LTC4371 datasheet(9/22 Pages) LINER

9 / 22 page

LTC4371

4371f

For more information www.linear.com/LTC4371

applicaTions inForMaTion

Figure 4. MOSFET Cascode for High Voltage > 250V

Applications with 5mA Gate Pull-Up Current

Figure 3. VDD Connected to VZ with NPN for Repetitive

5mA Gate Pull-Up Current

LTC4371

VDD

VSS

0.1F

RTN

VOUT

4371 F03

4371 F04

LTC4371

VDD

VSS

0.1F

RZ1

RZ2 RG

RTN

VOUT

2N3904

BSP125 (600V)

pulses from the 5mA fast pull-up, applications where the

input operating voltage exceeds 72V and applications with

a wide range of input voltage, particularly those where the

minimum input voltage approaches the operating voltage

of the LTC4371. A wide input voltage range may also result

in a situation where the maximum VZ current calculated

in Equation 3 exceeds 10mA. For these cases an NPN

transistor can be used to buffer the shunt regulator and

power VDD, as shown in Figure 3. Equation 1 becomes:

RZ <

VIN(MIN) – VZ(MIN)

50µA +

IDD(MAX)

(4)

RZ (or RZ2) may be split into multiple segments in order

to achieve the desired standoff voltage or dissipation.

Whereas 1206 size resistors are commonly rated for 200V

working and 400V peak, pad spacing and circuit board de-

sign rules may limit the working rating to as little as 100V.

In Figure 3, the voltage drop and power dissipation of Q1

may be augmented by the use of one or more resistors

in series with the collector. The same applies for M1 in

Figure 4.

For all applications RZ (or RZ1 + RZ2) must limit the maxi-

mum VZ current to less than 10mA, as calculated using

Equation 3. If voltage transients are anticipated, VIN(MAX)

becomes the peak transient voltage. Transient require-

ments may force the use of Figure 3 or Figure 4 instead

of Figure 2. The peak VZ current may also be reduced

to less than 10mA by filtering, e.g. split RZ (or RZ2) into

two equal parts and connect a bypass capacitor from the

central node to VSS.

Strong Gate Pull-Up

For fast turn-on, a strong 5mA driver pulls up on the gate

when the MOSFET forward drop (∆VSD) is large. In simple

shunt-regulated applications such as shown in Figure 2,

the bias resistor RZ may be incapable of supplying 5mA.

In this case, a 2.2µF bypass capacitor is required to

momentarily provide the strong pull-up current to fully

charge the MOSFET gate. In normal operation the 5mA

drive is not a DC condition, as it flows only long enough

to deliver gate charge to the MOSFET. The amount of

where 50μA represents the minimum VZ shunt regulator

operating current and β is Q1’s DC current gain.

The maximum power dissipation in RZ and the maximum

VZcurrent are calculated from Equations 2 and 3. Dissipa-

tion in emitter follower Q1 is given by:

PD(Q1) =(VIN(MAX) +VBE – VZ(MIN))•IDD(MAX)

(5)

In buffered applications, bypass VZ with a 100nF capacitor

to VSS. Bypassing VDD is unnecessary.

For applications at very high voltages, beyond 300V, small

high voltage MOSFETs are more readily available than

bipolar devices and the circuit of Figure 4 is preferred.

RZ, calculated using Equation 4, is split into two parts,

RZ1 and RZ2. RZ1 is sized to produce a 3V drop when

operating at VIN(MIN).

LTC4371 Datasheet(PDF) 9 Page - Linear Technology

LTC4371 Datasheet(HTML) 9 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL