LT1676C View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1676C

Wide Input Range, High Efficiency, Step-Down Switching Regulator

Linear Technology 

LT1676

APPLICATIONS INFORMATION

which tantalum capacitors are generally unavailable. Rela-

tively bulky “high frequency” aluminum electrolytic types,

specifically constructed and rated for switching supply

applications, may be the only choice.

Minimum Load Considerations

As discussed previously, a lightly loaded LT1676 with VC

pin control voltage below the boost threshold will operate

in low dV/dt mode. This affords greater controllability at

light loads, as minimum tON requirements are relaxed. In

many applications, it is possible to operate the LT1676

down to zero external load without “pulse skipping”!

In these cases, the LT1676’s modest VCC current

requirement of several milliamperes provides enough of a

load to avoid pulse skipping.

However, some users may be indifferent to pulse skipping

behavior, but instead may be concerned with maintaining

maximum possible efficiency at light loads. This require-

ment can be satisfied by forcing the part into Burst ModeTM

operation. The use of an external comparator whose

output controls the shutdown pin allows high efficiency at

light loads through Burst Mode operation behavior (see

Typical Applications and Figure 8).

Maximum Load/Short-Circuit Considerations

The LT1676 is a current mode controller. It uses the VC

node voltage as an input to a current comparator which

turns off the output switch on a cycle-by-cycle basis as

this peak current is reached. The internal clamp on the VC

node, nominally 2V, then acts as an output switch peak

current limit. This action becomes the switch current limit

specification. The maximum available output power is

then determined by the switch current limit.

A potential controllability problem could occur under

short-circuit conditions. If the power supply output is

short circuited, the feedback amplifier responds to the low

output voltage by raising the control voltage, VC, to its

peak current limit value. Ideally, the output switch would

be turned on, and then turned off as its current exceeded

the value indicated by VC. However, there is finite response

time involved in both the current comparator and turnoff

of the output switch. These result in a minimum on time

Burst Mode is a trademark of Linear Technology Corporation.

tON(MIN). When combined with the large ratio of VIN to

(VF + I • R), the diode forward voltage plus inductor I • R

voltage drop, the potential exists for a loss of control.

Expressed mathematically the requirement to maintain

control is:

f

•

tON

≤

VF

+I•R

VIN

where:

f = switching frequency

tON = switch ON time

VF = diode forward voltage

VIN = Input voltage

I • R = inductor I • R voltage drop

If this condition is not observed, the current will not be

limited at IPK, but will cycle-by-cycle ratchet up to some

higher value. Using the nominal LT1676 clock frequency

of 100KHz, a VIN of 48V and a (VF + I • R) of say 0.7V, the

maximum tON to maintain control would be approximately

140ns, an unacceptably short time.

The solution to this dilemma is to slow down the oscillator

when the FB pin voltage is abnormally low thereby indicat-

ing some sort of short-circuit condition. Figure 2 shows

the typical response of Oscillator Frequency vs FB divider

Thevenin voltage and impedance. Oscillator frequency is

unaffected until FB voltage drops to about 2/3 of its normal

value. Below this point the oscillator frequency decreases

roughly linearly down to a limit of about 25kHz. This lower

120

100

RTH = 22k

80

RTH = 10k

RTH = 4.7k

60

40

RTH LT1676

FB

20

0

0 0.25 0.50 0.75 1.00 1.25

FB DIVIDER THEVENIN VOLTAGE (V)

1676 F02

Figure 2. Oscillator Frequency vs FB Divider

Thevenin Voltage and Impedance

9

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