LM25018 datasheet(13/18 Pages) TI

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VIN

BST

RON

RTN

VCC

UVLO

VIN

VOUT1

VOUT2

RFB1

RUV1

RON

COUT1

CBST

CIN

COUT2

RFB2

RUV2

Rr NP

LM25018

CVCC

20V-48V

Cac

VOUT2 = VOUT1 x

- VF

LM25018

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SNVS953 – DECEMBER 2012

ISOLATED DC-DC CONVERTER USING LM25018

An isolated supply using LM25018 is shown in Figure 17 below. Inductor (L) in a typical buck circuit is replaced

with a coupled inductor (X1). A diode (D1) is used to rectify the voltage on a secondary output. The nominal

voltage at the secondary output (VOUT2) is given by:

(15)

where VF is the forward voltage drop of D1, and NP, NS are the number of turns on the primary and secondary

of coupled inductor X1. For output voltage (VOUT1) above the maximum VCC (8.55V), the VCC pin can be diode

connected to VOUT1 for higher efficiency and low dissipation in the IC. See AN-2292 for a complete isolated bias

design.

Figure 17. Typical Isolated Application Schematic

RIPPLE CONFIGURATION

LM25018 uses Constant-On-Time (COT) control scheme, in which the on-time is terminated by an on-timer, and

the off-time is terminated by the feedback voltage (VFB) falling below the reference voltage (VREF). Therefore, for

stable operation, the feedback voltage must decrease monotonically, in phase with the inductor current during

the off-time. Furthermore, this change in feedback voltage (VFB) during off-time must be large enough to

suppress any noise component present at the feedback node.

Table 1 shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1

and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output

voltage ripple has two components:

1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor.

2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor.

The capacitive ripple is not in phase with the inductor current. As a result, the capacitive ripple does not

decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and

decreases monotonically during the off-time. The resistive ripple must exceed the capacitive ripple at the output

node (VOUT) for stable operation. If this condition is not satisfied unstable switching behavior is observed in COT

converters, with multiple on-time bursts in close succession followed by a long off-time.

Type 3 ripple method uses Rr and Cr and the switch node (SW) voltage to generate a triangular ramp. This

triangular ramp is ac coupled using Cac to the feedback node (FB). Since this circuit does not use the output

voltage ripple, it is ideally suited for applications where low output voltage ripple is required. See application note

AN-1481 for more details for each ripple generation method.

Type 1

Type 2

Type 3

Lowest Cost Configuration

Reduced Ripple Configuration

Minimum Ripple Configuration

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