch
The MIC2876 includes an anti-ringing switch that
eliminates the ringing on the SW node of a
conventional boost converter operating in the
discontinuous conduction mode (DCM). At the end of a
switching cycle during DCM operation, both the NMOS
and PMOS are turned off. The anti-ringing switch in the
MIC2876 clamps the SW pin voltage to IN to dissipate
the remaining energy stored in the inductor and the
parasitic elements of the power switches.
5.4 Automatic Bypass Mode
The MIC2876 automatically operates in bypass mode
when the input voltage is higher than the target output
voltage. In bypass mode, the NMOS is turned off while
the PMOS is fully turned on to provide a very low
impedance path from IN to OUT.
5.5 Soft-Start
The MIC2876 integrates an internal soft-start circuit to
limit the inrush current during start-up. When the device
is enabled, the PMOS is turned-on slowly to charge the
output capacitor to a voltage close to the input voltage.
Then, the device begins boost switching cycles to
gradually charge up the output voltage to the target
V_OUT.
MIC2876
5.6 Output Voltage Programming
The MIC2876 has an adjustable version that allows the
output voltage to be set by an external resistor divider
R2 and R3. The typical feedback voltage is 900 mV, the
recommended maximum and minimum output voltage
is 5.5V and 3.2V, respectively. The current through the
resistor divider should be significantly larger than the
current into the FB pin (typically 0.01 µA). It is
recommended that 0.1% tolerance feedback resistors
must be used and the total resistance of R2 + R3
should be around 1 MΩ. The appropriate R2 and R3
values for the desired output voltage are calculated as
in Equation 5-1:
EQUATION 5-1:
R2
=
R3



V_{----O----U----T-}
0.9V
–
1_
Example 1:
With a V_OUT of 3.3V and an R3 value of 281.2 kΩ
(standard value is 280 kΩ), R2 calculates out to
750 kΩ.
Example 2:
With a V_OUT of 5V and an R3 value of 200 kΩ, R2
calculates out to 911.1 kΩ (standard value is 910 kΩ).
5.7 Current Limit Protection
The MIC2876 has a current limit feature to protect the
part against heavy load conditions. When the current
limit comparator determines that the NMOS switch has
a peak current higher than 4.8A, the NMOS is turned off
and the PMOS is turned on until the next switching
cycle. The current limit protection is reset cycle by
cycle.
 2016 Microchip Technology Inc.
DS20005572A-page 13