LTC1266CS datasheet(12/20 Pages) LINER

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LTC1266

LTC1266-3.3/LTC1266-5

APPLICATIO S I FOR ATIO

Driving N-Channel Topside MOSFETs

Driving an N-channel topside MOSFET (PINV, Pin 3, tied to

PWR VIN) is a little trickier than driving a P-channel since

the gate voltage must be positive with respect to the

source to turn it on, which means that the gate voltage

must be higher than VIN. This requires either a second

supply at least VGS(ON) above VIN or a bootstrapping circuit

to boost the VIN to the proper level. The easiest method is

using a higher supply (see Figure 14) but if one is not

available, the bootstrap method can be used at the ex-

pense of an additional diode (see Figure 1). The bootstrap

works by charging the bootstrap capacitor to VIN during

the off-time. During the on-time, the bottom plate of the

capacitor is pulled up to VIN so that the voltage at Pin 2 is

now twice VIN (plus any ringing on the switch node).

Since the maximum allowable voltage at Pin 2 is 20V, the

Figure 1 bootstrap circuit limits VIN to less than 10V. A

higher VIN can be achieved if the bootstrap capacitor is

charged to a voltage less than VIN, in which case

VIN(MAX) = 20 – VCAP.

N-channel mode, internal circuitry limits the maximum

on-time to 60

µs to guarantee start-up of the bootstrap

circuit. This maximum on-time reduces the maximum

duty cycle to:

Max Duty Cycle =

µs

µs + tOFF

which slightly increases the minimum input voltage at

which dropout occurs. However, because of the superior

on-conductance of the N-channel, the dropout perfor-

mance of an all N-channel regulator is still better (see

Figure 5) even with the duty cycle limitation, except at light

loads.

Low-Battery Comparator

The LTC1266 has an on-chip low-battery comparator

which can be used to sense a low-battery condition when

implemented as shown in Figure 6. The resistor divider

R1, R2 sets the comparator trip point as follows:

VTRIP = 1.25

)

)1+ R2

(VIN – VOUT) VOLTAGE (V)

600

800

1000

LTC1266 • F04

400

200

L = 50

µH

RSENSE = 0.02Ω

L = 25

µH

RSENSE = 0.02Ω

L = 50

µH

RSENSE = 0.05Ω

Figure 4. Minimum Value of COUT

series would normally be in continuous operation. The

output remains in regulation at all times. This minimum

capacitance requirement may be relaxed if Burst Mode

operation is disabled.

N-Channel vs P-Channel MOSFETs

The LTC1266 has the capability to drive either an

N-channel or a P-channel topside switch to give the user

more flexibility. N-channel MOSFETs are superior in per-

formance to P-channel due to their lower RDS(ON) and

lower gate capacitance and are typically less expensive;

however, they do have a slightly more complicated gate

drive requirement and a more limited input voltage range

(see following sections).

Driving P-Channel Topside MOSFETs

The P-channel topside switch circuit configuration is the

most straightforward due to the requirement of only one

supply voltage level. This is due to the negative gate

threshold of the P-channel MOSFET which allows the

MOSFET to be switched on and off by swinging the gate

between VIN and ground. The phase invert (Pin 3) is tied

to ground to choose this operating mode. Normally, the

converter input (VIN) is connected to the LTC1266 supply

Pins 2 and 5 and can go as high as 20V. Pin 2 supplies the

high frequency current pulses to switch the MOSFETs and

should be decoupled with a 0.1

µF to 1µF ceramic capaci-

tor. Pin 5 supplies most of the quiescent power to the rest

of the chip.

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