Datasheet

200 kHz
R
T
= 32.4 k: x
F
SW
(kHz)
- 0.8 k:
LM5015
www.ti.com SNVS538C –NOVEMBER 2007–REVISED APRIL 2013
High Side Bootstrap Bias
The high side bootstrap bias provides power to drive the high side power MOSFET. An external capacitor is
required between the BST and the HO pins. A minimum capacitor value of 0.022 µF is recommended. The
capacitor is charged from VCC via an internal diode during each power MOSFET off-time.
Oscillator
A single external resistor connected between RT and AGND pins sets the LM5015 oscillator frequency. To set a
desired oscillator frequency (F
SW
), the necessary value for the R
T
resistor can be calculated from the following
equation:
(1)
The tolerance of the external resistor and the frequency tolerance indicated in the Electrical Characteristics table
must be taken into account when determining the total variation of the switching frequency.
External Synchronization
The LM5015 can be synchronized to the rising edge of an external clock. Because the oscillator uses a divide-
by-two circuit, the switching frequency F
SW
in the above equation is actually half the native oscillator frequency.
Therefore, in order to synchronize, the external clock must have a frequency higher than twice the free running
F
SW
set by the R
T
resistor. The clock signal should be coupled through a 100 pF capacitor into the RT pin. A
peak voltage level greater than 3.2V at the RT pin is required for detection of the sync pulse. The DC voltage
across the R
T
resistor is internally regulated at 1.5 volts. The negative portion of the AC voltage of the
synchronizing clock is clamped to 1.5V by an amplifier inside the LM5015 with approximately 100Ω output
impedance. Therefore, the AC pulse superimposed on the R
T
resistor must have positive pulse amplitude of 1.7V
or greater to successfully synchronize the oscillator. The sync pulse width measured at the RT pin should have a
duration greater than 15 ns and less than 5% of the switching period. The R
T
resistor is always required, whether
the oscillator is free running or externally synchronized. The R
T
resistor should be located very close to the
device and connected directly to the RT and AGND pins of the LM5015.
Enable / Standby
The LM5015 contains a dual level Enable circuit. When the EN pin voltage is below 0.45V, the IC is in a low
current shutdown mode with the VCC LDO disabled. When the EN pin voltage is raised above the 0.45V
shutdown threshold but below the 1.26V standby threshold, the VCC LDO regulator is enabled, while the
remainder of the IC is disabled. When the EN pin voltage is raised above the 1.26V standby threshold, all
functions are enabled and normal operation begins. An internal 6 µA current source pulls up the EN pin to
activate the IC when the EN pin is left disconnected.
An external set-point resistor divider from VIN to AGND can be used to determine the minimum operating input
voltage of the regulator. The divider must be designed such that the EN pin exceeds the 1.26V standby threshold
when VIN is in the desired operating range. The internal 6 µA current source should be included when
determining the resistor values. The shutdown and standby thresholds have 100 mV hysteresis to prevent noise
from toggling between modes. The EN pin is internally protected by a 6V Zener diode through a 1 kΩ resistor.
The enabling voltage may exceed the Zener voltage, however the Zener current should be limited to less than 4
mA.
Error Amplifier and PWM Comparator
An internal high gain error amplifier generates an error signal proportional to the difference between the
regulated output voltage and an internal precision reference. The output of the error amplifier is connected to the
COMP pin allowing the user to add loop compensation, typically a Type II network, as illustrated in Figure 15.
This network creates a pole at the origin that rolls off the high DC gain of the amplifier, which is necessary to
accurately regulate the output voltage. A zero provides phase boost near the open loop unity gain frequency, and
a high frequency pole attenuates switching noise. The PWM comparator compares the current sense signal from
the current sense amplifier to the error amplifier output voltage at the COMP pin.
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