Rainbow Electronics MAX15054 User Manual

Page 7

High-Side MOSFET Driver for HB LED Drivers

and DC-DC Applications

MAX15054

_______________________________________________________________________________________ 7

Undervoltage Lockout

The MAX15054 drives an external high-side MOSFET.

Both the high- and low-side supplies feature separate

UVLO protection that monitors each driver’s input supply

voltage (BST-LX and V

). The low-side supply UVLO

threshold (V

DD_UVLO

) is referenced to GND and pulls

the driver output low when V

falls below 4V (typ)

irrespective of the high-side UVLO condition.
The high-side driver UVLO threshold (V

BST_UVLO

) is

referenced to LX, and pulls HDRV low when V

BST

falls

below 3.6V (typ) with respect to LX. After the MAX15054 is

first energized (V

> V

DD_UVLO

), the bootstrap capacitor

BST

) between BST and LX is not charged, and HDRV

does not switch since the BST-to-LX voltage is below

BST_UVLO

. An internal charging circuit charges the

BST-LX supply through an external Schottky diode and

within a short time C

BST

charges through V

and causes

BST

to exceed V

BST_UVLO

. HDRV then starts switching

and follows HI. The hysteresis is 0.2V (typ) for both UVLO

thresholds.

Output Driver

The MAX15054 driver contains low on-resistance

p-channel and n-channel devices in a totem pole

configuration for the driver output stage. This allows for

rapid turn-on and turn-off of high gate-charge (Qg)

external switching MOSFETs. The driver exhibits low

drain-to-source resistance (R

DS(ON)

) that decreases for

lower operating temperatures. Lower R

DS(ON)

means

higher source and sink currents from the device as

the external MOSFET gate capacitance charges and

discharges at a quicker rate, resulting in faster switching

speeds. The peak source and sink current provided by

the driver is 2.5A (typ).
Propagation delay from the logic input (HI) to the driver

output is 12ns (typ) (

Figure 1

). The internal driver also

contains break-before-make logic to eliminate shoot-

through conditions that cause unnecessarily high

operating supply currents, efficiency reduction, and

voltage spikes at V

Voltage from HDRV to LX is approximately equal to V

minus the diode drop of the bootstrap diode when in a

high state, and zero when in a low state.

Bootstrap Diode

Connect an external Schottky diode between V

and

BST, in conjunction with an external bootstrap capacitor

BST

), to provide the voltage required to turn on the

MOSFET (see the Typical Operating Circuits). The diode

charges the bootstrap capacitor from V

when the

high-side switch is off, and isolates V

when HDRV is

pulled high when the driver turns on.

Bootstrap Capacitor

The bootstrap capacitor (C

BST

) between BST and LX is

used to ensure adequate charge is available to switch

the high-side MOSFET. This capacitor is charged from

by an external bootstrap diode when the MOSFET

is off. The bootstrap capacitor value should be selected

carefully to avoid oscillations during turn-on and turn-

off at the HDRV output. Choose a capacitor value at

least 20 times greater than the total gate capacitance

of the MOSFET being switched. Use a low-ESR ceramic

capacitor (typically a minimum 0.1FF is needed). The

high-side MOSFET’s continuous on-time is limited due

to the charge loss from the high-side driver’s quiescent

current. The maximum on-time is dependent on the size

of C

BST

, I

BST

(125FA, max), and V

BST_UVLO

Driver Logic Input (HI)

The MAX15054 features a 5V CMOS logic input. The

required logic-input levels are independent of V

and

are capable of withstanding up to 13.5V. For example,

the MAX15054 can be powered by a 5V supply while the

logic inputs are provided from 12V logic. Additionally, HI

is protected against voltage spikes up to 15V, regardless

of the V

voltage. The logic input has 900mV hysteresis

to avoid double pulsing during signal transition. The logic

input is a high-impedance input (300kI, typ) and should

not be left unconnected to ensure the input logic state is

at a known level. With the logic input unconnected, HDRV

pulls low as V

rises above the UVLO threshold. The

PWM output from the controller must assume a proper

state while powering up the device.