L6738ATR STMicroelectronics, L6738ATR Datasheet - Page 20

• Manufacturer Part Number: L6738ATR
• Description: IC PWM CTLR 1PH STPDN 16-VFQFPN
• Manufacturer: STMicroelectronics

Specifications of L6738ATR

• Frequency - Max: 220kHz
• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Duty Cycle: 100%
• Voltage - Supply: 5 V ~ 12 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 32-VFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-11012-2

High Current embedded drivers
8
8.1
8.2
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High Current embedded drivers
The L6738A provides high-current driving control. The driver for the high-side MOSFET
uses the BOOT pin for supply and the PHASE pin for return. The driver for the low-side
MOSFET uses the VCCDR pin for supply and the GND pin for return.
The embedded driver embodies an anti-shoot-through and adaptive dead-time control to
minimize the low-side body diode conduction time maintaining good efficiency and saving
the use of Schottky diodes: when the high-side MOSFET turns off, the voltage on its source
begins to fall; when the voltage reaches about 2 V, the low-side MOSFET gate drive voltage
is suddenly applied. When the low-side MOSFET turns off, the voltage at the LGATE pin is
sensed. When it drops below about 1 V, the high-side MOSFET gate drive voltage is
suddenly applied. If the current flowing in the inductor is negative, the source of the high-
side MOSFET never drops. To allow the low-side MOSFET to turn on even in this case, a
watchdog controller is enabled: if the source of the high-side MOSFET doesn't drop, the low-
side MOSFET is switched on, so allowing the negative current of the inductor to recirculate.
This mechanism allows the system to regulate even if the current is negative.
Boot capacitor design
The bootstrap capacitor needs to be designed in order to show a negligible discharge due to
the high-side MOSFET turn on. In fact, it must give a stable voltage supply to the high-side
driver during the MOSFET turn on, also minimizing the power dissipated by the embedded
boot diode.
bootstrap according to the desired discharge and depending on the selected MOSFET.
To prevent the bootstrap capacitor to extra-charge as a consequence of large negative
spikes, an internal 2.2 Ohms series resistance R
diode pin.
Figure 8.
Power dissipation
It is important to consider the power that the device is going to dissipate in driving the exter-
nal MOSFETs in order to avoid surpassing the maximum junction operative temperature.
2.5
2.0
1.5
1.0
0.5
0.0
0
10
Cboot = 47nF
Cboot = 100nF
Cboot = 220nF
Cboot = 330nF
Cboot = 470nF
20
Figure 8
Bootstrap capacitor design
High -Side MOSFET Gate Charge [nC]
30
40
gives some guidelines on how to select the capacitance value for the
50
60
Doc ID 18134 Rev 1
70
80
90
100
2500
2000
1500
1000
500
0
0.0
BOOT
is provided in series to the BOOT
0.2
Boot Cap Delta Voltage [V]
0.4
0.6
Qg = 10nC
Qg = 25nC
Qg = 50nC
Qg = 100nC
0.8
L6738A
1.0