NCV8851-1DBR2G データシートの表示（PDF） - ON Semiconductor

部品番号

コンポーネント説明

メーカー

NCV8851-1DBR2G

Automotive Grade Synchronous Buck Controller

ON Semiconductor 

NCV8851−1

This sequence begins once VIN_IC surpasses its UVLO

threshold when the part is enabled and the LDO output has

risen. After an initial delay to assure a clean start−up,

switching begins, the output initially rises quickly and then

rises monotonically. The duty cycle is gradually increased

until VOUT has reached its set point or until maximum duty

cycle is reached.

Normal Shutdown Behavior and Sleep Mode

Normal shutdown occurs when the IC stops switching

because the input supply drops below the UVLO threshold,

the part enters TSD or the part is disabled. When disabled,

the part enters sleep mode.

In sleep mode, the LDO turns off and its output capacitor

discharges, causing switching to stop, the internal soft−start

capacitor to discharge and GH and GL to go low. The switch

node enters a high impedance state and the output inductor

and capacitors discharge through the load. The supply

current reduces to the sleep mode quiescent current.

Internal Linear Regulator (LDO)

The NCV8851−1 has an onboard low−dropout linear

regulator (LDO) internally connected to drive the low−side

gate. The 6VOUT pin should be externally connected to the

VIN_IC pin to power the internal rails. Typically, a RC filter

is used from 6VOUT to VIN_IC to further decrease noise on

the internal rails.

The 6VOUT pin should be externally connected through a

low leakage (< 100 mA at Tmax) diode to the BST pin,

charging the BST capacitor during off−time to generate a

voltage for the high−side driver. When the part is enabled

and VIN is below the LDO regulated value, the LDO is in

dropout and it tracks VIN. The LDO regulates its output once

VIN is above the output set point plus the dropout voltage.

An external bypass capacitor must be connected from

6VOUT to ground. A short to ground or overcurrent

condition on the 6VOUT pin will be mitigated by the LDO

current limit and internal thermal shutdown (TSD) circuitry

which disables all outputs. A normal soft−start will occur

when the die temperature falls below the TSD threshold.

Drivers

The NCV8851−1 includes 1.5 A gate drivers to switch

external N−Channel MOSFETs. This allows the

NCV8851−1 to address high−power, as well as low−power

conversion requirements. The gate drivers also include

adaptive non−overlap circuitry. The non−overlap circuitry

increases efficiency, which minimizes power dissipation, by

minimizing the body diode conduction time, while

protecting against cross−conduction (shoot−through) of the

MOSFETs. A detailed block diagram of the non−overlap

and gate drive circuitry used in the chip and related external

components is shown in Figure 23.

MainFault

GL to GH

Delay

PWM

Output

GH to GL

Delay

Main

Fault

GL

Threshold

VSW

Threshold

BST

GH

VSW

6VOUT

GL

PGND

VSW

Figure 23. Gate Driver Block Diagram

A capacitor is placed from VSW to BST and a diode is

placed from 6VOUT to BST to create a bootstrap supply on

the BST pin for the high−side floating gate driver. This

ensures that the voltage on BST is about 6VOUT higher than

VSW, less a diode drop, yielding a gate drive voltage high

enough to enhance the high−side MOSFET. The BST

capacitor supplies the charge used by the gate driver to

charge up the input capacitance of the high−side MOSFET,

and is typically chosen to be at least a decade larger than this

capacitance. A 0.1 mF BST capacitor is recommended.

Since the BST capacitor only recharges when the

low−side MOSFET is on, pulling VSW down to ground, the

NCV8851−1 has a minimum off−time. This also means that

the BST capacitor cannot be arbitrarily large, since 6VOUT

needs to be able to charge it up during this minimum

off−time so the high−side gate driver doesn’t run out of

headroom. 6VOUT needs to supply charge both to the BST

capacitor and also the low−side driver, so the LDO capacitor

must be sufficiently larger than the BST capacitor. A 1 mF

LDO capacitor is recommended.

www.onsemi.com

10

Share Link: