LM34927SD Datasheet, PDF (8/26 Pages) Texas Instruments – LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters

LM34927
SNVS799F – APRIL 2012 – REVISED DECEMBER 2013
Functional Description
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The LM34927 step-down switching regulator features all the functions needed to implement a low cost, efficient,
isolated bias supply. This high voltage regulator contains 100 V, N-channel buck and synchronous switches, is
easy to implement, and is provided in thermally enhanced SO PowerPAD-8 and WSON-8 packages. The
regulator operation is based on a constant on-time control scheme using an on-time inversely proportional to V_IN.
This control scheme does not require loop compensation. Current limit is implemented with forced off-time
inversely proportional to V_OUT. This scheme ensures short circuit protection while providing minimum foldback.
The simplified block diagram of the LM34927 is shown in Figure 12.
The LM34927 can be applied in numerous applications to efficiently regulate down higher voltages. This
regulator is well suited for 48 V telecom and 42 V automotive power bus ranges. Protection features include:
thermal shutdown, Undervoltage Lockout, minimum forced off-time, and an intelligent current limit.
Control Overview
The LM34927 regulator employs a control principle based on a comparator and a one-shot on-timer, with the
output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the
reference the internal buck switch is switched on for the one-shot timer period, which is a function of the input
voltage and the programming resistor (RT). Following the on-time the switch remains off until the FB voltage falls
below the reference, and the forced minimum off-time has expired. When the FB pin voltage falls below the
reference and the off-time one-shot period expires, the buck switch is then turned on for another on-time one-
shot period. This will continue until regulation is achieved and the FB voltage is approximately equal to 1.225 V
(typ).
In a synchronous buck converter, the low side (sync) FET is ‘on’ when the high side (buck) FET is ‘off’. The
inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’.
There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative
direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of
the output loading. The operating frequency remains relatively constant with load and line variations. The
operating frequency can be calculated as:
Csw
=
V_OUT
10^-10 x R_ON
(1)
The output voltage (V_OUT) is set by two external resistors (R_FB1, R_FB2). The regulated output voltage is calculated
as:
V_OUT
=
1.225V
x
R_FB2 + R_FB1
R_FB1
(2)
R_FB2
R_FB1
=
V_OUT - 1.225V
1.225V
(3)
This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor (C_OUT). A minimum of 25 mV of ripple voltage at the feedback pin (FB) is
required for the LM34927. In cases where the capacitor ESR is too small, additional series resistance may be
required (R_C in Figure 13).
For applications where lower output voltage ripple is required the output can be taken directly from a low ESR
output capacitor, as shown in Figure 13. However, R_C slightly degrades the load regulation.
V_CC Regulator
The LM34927 contains an internal high voltage linear regulator with a nominal output of 7.6 V. The input pin (V_IN)
can be connected directly to the line voltages up to 100 V. The V_CC regulator is internally current limited to 30mA.
The regulator sources current into the external capacitor at V_CC. This regulator supplies current to internal circuit
blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the V_CC pin
reaches the Undervoltage Lockout threshold of 4.5 V, the IC is enabled.
The V_CC regulator contains an internal diode connection to the BST pin to replenish the charge in the gate drive
boot capacitor when SW pin is low.
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