VIN

DR

FA/SD

FB

COMP

PGND

Under Voltage

Lockout

7.2V

LDO

internal Vcc

S

R

Q

DRIVER

Fixed Frequency

Detect

Oscillator

PWM

Isen

AGND

Softstart

logic

325mV

Short Circuit

Comparator

Vfb+Vovp

OVP

1.26V Reference

Gm

internal slope

compensation

THERMAL

LIMIT

(165°C)

slope compensation

ramp adjust current

source

Error

Amplifier

12

LM3478

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Product Folder Links: LM3478

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Copyright © 2000–2017, Texas Instruments Incorporated

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Overvoltage Protection

The LM3478 has over voltage protection (OVP) for the output voltage. OVP is sensed at the feedback pin (pin 3).

If at anytime the voltage at the feedback pin rises to VFB+ VOVP, OVP is triggered. See Electrical Characteristics

section for limits on VFB and VOVP.

OVP will cause the drive pin to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage

will drop. The LM3478 will begin switching again when the feedback voltage reaches VFB + (VOVP - VOVP(HYS)).

See Electrical Characteristics for limits on VOVP(HYS).

OVP can be triggered if the unregulated input voltage crosses 7.2 V, the output voltage will react as shown in

Figure 21. The internal bias of the LM3478 comes from either the internal LDO as shown in the block diagram or

the voltage at the Vin pin is used directly. At Vin voltages lower than 7.2 V the internal IC bias is the Vin voltage

and at voltages above 7.2V the internal LDO of the LM3478 provides the bias. At the switch over threshold at 7.2

V a sudden small change in bias voltage is seen by all the internal blocks of the LM3478. The control voltage

shifts because of the bias change, the PWM comparator tries to keep regulation. To the PWM comparator, the

scenario is identical to a step change in the load current, so the response at the output voltage is the same as

would be observed in a step load change. Hence, the output voltage overshoot here can also trigger OVP. The

LM3478 will regulate in hysteretic mode for several cycles, or may not recover and simply stay in hysteretic

mode until the load current drops or Vin is not crossing the 7.2 V threshold anymore. Note that the output is still

regulated in hysteretic mode.