LTC1625IGN#TR Linear Technology, LTC1625IGN#TR Datasheet - Page 16

• Manufacturer Part Number: LTC1625IGN#TR
• Description: IC SW REG STEP-DOWN SYNC 16-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1625IGN#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.19 ~ 36 V
• Current - Output: 50mA
• Frequency - Switching: 150kHz
• Voltage - Input: 3.7 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LTC1625IGNTR; Q1127755

APPLICATIONS
LTC1625
where L1, L2, etc. are the individual losses as a percentage
of input power. It is often useful to analyze individual
losses to determine what is limiting the efficiency and
which change would produce the most improvement.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1625 circuits:
1. INTV
2. DC I
3. Transition losses apply only to the topside MOSFET,
16
and control currents. The driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched on and then off,
a packet of gate charge Q
ground. The resulting current out of INTV
much larger than the control circuit current. In continu-
ous mode, I
By powering EXTV
the additional V
control currents will be scaled by a factor of Duty Cycle/
Efficiency. For example, in a 20V to 5V application at
400mA load, 10mA of INTV
proximately 3mA of V
from 10% (if the driver was powered directly from V
to about 3%.
tor, DC I
MOSFETs and inductor. In continuous mode the aver-
age output current flows through L, but is “chopped”
between the top MOSFET and the bottom MOSFET. If
the two MOSFETs have approximately the same R
then the resistance of one MOSFET can simply be
summed with the resistance of L to obtain the DC I
loss. For example, if each R
0.15 , then the total resistance is 0.2 . This results in
losses ranging from 2% to 8% as the output current
increases from 0.5A to 2A for a 5V output. I
cause the efficiency to drop at high output currents.
and only when operating at high input voltages (typi-
cally 20V or greater). Transition losses can be esti-
mated from:
Transition Loss = (1.7)(V
2
CC
R Losses. Since there is no separate sense resis-
current. This is the sum of the MOSFET driver
2
R losses arise only from the resistances of the
GATECHG
IN
U
current resulting from the driver and
CC
= f(Q
IN
INFORMATION
from an output-derived source,
U
current. This reduces the loss
g(TOP)
g
IN
DS(ON)
CC
2
moves from INTV
)(I
current results in ap-
+ Q
W
O(MAX)
= 0.05 and R
g(BOT)
)(C
CC
).
RSS
is typically
U
2
R losses
)(f)
DS(ON)
CC
L
IN
2
to
R
=
)
,
4. LTC1625 V
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, V
by an amount equal to ( I
effective series resistance of C
charge or discharge. The regulator loop acts on the
resulting feedback error signal to return V
state value. During this recovery time V
tored for overshoot or ringing which would indicate a
stability problem. The I
in Figure 1 will provide adequate compensation for most
applications.
A second, more severe transient is caused by connecting
loads with large (> 1 F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive in order
to limit the inrush current to the load.
Automotive Considerations: Plugging into the
Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during opera-
tion. But before you connect, be advised: you are plug-
ging into the supply from hell. The main battery line in an
supply current to the controller excluding MOSFET gate
drive current. Total supply current is typically about
850 A. If EXTV
draw only 330 A from V
come from EXTV
(< 1%) loss which increases with V
Other losses including C
losses, Schottky conduction losses during dead time
and inductor core losses, generally account for less
than 2% total additional loss.
OUT
, causing a rapid drop in V
IN
supply current. The V
CC
is connected to 5V, the LTC1625 will
CC
TH
. V
pin external components shown
IN
IN
LOAD
IN
and the remaining 520 A will
current results in a small
and C
)(ESR), where ESR is the
OUT
OUT
, and C
OUT
OUT
IN
IN
immediately shifts
. No regulator can
OUT
.
current is the DC
OUT
ESR dissipative
OUT
can be moni-
to its steady-
begins to