LM34919C-Q1 Datasheet, PDF (14/29 Pages) Texas Instruments – LM34919C-Q1 Ultra Small 50V, 600 mA Constant On-Time Buck Switching Regulator

LM34919C-Q1
SNVS831A – SEPTEMBER 2013 – REVISED DECEMBER 2013
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LM34919C
VCC
C3
BST
C4
L1
SW
D1
ISEN
SGND
FB
D2
V_OUT
R1
R3
R2
C2
Figure 7. Self Biased Configuration
Regulation Comparator
The feedback voltage at FB is compared to the voltage at the soft-start pin. In normal operation (the output
voltage is regulated), an on-time period is initiated when the voltage at FB falls below 2.52 V. The buck switch
stays on for the programmed on-time causing the FB voltage to rise above 2.52 V. After the on-time period, the
buck switch stays off until the FB voltage falls below 2.52 V. Input bias current at the FB pin is less than 100 nA
over temperature.
Overvoltage Comparator and Undervoltage Comparator
The voltage at FB is compared to an internal overvoltage comparator reference (120% of internal reference
voltage). If the voltage at FB rises above this reference, the on-time pulse is immediately terminated. This
condition can occur if the input voltage or the output load changes suddenly, or if the inductor (L1) saturates. The
buck switch remains off until the voltage at FB falls below 2.52 V.
When the FB pin voltage rises above the undervoltage comparator voltage reference (92% of the internal
reference voltage), the PGD pin is released and is pulled high by the external pull-up resistor. When the FB pin
voltage measures less than 90% of the internal reference voltage, the PGD pin switches low.
ON-Time Timer
The on-time is determined by the R_ON resistor and the input voltage (V_IN), and is calculated from:
T_ON

R_ON K 35.5 K10^-12
V_IN
s
(4)
The inverse relationship with V_IN results in a nearly constant frequency as V_IN is varied. To set a specific
continuous conduction mode switching frequency (f_S), the R_ON resistor is determined from the following:
R_ON
V_OUT
F_S u 35.5 u10^-12
:
(5)
The minimum off-time limits the maximum duty cycle achievable with a low voltage at V_IN. The minimum on-time
is limited to ≊ 90 ns.
Enable
The LM34919C can be remotely shut down by forcing the Enable (EN) pin low. The bias and control circuits are
turned off when EN is pulled below the enable shutdown falling threshold of 1.3 V (typ). In the shutdown mode
the input current falls below 10 µA. If remote shutdown feature is not needed the EN pin can be connected to the
input voltage or any voltage greater than 3 V. In this case the device is enabled and disabled based on the VCC
UVLO threshold.
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