LTC1702ACGN#TR-SMI Linear Technology, LTC1702ACGN#TR-SMI Datasheet - Page 14

• Manufacturer Part Number: LTC1702ACGN#TR-SMI
• Manufacturer: Linear Technology

Specifications of LTC1702ACGN#TR-SMI

• Lead Free Status / RoHS Status: Not Compliant

LTC1702A
APPLICATIONS
LTC1702A remains in continuous mode. See the Inductor
Selection section for a detailed description of ripple
current.
As the output load current decreases in continuous mode,
the average current in the inductor will reach a point where
it drops below half the ripple current. At this point, the
inductor current will reverse during a portion of the
switching cycle, or begin to flow from the output back to
the input. This does not adversely affect regulation, but
does cause additional losses as a portion of the inductor
current flows back and forth through the resistive power
switches, giving away a little more power each time and
lowering the efficiency. There are some benefits to allow-
ing this reverse current flow: the circuit will maintain
regulation even if the load current drops below zero (the
load supplies current to the LTC1702A) and the output
ripple voltage and frequency remain constant at all loads,
easing filtering requirements. Circuits that take advantage
of this behavior can force the LTC1702A to operate in
continuous mode at all loads by tying the FCB (Force
Continuous Bar) pin to ground.
Discontinuous Mode
To minimize the efficiency loss due to reverse current flow
at light loads, the LTC1702A switches to a second mode of
14
I
RIPPLE
Figure 5b. Discontinuous Mode
Figure 5a. Continuous Mode
U
I
RIPPLE
INFORMATION
TIME
U
TIME
W
I
AVERAGE
I
AVERAGE
1702A F05b
1702A F05a
U
operation: discontinuous mode (Figure 5b). In discontinu-
ous mode, the LTC1702A detects when the inductor
current approaches zero and turns off QB for the remain-
der of the switch cycle. During this time, the voltage at the
SW pin will float about V
inductor will be zero, and the inductor current remains
zero until the next switching cycle begins and QT turns on
again. This prevents current from flowing backwards in
QB, eliminating that power loss term. It also reduces the
ripple current in the inductor as the output current ap-
proaches zero.
The LTC1702A detects that the inductor current has reached
zero by monitoring the voltage at the SW pin while QB is
on. Since QB acts like a resistor, SW should ideally be right
at 0V when the inductor current reaches zero. In reality, the
SW node will ring to some degree immediately after it is
switched to ground by QB, causing some uncertainty as to
the actual moment the average current in QB goes to zero.
The LTC1702A minimizes this effect by ignoring the SW
node for a fixed 50ns after QB turns on when the ringing
is most severe, and by including a few millivolts offset in
the comparator that monitors the SW node. Despite these
precautions, some combinations of inductor and layout
parasitics can cause the LTC1702A to enter discontinuous
mode erratically. In many cases, the time that QB turns off
will correspond to a peak in the ringing waveform at the
SW pin (Figure 6). This erratic operation isn’t pretty, but
retains much of the efficiency benefit of discontinuous
mode and maintains regulation at all times.
Figure 6. Ringing at SW Causes Discontinuous
Comparator to Trip Early
V
SW
0V
5V
0V
BLANK
50ns
TIME
DISCONTINUOUS
COMPARATOR
TURNS OFF BG
OUT
TIME
, the voltage across the
TIME
1702A F06
1702afa