Datasheet

LTC4010
13
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the average current through R
SENSE
. The I
TH
output of
the error amplifier is a scaled control current for the input
of the PWM comparator CC. The I
TH
• R3 product sets a
peak current threshold for CC such that the desired aver-
age current through R
SENSE
is maintained. The current
comparator output does this by switching the state of the
SR latch at the appropriate time.
At the beginning of each oscillator cycle, the PWM clock
sets the SR latch and the external P-channel MOSFET is
switched on (N-channel MOSFET switched off) to refresh
the current carried by the external inductor. The inductor
current and voltage drop across R
SENSE
begin to rise
linearly. During normal operation, the PFET is turned
off (NFET on) during the cycle by CC when the voltage
difference across R
SENSE
reaches the peak value set by
the output of EA. The inductor current then ramps down
linearly until the next rising PWM clock edge. This closes
the loop and maintains the desired average charge current
in the external inductor.
Low Dropout Charging
After charging is initiated, the LTC4010 does not require
that V
CC
remain at least 500mV above BAT because situ-
ations exist where low dropout charging might occur. In
one instance, parasitic series resistance may limit PWM
headroom (between V
CC
and BAT) as 100% charge is
reached. A second case can arise when the DC adapter
selected by the end user is not capable of delivering the
current programmed by R
SENSE
, causing the output volt-
age of the adapter to collapse. While in low dropout, the
LTC4010 PWM runs near 100% duty cycle with a frequency
that may not be constant and can be less than 550kHz.
The charge current will drop below the programmed value
to avoid generating audible noise, so the actual charge
delivered to the battery may depend primarily on the
LTC4010 charge timer.
Internal Die Temperature
The LTC4010 provides internal overtemperature detection
to protect against electrical overstress, primarily at the
FET driver outputs. If the die temperature rises above this
thermal limit, the LTC4010 stops switching and indicates
a fault as previously discussed.
operaTion
(Refer to Figure 1)