LT3956
3956f
block DiagraM
+–
+
–
+
–
+
–
+
–
+
+
–
A1
A3
A6
+
+
–
FREQ
PROG
1.25V
SSCLAMP
1.1V
CTRL
V
REF
EN/UVLO
ISP
ISN
Q2
350mV
20k
170k
140µA
2.1µA
CTRL
BUFFER
g
m
EAMP
PWM
COMPARATOR
DRIVER
1mA (MAX)
I
SENSE
A4
g
m
A10
A5
OVFB
COMPARATOR
1.25V
FB
SHORT-CIRCUIT
DETECT
SCILMB
SCILMB
5k
PWMOUT PWM
1.25V
V
IN
INTV
CC
VC
+
–
+
–
A2
R
Q
S
RAMP
GENERATOR
100kHz TO 1MHz
OSCILLATOR
+
–
+
–
A8
7.15V
LDO
SW
PGND
3956 BD
VMODE
GND
1.2V
FB
+
–
1.22V
+
–
2V
1.31V
RT
SS
SHDN
A7
10µA
10µA AT
FB = 1.25V
VC
TSD
165°C
FAULT
LOGIC
10µA
10µA AT
A1
+
= A1
–
V
REF
:
Voltage Reference Output Pin (typically 2V). This
pin drives a resistor divider for the CTRL pin, either for
analog dimming or for temperature limit/compensation
of LED load. Can supply up to 100μA.
PWM:
A signal low turns off switcher, idles oscillator and
disconnects V
C
pin from all internal loads. PWMOUT pin
follows PWM pin. PWM has an internal pull-down resistor.
If not used, connect to INTV
CC
.
VMODE
:
An open-collector pull-down on
VMODE
asserts
if the FB input is greater than the FB regulation threshold
minus 50mV (typical). To function, the pin requires an
external pull-up resistor. When the PWM input is low
and the DC/DC converter is idle, the
VMODE
condition is
latched to the last valid state when the PWM input was
high. When PWM input goes high again, the
VMODE
pin
will be updated. This pin may be used to report an open
LED fault. Use a pull-up current less than 1mA.
SS:
Soft-Start Pin. This pin modulates oscillator fre-
quency and compensation pin voltage (V
C
) clamp. The
soft-start interval is set with an external capacitor. The
pin has a 10µA (typical) pull-up current source to an
internal 2.5V rail. The soft-start pin is reset to GND by
an undervoltage condition (detected by EN/UVLO pin)
or thermal limit.
RT:
Switching Frequency Adjustment Pin. Set the fre-
quency using a resistor to GND (for resistor values, see
the Typical Performance curve or Table 1). Do not leave
the RT pin open.
pin FuncTions