MP3435 – 19A, 600kHz, 22V SYNCHRONOUS BOOST WITH INPUT DISCONNECT

MP3435 Rev. 1.0

MonolithicPower.com

16

11/30/2021

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If internal current sensing is selected, OCP is

achieved by limiting IL(MAX) in every switching

cycle (without hiccup mode), unless VOUT is

pulled below VIN. After VSS exceeds about 0.7V,

the MP3435 can work in hiccup mode if it

detects that VOUT is below VIN. This prevents the

MP3435 from damage, even if there is no input

disconnect

MOSFET

under

a

heavy-load

condition.

If external current sensing is selected, GATE is

charged by a typical 13µA current from the

internal charge pump. Once VGATE reaches the

MOSFET threshold (VTH), the input current (IIN)

is generated, and the output capacitors are

charged.

VOUT

follows

VGATE

with

a

VTH

difference. The MP3435 has a current feedback

loop to control VGATE and VCOMP, so IIN does not

exceed VCL (mV) / RSENSE (mΩ).

During start-up with external current sensing (if

VGATE is below VIN + 1.6V), the linear charge

current limit works with the VCL / RSENSE

limitation, and VGATE is regulated to limit the

current. The MP3435 shuts down if the linear

charge current limit is triggered for over 0.5ms

by pulling GATE down to ground. The MP3435

waits for 20ms to 70ms (the hiccup time

depends on VIN and VOUT) to restart if it is not

reset by VIN or EN. A normal load does not lead

to hiccup protection during start-up.

If VGATE exceeds VIN + 1.6V, boost switching is

enabled.

SS

is

charged,

and

the

power

MOSFET periodically turns on and off to

regulate VOUT following the SS signal. When the

MP3435 starts switching and VOUT is below VIN,

both the linear charge current limit (regulated by

VGATE) and the boost input average current limit

(regulated by VCOMP) begin to work. Both control

loops work with the VCL / RSENSE limit.

After VOUT is charged above VIN in boost mode,

only the boost input average current limit works

(regulated by VCOMP). The MP3435 does not

trigger hiccup OCP unless VSS exceeds 0.7V,

and VOUT drops below VIN. If hiccup protection is

triggered in switching mode, the switching

stops, and GATE is pulled low. It will restart

after 20ms to 70ms, depending on VIN and VOUT.

The recovery process is the same as the start-

up process.

Table 1 shows the detailed OCP mode when

using an external current-sense resistor.

Table 1: OCP Modes with an External Current-Sense Resistor

Condition

Work Mode

OCP Action

VGATE < VIN + 1.6V

Linear charge mode,

no boost switching

The linear charge current limit works when:

1.

VGATE is regulated down to keep IIN at

VCL / RSENSE.

2.

If the linear charge OCP lasts 0.5ms, the MP3435

triggers hiccup protection.

The boost input average current limit does not work.

VGATE

≥ VIN + 1.6V (9)

VOUT

≤ VIN

VSS

≤ 0.7V

Boost switching

The linear charge current limit works when:

1.

VGATE is regulated down to keep IIN at

VCL / RSENSE.

2.

If the linear charge OCP lasts 0.5ms, the MP3435

triggers hiccup protection.

The boost input average current limit works when VCOMP is

regulated to keep the input average current at

VCL / RSENSE.

VGATE

≥ VIN + 1.6V (9)

VOUT

≤ VIN

VSS > 0.7V

Boost switching

Enters hiccup protection without a delay.

VGATE

≥ VIN + 1.6V (9)

VOUT > VIN

Boost switching

The linear charge current limit does not work, while VGATE

remains high. The boost input average current limit works

when VCOMP is regulated to keep the input average current at

VCL / RSENSE. There is no hiccup mode.

Note:

9) After start-up, the VGATE ≥ VIN + 1.6V condition is registered if VGATE exceeds VIN + 1.6V one time. This means the MP3435 treats the

condition as VGATE ≥ VIN + 1.6V, even if VGATE falls below VIN + 1.6V again in protection mode (unless the device turns off due to hiccup

mode or if the power is cycled.