LTC1703CG#TRPBF Linear Technology, LTC1703CG#TRPBF Datasheet - Page 14

• Manufacturer Part Number: LTC1703CG#TRPBF
• Description: IC REG SW DUAL SYNC VID 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1703CG#TRPBF

• Applications: Controller, Mobile Intel Pentium® III
• Voltage - Input: 3 ~ 7 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 2 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
LTC1703
LTC1703 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
current in the inductor 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 LTC1703) 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 LTC1703 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 LTC1703 switches to a second mode of
operation: discontinuous mode (Figure 5b). In discontinu-
ous mode, the LTC1703 detects when the inductor current
approaches zero and turns off QB for the remainder of the
switch cycle. During this time, the voltage at the SW pin
will float about V
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, eliminat-
ing that power loss term. It also reduces the ripple current
in the inductor as the output current approaches zero.
The LTC1703 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 LTC1703 minimizes this effect by ignoring the SW
node for a fixed 50ns after QB turns on when the ringing
14
OUT
U
, the voltage across the inductor will
U
W
U
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 LTC1703 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
I
RIPPLE
0V
5V
0V
Figure 5b. Discontinuous Mode
Figure 5a. Continuous Mode
BLANK
I
TIME
RIPPLE
50ns
DISCONTINUOUS
TURNS OFF BG
COMPARATOR
TIME
TIME
TIME
TIME
1703 F06
I
AVERAGE
I
AVERAGE
1703 F05b
1703 F05a
1703fa