Detailed Operating Description

The LM22677 switching regulator features all of the functions

necessary to implement an efficient high voltage buck regu-

lator using a minimum of external components. This easy to

use regulator integrates a 42V N-Channel switch with an out-

put current capability of 5A. The regulator control method is

based on voltage mode control with input voltage feed for-

ward. The loop compensation is integrated into the LM22677

so that no external compensation components need to be se-

lected or utilized. Voltage mode control offers short minimum

on-times allowing short duty-cycles necessary in high input

voltage applications. The operating frequency is fixed at

500kHz to allow for small external components while avoiding

excessive switching losses. The switching frequency can be

adjusted with an external resistor from 200 kHz to 1 MHz or

it can be synchronized to an external clock up to 1 MHz. The

output voltage can be set as low as 1.285V with the -ADJ

device. Fault protection features include current limiting, ther-

mal shutdown and remote shutdown capability. The device is

available in the TO-263 THIN package featuring an exposed

pad to aid thermal dissipation.

The functional block diagram with typical application of the

LM22677 are shown in Figure 1.

The internal compensation of the -ADJ option of the LM22677

is optimized for output voltages up to 5V. If an output voltage

of 5V or higher is needed, the -5.0 fixed output voltage option

with an additional external resistive feedback voltage divider

may also be used.

Precision Enable

The precision enable pin (EN) can be used to shut down the

power supply. Connecting this pin to ground or to a voltage

less than typical 1.6V will completely turn off the regulator.

The current drain from the input supply when off is typically

25 µA with 12V input voltage. The power consumed during

this off state is mostly defined by an internal 2 M

Ω resistor to

VIN. The enable pin has an internal pull-up current source of

approximately 6 µA. When driving the enable pin, the high

voltage level for the on condition should not exceed the 6V

absolute maximum limit. When enable control is not required,

the EN pin should be left floating. The precision feature en-

ables simple sequencing of multiple power supplies with a

resistor divider from another power supply.

The EN pin can also be used as an external UVLO to disable

the part when input voltage falls below a lower boundary of

operation. This is often used to prevent excessive battery dis-

charge. It can also be used to prevent early turn-on as Vin is

rising which can cause undesirable on-off toggling if Vin

droops below 4.5V during startup. Using EN as en external

UVLO is also recommended to prevent abnormal device op-

eration in applications where the input voltage falls below the

minimum operating voltage of 4.5V, during power down for

example.

Maximum Duty-Cycle / Dropout

Voltage

The typical maximum duty-cycle is 90% at 500 kHz switching

frequency. This corresponds to a typical minimum off-time of

200 ns. When operating at switching frequencies higher than

500 kHz, the 200 ns minimum off-time results in a lower max-

imum duty-cycle limit than 90%. This forced off-time is impor-

tant to provide enough time for the Cboot capacitor to charge

during each cycle.

The lowest input voltage required to maintain operation is:

Where V

Schottky diode and V

power N-FET of the LM22677. The R

specified in the electrical characteristics section of this

datasheet to calculate V

the switching frequency.

Minimum Duty-Cycle

Besides a minimum off-time, there is also a minimum on-time

which will take effect when the output voltage is adjusted very

low and the input voltage is very high. Should the operation

require an on-time shorter than minimum, individual switching

pulses will be skipped.

Pulse skipping is a normal mode of operation which appears

as a decrease in switching frequency. It has no effect on op-

eration or regulation except for an increase in output ripple

voltage. The pulse skipping function is required to maintain

proper regulation and overcurrent protection under the full

range of operating conditions.

The specified typical minimum on time of 100 ns is based on

the blanking time during current limit operation. During normal

operation, the minimum on-time will also include the effect of

propagation delay. Assume approximately 150 ns as a typical

operating minimum on time.

where D is the duty-cycle.

Current Limit

When the power switch turns on, the slight capacitance load-

ing of the Schottky diode, D1, causes a leading-edge current

spike with an extended ringing period. This spike can cause

the current limit comparator to trip prematurely. A leading

edge blanking time (T

sampling the spike.

When the switch current reaches the current limit threshold

the switch is immediately turned off. If T

minimum (100 ns typical) the switcher will hold the output

current flat at the set current limit value. But if T

decreases to the minimum T

frequency decreases to 1/5 the typical frequency. This effec-

tively causes the output current to fold back to a lower and

safe value. When the current limit condition is removed the

switching frequency is restored to nominal. This 5X frequency

fold back will result in a lower duty cycle pulse of the power

switch to minimize the overall fault condition power dissipa-

tion.

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