PIFE32251B-R68MS datasheet(131/157 Pages) TI1

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IOCL

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131

TPS650830

www.ti.com

SLVSCF4A – DECEMBER 2014 – REVISED JULY 2016

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Product Folder Links: TPS650830

Application and Implementation

7.2.2.1.4 Bootstrap Capacitor

To make sure that the internal high side gate drivers are supplied with a stable low noise supply voltage, a

capacitor must be connected between the VBSTVRx pins and the respective SWVRx pins. Using ceramic

capacitors with the value of 0.1 µF are recommended for the converters and the controllers, respectfully.

For testing, a 0.1-µF, size 0402, 10-V capacitor was used for the controllers.

It is recommended to reserve a small resistor in series with the bootstrap capacitor in case the turn on / off

of the FETs need to be slowed in order to reduce voltage ringing on the switch node. This is common

practice for controller design.

7.2.2.1.5 Setting the Current Limits

The controller has a Valley Current Limit topology, also known as a Low Side Current Limit. This type of

current limit works by limiting the current only when the low side FET is on. If the current being sourced by

the low side FET is greater than the set low side current limit, ILS, the controller will hold the low side FET

on and the high side off until the current through the low side FET decreases below the set ILS. Only if the

current through the low side FET is less than the ILS will the low side FET be allowed to turn off and the

high side FET to turn on.

A fast current increase is limited by the maximum on time for the high side FET. This forces the low side

FET to turn on every period. Once the low side FET turns on, the Low Side Current Limit can control the

FETs until the current decreases below the ILS. The maximum on time for the high side FET limits the

current increase to maximum on time multiplied by the di/dt of the inductor until the low side FET is

switched on.

IOCL is the average current when the valley current is consistently the ILS.

Figure 7-3. IOCL Depiction

The low side current limit for the controllers is set by a resistor, RCS, at the ILIMx pin. A current, ITRIP, is

sourced across the RCS to set the voltage for the current limit comparator. Use the equation below to

determine the RCS resistor. It is recommended to set IOCL to 130% of IOUTmax and use a resistor with ±1%

or less tolerance for best results. Since the current limit is when the inductor current is near its maximum it

is recommended to use the saturation derating of the inductor when calculating the RCS.

(3)

There is a minimum and a maximum IOCL that can be achieved for the given parameters used in the

equation above. To ensure that the RCS has been sized correctly, the following equation must be true

across the application temperature range.

(4)

PIFE32251B-R68MS 데이터시트(PDF) 131 Page - Texas Instruments

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