DS6203B Datasheet, PDF (20/22 Pages) Richtek Technology Corporation – 5A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter with VID Control

RT6203B
output voltage drops below 87.5% of its nominal voltage,
PGOOD will be pulled low. It will be held low until the
output voltage rises to 92.5% of the nominal voltage. During
soft-start and in shutdown mode (EN pin pull low), PGOOD
is actively held low. When the output voltage has reached
90% of its nominal voltage and the soft-start sequence is
finished, PGOOD is high impedance and will be pulled
high by the external pull-up resistor.
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT and SW pins. This capacitor provides the gate driver
voltage for the high-side MOSFET. It is recommended to
add an external bootstrap diode between an external 5V
and BOOT pin for efficiency improvement when input
voltage is lower than 5.5V. The bootstrap diode can be a
low cost one such as IN4148 or BAT54. The external 5V
can be a 5V fixed input from system or a 5V output of the
RT6203B Note that the external boot voltage must be lower
than 5.5V.
External BOOT Capacitor Series Resistor
The internal power MOSFET gate driver is not only
optimized to turn the switch on fast enough to minimize
switching loss, but also slow enough to reduce EMI. Since
the switch rapidly turn-on will induce high di/dt noise which
let EMI issue much worse. During switch turn-off, SW is
discharged relatively slowly by the inductor current during
the dead time between high-side and low-side switch on-
times. In some cases it is desirable to reduce EMI further,
at the expense of some additional power dissipation. The
switch turn-on can be slowed by placing a small (<47Ω)
resistance between BOOT and the external bootstrap
capacitor. This will slow the high-side switch turn-on speed
and V_SW's rise. The recommended external diode
connection is shown in Figure 5, using external diode to
charge the BOOT capacitor, and place a resistor between
BOOT and the capacitor/diode connection to reduce turn-
on speed for any EMI issue consideration.
5V
BOOT
RT6203B
SW
R
0.1µF
Figure 5. External Bootstrap Diode and BOOT Capacitor
Series Resistor
Thermal Considerations
The junction temperature should never exceed the
absolute maximum junction temperature T_J(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
dissipation depends on the thermal resistance of the IC
package, the PCB layout, the rate of surrounding airflow,
and the difference between the junction and ambient
temperatures. The maximum power dissipation can be
calculated using the following formula :
P_D(MAX) = (T_J(MAX) − T_A) / θ_JA
where T_J(MAX) is the maximum junction temperature, T_A is
the ambient temperature, and θ_JA is the junction-to-ambient
thermal resistance.
For continuous operation, the maximum operating junction
temperature indicated under Recommended Operating
Conditions is 125°C. The junction-to-ambient thermal
resistance, θ_JA, is highly package dependent. For a
WQFN-20L 4x4 package, the thermal resistance, θ_JA, is
28°C/W on a standard JEDEC 51-7 high effective-thermal-
conductivity four-layer test board. The maximum power
dissipation at T_A = 25°C can be calculated as below :
P_D(MAX) = (125°C − 25°C) / (28°C/W) = 3.57W for a
WQFN-20L 4x4 package.
The maximum power dissipation depends on the operating
ambient temperature for the fixed T_J(MAX) and the thermal
resistance, θ_JA. The derating curves in Figure 6 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
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DS6203B-00 September 2016