LTC1702IGN Linear Technology, LTC1702IGN Datasheet - Page 14

• Manufacturer Part Number: LTC1702IGN
• Description: IC REG SW DUAL SYNC 2PH 24SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC1702IGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Current - Output: 1A, 25A
• Frequency - Switching: 550kHz
• Voltage - Input: 3 ~ 7 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Power - Output: -

LTC1702
APPLICATIONS
ripple voltage and frequency remain constant at all loads,
easing filtering requirements. Circuits that take advantage
of this behavior can force the LTC1702 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 LTC1702 switches to a second mode of
operation: discontinuous mode (Figure 5b). In discontinu-
ous mode, the LTC1702 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 LTC1702 detects that the inductor current has reached
zero by monitoring the voltage at the SW pin while QB is
14
I
RIPPLE
Figure 5b. Discontinuous Mode
Figure 5a. Continuous Mode
OUT
U
I
, the voltage across the inductor will
RIPPLE
INFORMATION
TIME
U
TIME
W
I
AVERAGE
I
AVERAGE
1702 F05b
1702 F05a
U
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 LTC1702 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 LTC1702 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.
Burst Mode Operation
Discontinuous mode removes the resistive loss drop term
in QB, but the LTC1702 is still switching QT and QB on and
off once a cycle. Each time an external MOSFET is turned
on, the internal driver must charge its gate to V
time it is turned off, that charge is lost to ground. At the
high switching frequencies that the LTC1702 operates at,
the charge lost to the gates can add up to tens of milliamps
from V
quickly become the dominant power loss term, reducing
efficiency once again.
CC
Figure 6. Ringing at SW Causes Discontinuous
Comparator to Trip Early
. As the load current continues to drop, this
V
SW
0V
5V
0V
BLANK
TIME
50ns
DISCONTINUOUS
TURNS OFF BG
COMPARATOR
TIME
TIME
1702 F06
CC
. Each
1702fa