N - Channel Buck Switch and

Driver

The LM3100 integrates an N-Channel buck (high-side)

switch and associated floating high voltage gate driver. The

gate drive circuit works in conjunction with an external boot-

strap capacitor and an internal high voltage diode. A 33 nF

capacitor (C

vides voltage to the high-side driver during the buck switch

on-time. During each off-time, the SW pin falls to approxi-

mately -1V and C

the internal diode. The minimum off-time of 260ns ensures

adequate time each cycle to recharge the bootstrap capaci-

tor.

Softstart

The soft-start feature allows the converter to gradually reach

a steady state operating point, thereby reducing start-up

stresses and current surges. Upon turn-on, after V

CC

reaches the under-voltage threshold, an internal 8µA current

source charges up the external capacitor at the SS pin. The

ramping voltage at SS (and the non-inverting input of the

regulation comparator) ramps up the output voltage in a

controlled manner.

An internal switch grounds the SS pin if V

under-voltage lockout threshold, if a thermal shutdown oc-

curs, or if the EN pin is grounded. Using an externally

controlled switch, the output voltage can be shut off by taking

the SS pin to ground. Releasing the switch allows the SS pin

to ramp up, and the output voltage to return to normal. The

shut-down configuration is shown in Figure 3.

Thermal Protection

The LM3100 should be operated so the junction temperature

does not exceed the maximum limit. An internal Thermal

Shutdown circuit, which activates (typically) at 165˚C, takes

the controller to a low power reset state by disabling the buck

switch and the on-timer, and grounding the Softstart pin. This

feature helps prevent catastrophic failures from accidental

device overheating. When the junction temperature falls

back below 145˚C (typical hysteresis = 20˚C), the Softstart

pin is released and normal operation resumes.

Applications Information

EXTERNAL COMPONENTS

The following guidelines can be used to select the external

components.

R

from:

R

R

tors in the range of 1.0 k

Ω -10kΩ which satisfy the above

ratio.

For V

directly. However, the converter operation needs a minimum

inductor current ripple to maintain good regulation when no

load is connected. This minimum load is about 10 µA and

can be implemented by adding a pre-load resistor to the

output.

R

Equation 1 can be used to select R

is desired as long as the above limitation is met.

L: The main parameter effected by the inductor is the output

current ripple amplitude (I

must be determined at both the minimum and maximum

nominal load currents. At minimum load current, the lower

peak must not reach 0 mA. At maximum load current, the

upper peak must not exceed the current limit threshold

(1.9A). The allowable ripple current is calculated from the

following equations:

I

or

I

The lesser of the two ripple amplitudes calculated above is

then used in the following equation:

(7)

where V

mined from equation 1. This provides a value for L. The next

larger standard value should be used. L should be rated for

the I

Inductor Selector for V

20174736

20174706

FIGURE 3. Alternate Shutdown Implementation

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