ISL6622A Datasheet, PDF (7/11 Pages) Intersil Corporation – VR11.1 Compatible Synchronous Rectified Buck MOSFET Drivers

ISL6622A
protection to the load if the upper MOSFET(s) is or becomes
shorted. If the PHASE node goes higher than the gate
threshold of the lower MOSFET, it results in the progressive
turn-on of the device and the effective clamping of the PHASE
node’s rise. The actual PHASE node clamping level depends
on the lower MOSFET’s electrical characteristics, as well as the
characteristics of the input supply and the path connecting it to
the respective PHASE node.
Internal Bootstrap Device
The ISL6622A features an internal bootstrap Schottky diode.
Simply adding an external capacitor across the BOOT and
PHASE pins completes the bootstrap circuit. The bootstrap
function is also designed to prevent the bootstrap capacitor
from overcharging due to the large negative swing at the
trailing-edge of the PHASE node. This reduces voltage
stress on the BOOT to PHASE pins.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2 _20nC
Q_GATE = 100nC
50nC
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
ΔV_{BOOT_CAP} (V)
FIGURE 2. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE
VOLTAGE
The bootstrap capacitor must have a maximum voltage
rating well above the maximum voltage intended for UVCC.
Its minimum capacitance value can be chosen from
Equation 1.
C _{B O O T _CAP}
≥
----------^Q-----^G----^A----^T---^E-----------
Δ V_{B O O T _CAP}
(EQ. 1)
Q_GATE=
Q_{-----G-----1----}•_-----U_----V_-----C_-----C_--
V_GS1
•
N_Q1
where Q_G1 is the amount of gate charge per upper MOSFET
at V_GS1 gate-source voltage and N_Q1 is the number of
control MOSFETs. The ΔV_{BOOT_CAP} term is defined as the
allowable droop in the rail of the upper gate drive. Select
results are exemplified in Figure 2.
Gate Drive Voltage Versatility
The ISL6622A provides the user flexibility in choosing the
gate drive voltage for efficiency optimization. The ISL6622A
upper gate drive is fixed to VCC [+12V] in the SOIC, but the
lower drive rail can be driven from 5V to 12V using the LVCC
pin. In the DFN package, a separate UVCC pin is available
for the upper gate drive voltage to be driven from 5V to 12V
for efficiency optimization, while the lower gate can be driven
independently using the LVCC pin from 5V to 12V.
Diode Emulation
Diode emulation allows for higher converter efficiency under
light-load situations. With diode emulation active, the
ISL6622A detects the zero current crossing of the output
inductor and turns off LGATE. This prevents the low side
MOSFET from sinking current and ensures that
discontinuous conduction mode (DCM) is achieved. The
LGATE has a minimum on-time of 350ns in DCM mode.
Power Dissipation
Package power dissipation is mainly a function of the
switching frequency (F_SW), the output drive impedance, the
external gate resistance, and the selected MOSFET’s internal
gate resistance and total gate charge. Calculating the power
dissipation in the driver for a desired application is critical to
ensure safe operation. Exceeding the maximum allowable
power dissipation level may push the IC beyond the maximum
recommended operating junction temperature. The DFN
package is more suitable for high frequency applications. See
“Layout Considerations” on page 8 for thermal transfer
improvement suggestions. When designing the driver into an
application, it is recommended that the following calculation is
used to ensure safe operation at the desired frequency for the
selected MOSFETs. The total gate drive power losses due to
the gate charge of MOSFETs and the driver’s internal circuitry
and their corresponding average driver current can be
estimated with Equations 2 and 3, respectively:
P_{Qg_TOT} = P_{Qg_Q1} + P_{Qg_Q2} + I_Q • VCC
(EQ. 2)
P_{Q g _Q1}
=
Q_{-----G-----1----}•_-----U_----V_-----C_-----C_----²_--
V_GS1
•
F_S
W
•
N_Q
1
P _{Q g _Q2}
=
Q_{-----G-----2----}•_-----L_---V_-----C_-----C_----²_--
V_GS2
•
F_SW
•
N_Q2
I_DR
=
⎛
⎜
⎝
Q_{-----G-----1----}•_-----U_----V_-----C_-----C_-----•_-----N_----Q-----1-
V_GS1
+
-^Q----^G-----²----^•-----^LV---^V--_G---^C-_S----^C₂-----^•-----^N----^Q-----²-_{⎠⎟⎞}
• F_SW + I_Q
(EQ. 3)
where the gate charge (Q_G1 and Q_G2) is defined at a
particular gate to source voltage (V_GS1and V_GS2) in the
corresponding MOSFET data sheet; I_Q is the driver’s total
quiescent current with no load at both drive outputs; N_Q1
and N_Q2 are number of upper and lower MOSFETs,
respectively; UVCC and LVCC are the drive voltages for
both upper and lower FETs, respectively. The I_Q*VCC
product is the quiescent power of the driver without
capacitive load.
7
FN6601.2
March 19, 2009